Category: Chemistry

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Chemical Properties of Aldehydes and Ketones

A. Nucleophilic Addition Reactions

This reaction is the most common reactions of aldehydes and ketones. The carbonyl carbon carries a small degree of positive charge. Nucleophile such as CN^– can attack the carbonyl carbon and uses its lone pair to form a new carbon – nucleophile ‘σ’ bond, at the same time two electrons from the carbon – oxygen double bond move to the most electronegative oxygen atom. This results in the formation of an alkoxide ion. In this process, the hybridisation of carbon changes from sp² to sp³.

The tetrahedral intermediate can be protonated by water or an acid to form an alcohol.

In general, aldehydes are more reactive than ketones towards nucleophilic addition reactions due to +I and steric effect of alkyl groups.

Examples

1. Addition of HCN

Attack of CN^– on carbonyl carbon followed by protonation gives cyanohydrins.

The cyanohydrins can be converted into hydroxy acid by acid hydrolysis. Reduction of cyanohydrins gives hydroxy amines.

2. Addition of NaHSO₃

This reaction finds application in the separation and purification of carbonyl compound. The bisulphate addition compound is water soluble and the solution is treated with mineral acid to regenerate the carbonyl compounds.

3. Addition of Alcohol

When aldehydes / ketones is treated with 2 equivalents of an alcohol in the presence of an acid catalyst to form acetals.

Example

When acetaldehyde is treated with 2 equivalent of methanol in presence of HCl, 1, 1, – dimethoxy ethane is obtained.

Mechanism

4. Addition of Ammonia and its Derivatives

When the nucleophiles, such as ammonia and its derivative image 6 is treated with carbonyl compound, nuceophilic addition takes place, the carbonyl oxygen atom is protonated and then elimination takes place to form carbon – nitrogen double bond image 7

When G – alkyl, aryl, OH, NH₂, C₆H₅NH, NHCONH₂ etc…

(i) Reaction with Hydroxyl Amine

Aldehyde and ketones react with hydroxylamine to form oxime.

Example:

(ii) Reaction with Hydrazine

Aldehydes and ketones react with hydrazine to form hydrazone.

Example:

(iii) Reaction with Phenyl Hydrazine

Aldehydes and ketones react with phenyl hydrazine to form phenyl hydrazone.

Example:

5. Reaction with NH₃

(i) Aliphatic aldehydes (except formaldehyde) react with an ethereal solution of ammonia to form aldimines.

(ii) Formaldehyde reacts with ammonia to form hexa methylene tetramine, which is also known as Urotropine.

Structure

Uses

1. Urotropine is used as a medicine to treat urinary infection.

2. Nitration of Urotropine under controlled condition gives an explosive RDX (Research and development explosive). It is also called cyclonite or cyclotri methylene trinitramine.

3. Acetone reacts with ammonia to form diacetone amine.

4. Benzaldehyde form a complex condensation product with ammonia.

B. Oxidation of Aldehydes and Ketones

(a) Oxidation of Aldehydes

Aldehydes are easily oxidised to carboxylic acid containing the same number of carbon atom, as in parent aldehyde. The common oxidising agents are acidified K₂Cr₂O₇, acidic or alkaline KMnO₄ or chromic oxide.

Example

(b) Oxidation of Ketone

Ketones are not easily oxidised. Under drastic condition or with powerful oxidising agent like Con. HNO₃, H⁺/KMnO₄, H⁺/K₂Cr₂O₇, cleavage of carbon-carbon bond takes place to give a mixture of carboxylic acids having less number of carbon atom than the parent ketone.

The oxidation of unsymmetrical ketones is governed by Popoff ’s rule. It states that during the oxidation of an unsymmetrical ketone, a (C-CO) bond is cleaved in such a way that the keto group stays with the smaller alkyl group.

C. Reduction Reactions

(i) Reduction to Alcohols

We have already learnt that aldehydes and ketones can be easily reduced to primary and secondary alcohols respectively. The most commonly used reducing agents are Lithium Aluminium hydride (LiAlH₄), and Sodium borohydride (NaBH₄).

(a) Aldehyde are Reduced to Primary Alcohols

Example

(b) Ketone are Reduced to Secondary Alcohols.

Example

The above reactions can also be carried out with hydrogen in the presence of metal catalyst like Pt, Pd, or Ni. LiAlH₄ and NaBH₄ do not reduce isolated carbon – carbon double bonds and double bond of benzene rings. In case of α, β unsaturated aldehyde and ketones, LiAlH₄ reduces only C = O group leaving C = C bond as such.

(ii) Reduction to Hydrocarbon

The carbonyl group of aldehydes and ketones can be reduced to methylene group using suitable reducing agents to give hydrocarbons.

(a) Clemmensen Reduction

Aldehydes and Ketones when heated with zinc amalgam and concentrated hydrochloric acid gives hydrocarbons.

Example

(b) Wolf Kishner Reduction

Aldehydes and Ketones when heated with hydrazine (NH₂NH₂) and sodium ethoxide, hydrocarbons are formed Hydrazine acts as a reducing agent and sodium ethoxide as a catalyst.

Example

Aldehyde (or) ketones is first converted to its hydrazone which on heating with strong base gives hydrocarbons.

(iii) Reduction to Pinacols:

Ketones, on reduction with magnesium amalgam and water, are reduced to symmetrical diols known as pinacol.

D. Haloform Reaction

Acetaldehyde and methyl ketones, containing image 26 group, when treated with halogen and alkali give the corresponding haloform. This is known as Haloform reaction.

E. Reaction Involving Alkylgroup

(i) Aldol Condensation

The carbon attached to carbonyl carbon is called α – carbon and the hydrogen atom attached to α – carbon is called α – hydrogen.

In presence of dilute base NaOH, or KOH, two molecules of an aldehyde or ketone having α – hydrogen add together to give β – hydroxyl aldehyde (aldol) or β – hydroxyl ketone (ketol). The reaction is called aldol condensation reaction. The aldol or ketol readily loses water to give α, β – unsaturated compounds which are aldol condensation products.

(a) Acetaldehyde when warmed with dil NaOH gives β – hydroxyl butyraldehyde (acetaldol)

Mechanism

The mechanism of aldol condensation of acetaldehyde takes place in three steps.

Step 1:

The carbanion is formed as the α – hydrogen atom is removed as a proton by the base.

Step 2:

The carbanion attacks the carbonyl carbon of another unionized aldehyde to form an alkoxide ion.

Step 3:

The alkoxide ion formed is protonated by water to form aldol.

The aldol rapidly undergoes dehydration on heating with acid to form α – β unsaturated aldehyde.

(ii) Crossed Aldol Condensation

Aldol condensation can also take place between two different aldehydes or ketones or between one aldehyde and one ketone such an aldol condensation is called crossed or mixed aldol condensation. This reaction is not very useful as the product is usually a mixture of all possible condensation products and cannot be separated easily.

Example

F. Some Important Reactions of Benzaldehyde

(i) Claisen – Schmidt Condensation

Benzaldehye condenses with aliphatic aldehyde or methyl ketone in the presence of dil. alkali at room temperature to form unsaturated aldehyde or ketone. This type of reaction is called Claisen – Schmidt condensation.

Example

(ii) Cannizaro Reaction

In the presence of concentrated aqueous or alcoholic alkali, aldehydes which do not have α – hydrogen atom undergo self oxidation and reduction (disproportionation) to give a mixture of alcohol and a salt of carboxylic acid. This reaction is called Cannizaro reaction.

Benzaldehyde on treatment with concentrated NaOH (50%) gives benzyl alcohol and sodium benzoate.

This reaction is an example disproportionation reaction

Mechanism of Cannizaro Reaction

Cannizaro reaction involves three steps.

Step 1:

Attack of OH^– on the carbonyl carbon.

Step 2:

Hydride ion transfer

Step 3:

Acid – base reaction.

Cannizaro reaction is a characteristic of aldehyde having no α – hydrogen.

Crossed Cannizaro Reaction

When Cannizaro reaction takes place between two different aldehydes (neither containing an α hydrogen atom), the reaction is called as crossed cannizaro reaction.

In crossed cannizaro reaction more reactive aldehyde is oxidized and less reactive aldehyde is reduced.

3. Benzoin Condensation

The Benzoin condensation involves the treatment of an aromatic aldehyde with aqueous alcoholic KCN. The products are a hydroxy ketone.

Example

Benzaldehyde reacts with alcoholic KCN to form benzoin

4. Perkins’ Reaction

When an aromatic aldehyde is heated with an aliphatic acid anhydride in the presence of the sodium salt of the acid corresponding to the anhydride, condensation takes place and an α, β unsaturated acid is obtained. This reaction is known as Perkin’s reaction.

Example

5. Knoevenagal Reaction

Benzaldehyde condenses with malonic acid in presence of pyridine forming cinnamic acid, Pyridine act as the basic catalyst.

6. Reaction with Amine

Aromatic aldehydes react with primary amines (aliphatic or aromatic) in the presence of an acid to form schiff’s base.

Example

7. Condensation with Tertiary Aromatic Amines

Benzaldehyde condenses with tertiary aromatic amines like N, N – dimethyl aniline in the presence of strong acids to form triphenyl methane dye.

Example

8. Electrophilic Substitution Reactions of Benzaldehyde

Electrophilic Substitution Reaction of Acetophenone

Acetophenone reacts with Nitrating mixture to form m – nitroacetophenone.

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Physical Properties of Aldehydes and Ketones

1. Physical State:

Formaldehyde is a gas at room temperature and acetaldehyde is a volatile liquid. All other aldehydes and ketones upto to C₁₁ are colourless liquids while the higher ones are solids.

2. Boiling Points

Aldehydes and ketones have relatively high boiling point as compared to hydrocarbons and ethers of comparable molecular mass. It is due to the weak molecular association in aldehydes and ketones arising out of the dipole-dipole interactions.

These dipole-dipole interactions are weaker than intermolecular H-bonding. The boiling points of aldehydes and ketones are much lower than those of corresponding alcohols and carboxylic acids which possess inter molecular hydrogen bonding.

3. Solubility

Lower members of aldehydes and ketones like formaldehyde, acetaldehyde and acetone are miscible with water in all proportions because they form hydrogen bond with water. Solubility of aldehydes and ketones decreases rapidly on increasing the length of alkyl chain.

4. Dipolemoment

The carbonyl group of aldehydes and ketones contains a double bond between carbon and oxygen. Oxygen is more electronegative than carbon and it attracts the shared pair of electron which makes the carbonyl group as polar and hence aldehydes and ketones have high dipole moments.

Solubility:

Aldehydes and ketones are soluble in water but their solubility decreases with increase in the length of the chain. Methanal, ethanal and propanone are those aldehydes and ketones which are of small size and are miscible with water in almost all proportions.

Physical Properties of Ketones

Ketones are highly reactive, although less so than aldehydes, to which they are closely related. Much of their chemical activity results from the nature of the carbonyl group. Ketones readily undergo a wide variety of chemical reactions.

Properties of Aldehydes

The reactivity of these compounds arises largely through two features of their structures:

The polarity of the carbonyl group and the acidity of any α-hydrogens that are present. Aldehydes are polar molecules, and many reagents seek atoms with a deficiency of electrons.

Both aldehydes and ketones contain a carbonyl group. That means that their reactions are very similar in this respect. An aldehyde differs from a ketone by having a hydrogen atom attached to the carbonyl group. This makes the aldehydes very easy to oxidise.

Carboxylic acids have high boiling points compared to other substances of comparable molar mass. Boiling points increase with molar mass. Carboxylic acids having one to four carbon atoms are completely miscible with water. Solubility decreases with molar mass.

Aldehydes and ketones have a much higher boiling point than the alkanes. As the molecules get larger, the difference between an aldehyde/ketone and its corresponding alkane gets smaller. The reason for this is that the non-polar region of the carbon chain is getting larger as the polar region (C=O) is staying the same.

Examples of Aldehydes

Aldehydes are given the same name but with the suffix – ic acid replaced by – aldehyde. Two examples are formaldehyde and benzaldehyde. As another example, the common name of CH₂=CHCHO, for which the IUPAC name is 2-propenal, is acrolein, a name derived from that of acrylic acid, the parent carboxylic acid.

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General Methods of Preparation of Aldehydes and Ketones

A. Preparation of Aldehydes and Ketones

1. Oxidation and Catalytic Dehydrogenation of Alcohols

We have already learnt that the oxidation of primary alcohol gives aldehydes and secondary alcohol gives a ketone. Oxidising agents such as acidified Na₂Cr₂O₇, KMnO₄, PCC are used for oxidation. Oxidation using PCC yield aldehydes. Other oxidising agents further oxidise the aldhydes / ketones in to carboxylic acids.

When vapours of alcohols are passed over heavy metal catalyst such as Cu, Ag, alcohols give aldehydes and ketons.

Catalytic Dehydrogenation of Alcohols

2. Ozonolysis of Alkenes

We have already learnt in XIth standard that the reductive ozonolysis of alkenes gives aldehydes and ketones. Alkenes react with ozone to form ozonide which on subsequent cleavage with zinc and water gives aldehydes and ketones. Zinc dust removes H₂O₂ formed, which otherwise can oxidise aldehydes/ketones.

Terminal olefies give formaldehyde as one of the product.

3. Hydration of Alkynes

We have already learnt in XI standard that the hydration of alkynes in presence of 40% dilute sulphuric acid and 1% HgSO₄ to give the corresponding aldehydes/ketones.

(a) Hydration of acetylene yields acetaldehyde

(b) Hydration of alkynes, other than acetylene gives ketones

4. From Calcium Salts of Carboxylic Acids

Aldehydes and ketones may be prepared by the dry distillation of calcium salts of carboxylic acids.

(a) Aldehydes are obtained when the mixture of calcium salt of carboxylic acid and calcium formate is subjected to dry distillation.

(b) Symmetrical ketones can be obtained by dry distillation of the calcium salt of carboxylic acid (except formic acid)

B. Preparation of Aldehydes

1. Rosenmund Reduction

(a) Aldehydes can be prepared by the hydrogenation of acid chloride, in the presence of palladium supported by barium sulphate. This reaction is called Rosenmund reduction.

2. Stephen’s Reaction

When alkylcyanides are reduced using SnCl₂/HCl, imines are formed, which on hydrolysis gives corresponding aldehyde.

3. Selective Reduction of Cyanides

Diisobutyl aluminium hydride (DIBAL -H) selectively reduces the alkyl cyanides to form imines which on hydrolysis gives aldehydes.

(c) Preparation of Benzaldehyde

1. Side chain oxidation of toluene and its derivatives by strong oxidising agents such as KMnO₄ gives benzoic acid. When chromylchloride is used as an oxidising agent, toluene gives benzaldehyde. This reaction is called Etard reaction. Acetic anhydride and CrO₃ can also be used for this reaction.

Oxidation of toluene by chromic oxide gives benzylidine diacetate which on hydrolysis gives benzaldehyde.

2. Gattermann – Koch Reaction

This reaction is a variant of Friedel – Crafts acylation reaction. In this method, reaction of carbon monoxide and HCl generate an intermediate which reacts like formyl chloride.

3. Manufacture of Benzaldehyde from Toluene

Side chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde.

This is the commercial method for the manufacture of benzaldehye.

(d) Preparation of Ketones

1. Ketones can be prepared by the action of acid chloride with dialkyl cadmium.

2. Preparation of Phenyl Ketones

Friedel – Craft Acylation

It is the best method for preparing alkyl aryl ketones or diaryl ketones. This reaction succeeds only with benzene and activated benzene derivatives.

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Structure of Carbonyl Group

The carbonyl carbon is sp² hybridised and the carbon – oxygen bond is similar to carbon – carbon double bond in alkenes. The carbonyl carbon forms three σ bonds using their three sp² hybridised orbital. One of the sigma bond is formed with oxygen and the other two with hydrogen and carbon (in aldehydes) or with two carbons (in ketones).

All the three ‘σ’ bonded atoms are lying on the same plane as shown in the fig (12.1). The fourth valence electron of carbon remains in its unhybridised ‘2p’ orbital which lies perpendicular to the plane and it overlaps with 2p orbital of oxygen to form a carbon – oxygen π bond. The oxygen atom has two nonbonding pairs of electrons, which occupy its remaining two p-orbitals.

Oxygen, the second most electro negative atom attracts the shaired pair of electron between the carbon and oxygen towards itself and hence the bond is polar. This polarisation contributes to the reactivity of aldehydes and ketones.

A carbonyl group is a chemically organic functional group composed of a carbon atom double-bonded to an oxygen atom → [C=O]. The simplest carbonyl groups are aldehydes and ketones usually attached to another carbon compound. These structures can be found in many aromatic compounds contributing to smell and taste.

Properties of Carbonyl Compounds:

1. These are to be polar in nature. They exhibit both positive and negative charge in slight form.
2. These compounds are reported to be insoluble in water but sometimes they dissolve other forms of polar molecules.
3. These are known to be as chemically reactive compounds.

Carbonyl group, in organic chemistry, a divalent chemical unit consisting of a carbon (C) and an oxygen (O) atom connected by a double bond.

Formula of Carbonyl Group

Both organic families have the general formula C_nH_2nO. The carbonyl functional group (C = O) consists of a carbon atom double-bonded to an oxygen atom. The position of the C=O. functional group in the carbon chain marks the difference between aldehydes and ketones.

Examples:

Examples of inorganic carbonyl compounds are carbon dioxide and carbonyl sulphide. A special group of carbonyl compounds are 1, 3-dicarbonyl compounds that have acidic protons in the central methylene unit. Examples are Meldrum’s acid, diethyl malonate and acetylacetone.

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Nomenclature of Aldehydes and Ketones

We have already learnt the IUPAC system of nomenclature of organic compounds in XI^th standard. Let us apply the rules to name the following compounds.

*PIN – Preferred IUPAC name

Aldehydes contain the carbonyl group bonded to at least one hydrogen atom. Ketones contain the carbonyl group bonded to two carbon atoms.

Naming Ketones

1. Ketones take their name from their parent alkane chains.
2. The common name for ketones are simply the substituent groups listed alphabetically +ketone.

They are named by finding the carbonyl group and identifying it with a location number, if necessary, then adding the suffix “- one.” The common name for ketones is determined by naming the alkyl groups attached to the carbonyl (in alphabetical order), then adding ‘ketone’.

For an aldehyde, drop the -e from the alkane name and add the ending -al. Methanal is the IUPAC name for formaldehyde, and ethanal is the name for acetaldehyde.

Nomeclature of ketone

The parent chain is numbered from the end that gives the carbonyl carbon the smaller number. The suffix -e of the parent alkane is changed to -one to show that the compound is a ketone.

Characteristics of Aldehydes and Ketones

Aldehydes and ketones are the class of organic compounds that have a carbonyl group i.e. carbon-oxygen double bond (-C=O). As they do not have any other reactive groups like -OH or -Cl attached to the carbon atom in the carbonyl group they are very simple compounds.

Functional Group of Ketone

Nomenclature of Aldehydes and Ketones. Aldehydes and ketones are organic compounds which incorporate a carbonyl functional group, C=O. The carbon atom of this group has two remaining bonds that may be occupied by hydrogen or alkyl or aryl substituents.

You will remember that the difference between an aldehyde and a ketone is the presence of a hydrogen atom attached to the carbon-oxygen double bond in the aldehyde. Ketones don’t have that hydrogen. Aldehydes are easily oxidized by all sorts of different oxidizing agents: ketones are not.

Aldehyde Formula

Aldehyde is a chemical compound with a functional group -CHO. The general formula of alkene is CnH_2n+1 so the general formula for aldehyde will be CnH_2n+1CHO or CnH_2nO.

An aldehyde is similar to a ketone, except that instead of two side groups connected to the carbonyl carbon, they have at least one hydrogen (RCOH). The simplest aldehyde is formaldehyde (HCOH), as it has two hydrogens connected to the carbonyl group.

Combined with other functional group aldehydes and ketone are widespread in nature. Compounds such as cinnamaldehyde (cinnamon bark), vanillin (vanilla bean), Citra (lemongrass), helminthosporal (a fungal toxin), carvone (spearmint and caraway), camphor (camphor trees) are found chiefly in microorganisms or plants.

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Iupac Nomenclature

We have already learnt about naming the organic compounds according to IUPAC guidelines in XI standard. Let us recall the basic rules to name the alcohols.

1. Select the longest continuous chain of carbon atoms (root word) containing the functional group (- OH).
2. Number the carbon atoms in the chain so that the carbon bearing the – OH group has the lowest possible number.
3. Name the substituent (if any)
4. Write the name of the alcohol as below.

The following table illustrates the IUPAC nomenclature of alcohols.

Structure of the Functional Group of Alcohol

The structure of -O-H group which is attached to a sp³ hybridised carbon is similar to the structure of -O-H group attached to a hydrogen in water. i.e., ‘V’ shaped.

In such alcohols, one of the sp³ hybridised orbital of oxygen linearly overlap with the sp³ hybridised orbital of carbon to form a C-O, ‘σ’ bond and another sp³ hybridised orbital linearly overlap with 1s orbital of hydrogen to form a O-H ‘σ’ bond. The remaining two sp³ hybridised orbitals of oxygen are occupied by two lone pairs of electrons. Due to the lone pair – lone pair repulsion, the C-O-H bond angle in methanol is reduced to 108.9° from the regular tetrahedral bond angle of 109.5°.

Preparation of Alcohols:

We have already learnt that the nucleophilic substitution reactions of alkyl halides with dilute alkali, conversion of alkenes to alcohols by hydration and the preparation of alcohols using Grignard reagent in XI standard. These reactions are summarised below.

1. From Alkyl Halides:

Alkyl halides on heating with dilute aqueous NaOH gives alcohols. Primary alkyl halides undergo substitution by SN² reaction. Secondary and tertiary alkyl halides usually undergo nucleophilic substitution by SN¹ mechanism.

If R = t-butyl, the reaction proceeds through the formation of t-butyl carbocation

2. From Alkenes:

Addition of water across the double bond of an alkene in presence of concentrated sulphuric acid gives alcohols. This addition reaction follows Markownikof ’s rule.

Example:

3. From Grignard Reagent:

Nucleophilic addition of Grignard reagent to aldehydes/ketones in presence of dry ether followed by the acid hydrolysis gives alcohols. Formaldehyde gives primary alcohol and other aldehydes give secondary alcohols. Ketones give tertiary alcohols.

4. Hydroboration:

Diborane reacts with an alkene to form trialkyl borane which on treatment with H₂O₂ in presence of NaOH
gives an alcohol. (Refer reactions of diborane) The overall reaction is hydration of an alkene. This reaction yields an anti-Markownikof ‘s product.

5. Reduction of Carbonyl Compounds:

Reduction of aldehydes/ketones with LiAlH₄ in the presence of solvents like THF (Tetrahydrofuran) followed by hydrolysis gives alcohols. Unlike other reducing agents such as Raney Ni, Na-Hg/H₂O, the lithium aluminium hydride does not reduce the carbon-carbon double bond present in unsaturated carbonyl compound and hence it is a best reagent to prepare unsaturated alcohols.

Preparation of Glycol

We have already learnt that the hydroxylation of ethylene using cold alkaline solution of potassium permanganate (Baeyer’s reagent) gives ethylene glycol.

Preparation of Glycerol

Glycerol occurs in many natural fats and it is also found in long chain fatty acids in the form of glyceryl esters (Triglycerides). The alkaline hydrolysis of these fats gives glycerol and the reaction is known as saponification.

Methods to Differentiate Primary, Secondary and Tertiary Alcohols.

The following tests are used to distinguish between 1°, 2° and 3° alcohols.

(a) Lucas Test:

When alcohols are treated with Lucas agent (a mixture of concentrated HCl and anhydrous ZnCl₂) at room temperature, tertiary alcohols react immediately to form a turbidity due to the formation of alkyl chloride which is insoluble in the medium. Secondary alcohols react within 10 minutes to form a turbidity of alkyl chloride where primary alcohols do not react at room temperature.

(b) Victor Meyer’s Test:

This test is based on the behaviour of the different nitro alkanes formed by the three types of alcohols with nitrous acid and it consists of the following steps.

• Alcohols are converted into alkyl iodide by treating it with I₂/P.
• Alkyl iodide so formed is then treated with AgNO₂ to form nitro alkanes.
• Nitro alkanes are fially treated with HNO₂ (mixture of NaNO₂/HCl) and the resultant solution is made alkaline with KOH.

Result:

• Primary alcohol gives red colour
• Secondary alcohol gives blue colour.
• No colouration will be observed in case of tertiary alcohol.

Properties of Alcohols

Physical Properties

1. Lower alcohols are colourless liquids and the higher members are waxy solids.
2. They have higher boiling points than the corresponding other organic compounds such as alkanes, aldehydes, ethers etc., this is due to the presence of intermolecular hydrogen bonding present in alcohols.
3. Among isomeric alcohols primary alcohols have higher boiling point and the tertiary alcohols have lower boiling points.
4. The lower members are highly soluble in water due to the formation of intermolecular hydrogen bonding with water.

Table: Boiling point of alcohols in comparision with other organic compounds.

Chemical Properties of Alcohols
Nucleophilic Substitution Reactions of Alcohols

Alcohol has a strong basic leaving group (OH^–). So, – OH group is first converted into – O⁺H₂ group by adding an acid. The – O⁺H₂ group in the protonated alcohol can be easily displaced by a nucleophile such as Br^– to give alkyl halides.

Example:

Alcohols undergo nucleophilic substitution reaction with hydro halic acids to form alkyl halides. In case of tertiary alcohols heating is required.

Alkyl Halide Formation from Primary Alcohols Follow SN₂ mechanism

Alkyl Halide Formation of Tertiary Alcohols Follow SN₁ mechanism.

The conversion of an alcohol to alkyl halide can also be effected using thionyl chloride

This reaction also follows the S_Ni mechanism in the presence of pyridine.

Elimination Reactions of Alcohols

When alcohols are heated with a suitable dehydrating agents like sulphuric acid, the H and OH present in the adjacent carbons of alcohols are lost, and it results in the formation of a carbon – carbon double bond. Phosphoric acid, anhydrous ZnCl₂, alumina etc., can also be used as dehydrating agents.

Example:

Mechanism

Primary alcohols undergo dehydration by E₂ mechanism

Tertiary alcohols undergo dehydration by E1 mechanism. It involves the formation of a carbocation.

Protonation of Alcohol

Step 1:

Step 2:
Dissociation of oxonium ion to form a carbonation.

Step 3:
Deprotonation of carbocation to form an alkene

Order of Reactivity:

The relative reactivities of alcohols in the dehydration reaction follows the order
primary < secondary < tertiary

Saytzef ‘s Rule

During intramolecular dehydration, if there is a possibility to form a carbon – carbon double bond at different locations, the preferred location is the one that gives the more (highly) substituted alkene i.e., the stable alkene.

For example, the dehydration of 3, 3 – dimethyl – 2 – butanol gives a mixture of alkenes. The secondary carbocation formed in this reaction undergoes rearrangement to form a more stable tertiary carbocation.

Oxidation of Alcohols

The important reactions of alcohols are their oxidation to give carbonyl compounds. The commonly used oxidising agent is acidified sodiumdichromate. Oxidation of primary alcohols give an aldehyde which on further oxidation gives the carboxylic acids. To stop the oxidation reaction at the aldehyde / ketone stage, pyridinium chlorochromate (PCC) is used as an oxidising agent.

Swern Oxidation

In this method, dimethyl sulfoxide (DMSO) is used as the oxidising agent, which converts alcohols to ketones / aldehydes. In this method an alcohol is treated with DMSO and oxalyl chloride followed by the addition of triethylamine.

Biological Oxidation

The fermentation of the food consumed by an animal produces alcohol. To detoxify the alcohol, the liver produces an enzyme called alcohol dehydrogenase (ADH). Nicotinamide adenine dinucleotide (NAD) present in the animals act as an oxidising agent and ADH catalyses the oxidation of toxic alcohols into non-toxic aldehyde.

Catalytic Dehydrogenation

When the vapours of a primary or a secondary alcohol are passed over heated copper at 573K, dehydrogenation takes place to form aldehyde or ketone.

Tertiary alcohols undergo dehydration reaction to give alkenes.

Esterification

Alcohols react with carboxylic acids in the presence of an acid to give esters

Reactions of Glycol

Ethylene glycol contains two primary alcoholic groups and it exhibits the usual reactions of hydroxyl group. Like other primary alcohols, it reacts with metallic sodium to form monosodium glycolate and disodium glycolate. The hydroxyl groups can be converted to the halide groups by treating glycol with halic acid (or with PCl₅ / PCl₃ / SOCl₂)

When ethylene glycol is treated with HI or P/I₂, 1, 2 – diiodoethane is first formed which decomposes to give ethene.

On heating with conc HNO₃ in the presence of Con. H₂SO₄, ethylene glycol forms dinitroglycol.

Dehydration Reaction

Ethyleneglycol undergoes dehydration reaction under different conditions to form different products.

1. When heated to 773K, it forms epoxides.

2. When heated with dilute sulphuric acid (or) anhydrous ZnCl₂ under pressure in a sealed tube, it gives acetaldehyde.

3. When distilled with Conc. H₂SO₄, glycol forms dioxane

Oxidation of Glycol

On oxidation, glycol gives a variety of products depending on the nature of oxidizing agent and other reaction conditions.

(i) When nitric acid (or) alkaline potassium permanganate is used as the oxidizing agent, the following products are obtained.

(ii) Oxidation of Glycol with Periodic Acid

Ethylene glycol on treatment with periodic acid gives formaldehyde. This reaction is selective for vicinal 1, 2 – diols and it proceeds through a cyclic periodate ester intermediate.

Reaction of Glycerol

Nitration:
Glycerol reacts with concentrated nitric acid in the presence of concentrated sulphuric acid to form TNG (nitroglycerine).

Dehydration

When glycerol is heated with dehydrating agents such as Con H₂SO₄, KHSO₄ etc…., it undergoes dehydration to form acrolein.

Oxidation

Glycerol can give rise to a variety of oxidation products depending on the nature of the oxidising agent used for oxidation.

• Oxidation of glycerol with dil. HNO₃ gives glyceric acid and tartronic acid.
• Oxidation of glycerol with Conc. HNO₃ gives mainly glyceric acid.
• Oxidation of glycerol with bismuth nitrate gives as meso oxalic acid.
• Oxidation of glycerol with Br₂/H₂O (or) NaOBr (or) Fenton’s reagent (FeSO₄ + H₂O₂) gives a mixture of glyceraldehyde and dihydroxy acetone(Ths mixture is named as glycerose).
• On oxidation with HIO₄ or Lead tetra acetate (LTA) it gives formaldehyde and formic acid.
• Acidified KMnO₄ oxidises glycerol into oxalicacid.

Uses of Alcohols

Uses of Methanol:

• Methanol is used as a solvent for paints, varnishes, shellac, gums, cement, etc.
• In the manufacture of dyes, drugs, perfumes and formaldehyde.

Uses of Ethanol:

1. It is also used in the preparation of

• Paints and varnishes.
• Organic compounds like ether, chloroform, iodoform, etc.,
• Dyes, transparent soaps.

2. As a substitute for petrol under the name power alcohol used as fuel for aeroplane
3. It is used as a preservative for biological specimens.

Uses of Ethylene Glycol:

1. Ethylene glycol is used as an antifreeze in automobile radiator
2. Its dinitrate is used as an explosive with TNG.

Uses of Glycerol

1. Glycerol is used as a sweetening agent in confectionary and beverages.
2. It is used in the manufacture of cosmetics and transparent soaps.
3. It is used in making printing inks and stamp pad ink and lubricant for watches and clocks.
4. It is used in the manufacture of explosive like dynamite and cordite by mixing it with china clay.

Acidity of Alcohols

According to Bronsted theory, an acid is defined as a proton donor and the acid strength is the tendency to give up a proton. Alcohols are weakly acidic and their acidity is comparable with water. Except methanol, all other alcohols are weaker acid than water. The K_a value for water is 1.8 × 10^-16 where as for alcohols, the K_a value in the order 10^-18 to 10^-16.

Alcohols react with active metals such as sodium, aluminium etc… to form the corresponding alkoxides with the liberation of hydrogen gas and similar reaction to give alkoxide is not observed in the reaction of alcohol with NaOH.

2C₂H₅ – OH + 2Na → 2C₂H₅ONa + H₂↑

The above reaction explains the acidic nature of alcohols.

Comparison of acidity of 1°, 2° and 3° alcohols

The acidic nature of the alcohol is due to the polar nature of O – H bond. When an electron withdrawing – I groups such as – Cl, – F etc… is attached to the carbon bearing the OH group, it withdraws the electron density towards itself and thereby facilitating the proton donation.

In contrast, the electron releasing group such as alkyl group increases the electron density on oxygen and decreases the polar nature of O – H bond, Hence it results in the decrease in acidity. On moving from primary to secondary and tertiary alcohols, the number of alkyl groups which attached to the carbon bearing -OH group increases, which results in the following order of acidity.

1° alcohol > 2° alcohol > 3° alcohol

Alcohols can also act as a Bronsted bases. It is due to the presence of unshared electron pairs on oxygen which make them proton acceptors.

Acidity of Phenol

Phenol is more acidic than aliphatic alcohols. Unlike alcohols it reacts with bases like sodium hydroxide to form sodium phenoxide. This explains the acidic behaviour of phenol let us consider the aqueous solution of phenol in which the following equilibrium exists.

C₆H₅ – OH + H.OH ⇄ C₆H₅ – 0^Θ + H₃O^⊕

K_a value for the above equilibrium is 1 × 10^-10 at 25°C. This Ka value indicates that it is more acidic than aliphatic alcohols. This increased acidic behaviour can be explained on the basis of the stability of phenoxide ion. We have already learnt in XI standard that the phenoxide is more stabilised by resonance than phenol.

In substituted phenols, the electron withdrawing groups such as – NO₂, – Cl enhances the acidic nature of phenol especially when they are present at ortho and para positions. In such cases, there is a possibility for the extended delocalisation of negative charge on the phenoxide ion. On the other hand the alkyl substituted phenols show a decreased acidity due to the electron releasing +I effect of alkyl group.

Table: pK_a Values of some Alcohols and Phenols

Phenols:

Phenols are organic compounds in which a – OH group is directly attached to a benzene ring. The carbon bearing the – OH group is sp² hybridized.

Table: Classification of Phenols

Preparation of Phenols

(a) From Halo Arenes (Dows Process)

When Chlorobenzene is hydrolysed with 6-8% N a O H at 300 bar and 633K in a closed vessel, sodium phenoxide is formed which on treatment with dilute HCl gives phenol.

(b) From Benzene Sulphonic Acid

Benzene is sulphonated with oleum and the benzene sulphonic acid so formed is heated with molten NaOH at 623K gives sodium phenoxide which on acidification gives phenol.

(c) From Aniline

Aniline is diazotized with nitrous acid (NaNO₂ + HCl ) at 273-278K to give benzene diazonium chloride which on further treatment with hot water in the presence of mineral acid gives phenol.

(d) From Cumene

A mixture of benzene and propene is heated at 523K in a closed vessel in presence of H₃PO₄ catalyst gives
cumene (isopropylbenzene). On passing air to a mixture of cumene and 5% aqueous sodium carbonate solution, cumene hydro peroxide is formed by oxidation. It is treated with dilute acid to get phenol and acetone. Acetone is also an important byproduct in this reaction.

Physical Properties

Phenol is colourless, needle shaped crystal, hygroscopic, corrosive and poisonous. It turns pink on exposure to air and light. The simplest phenols are liquids or low melting solids, they have quite high boiling points. Phenol is slightly soluble in water because of hydrogen bonding. However other substituted phenols are essentially insoluble in water.

Chemical Properties:
Reactions involving – OH group.

(a) Reaction with Zn Dust:

Phenol is converted to benzene on heating with zinc dust. In this reaction the hydroxyl group which is attached to the aromatic ring is eliminated.

(b) Reaction with Ammonia:

Phenol on heating with ammonia in presence of anhydrous ZnCl₂ gives aniline.

(c) Formation of Esters:

Schotten-Baumann Reaction:

Phenol on treatment with acid chlorides gives esters. The acetylation and benzoylation of phenol are called Schotten-Baumann reaction.

(d) Formation of Ethers:

Williamson Ether Synthesis:

An alkaline solution of phenol reacts with alkyl halide to form phenyl ethers. The alkyl halide undergoes nucleophilic substitution by the phenoxide ion in the presence of alkali.

(e) Oxidation:

Phenol undergoes oxidation with air or acidified K₂Cr₂O₇ with conc. H₂SO₄ to
form 1, 4 – benzoquinone.

(f) Reduction:

Phenol on catalytic hydrogenation gives cyclohexanol.

Reactions of Benzene Ring:

Electrophilic Aromatic Substitution:

We have already learnt in XI standard that the groups like etc., which when directly attached to the benzene ring, activate the ring towards electrophilic substitution reaction and direct the incoming electrophile to occupy either the ortho or para position.

Common electrophilic aromatic substitutions are as follows:

(i) Nitrosation:

Phenol can be readily nitrosoated at low temperature with HNO₂.

(ii) Nitration:

Phenol can be nitrated using 20% nitric acid even at room temperature, a mixture of ortho and para nitro phenols are formed.

The ortho and para isomers are separated by steam distillation, as o-nitro phenol is slightly soluble in water and more volatile due to intra molecular hydrogen bonding, whereas p-nitro phenol is more soluble in water and less volatile due to intermolecular hydrogen bonding.

Nitration with Conc. HNO₃ + con.H₂SO₄ gives picric acid.

(iii) Sulphonation:

Phenol reacts with con. H₂SO₄ at 280K to form o-phenol sulphonic acid as the major product. When the reaction is carried out at 373K the major product is p-phenol sulphonic acid.

(iv) Halogenation:

Phenol reacts with bromine water to give a white precipitate of 2, 4, 6-tri bromo phenol.

If the reaction is carried out in CS₂ or CCl₄ at 278K, a mixture of ortho and para bromo phenols are formed.

(v) Kolbe’s (or) Kolbe’s Schmit Reaction:

In this reaction, phenol is first converted into sodium phenoxide which is more reactive than phenol towards electrophilic substitution reaction with CO₂. Treatment of sodium phenoxide with CO₂ at 400K, 4-7 bar pressure followed by acid hydrolysis gives salicylic acid.

(vi) Riemer – Tiemann Reaction:

On treating phenol with CHCl₃/NaOH, a -CHO group is introduced at ortho position. This reaction proceeds through the formation of substituted benzal chloride intermediate.

(vii) Phthalein Reaction:

On heating phenol with phthalic anhydride in presence of con. H₂SO₄, phenolphthalein is obtained.

(viii) Coupling Reaction:

Phenol couples with benzene diazonium chloride in an alkaline solution to form p-hydroxy azobenzene(a red orange dye).

Test to Differentiate Alcohol and Phenols

• Phenol react with benzene diazonium chloride to form a red orange dye, but ethanol has no reaction with it.
• Phenol gives purple colouration with neutral ferric chloride solution, alcohols do not give such coloration with FeCl₃.
• Phenol reacts with NaOH to give sodium phenoxide. Ethyl alcohol does not react with NaOH.

Uses of Phenol

1. About half of world production of phenol is used for making phenol formaldehyde resin. (Bakelite).
2. Phenol is a starting material for the preparation of
3. Drugs such as phenacetin, Salol, aspirin, etc.
4. Phenolphthalein indicator.
5. Explosive like picric acid.

It is used as an antiseptic-carbolic lotion and carbolic soaps.

Ethers:

Ethers are a class of organic compound in which an oxygen atom is connected to two alkyl/aryl groups image 66. Ethers can be considered as the derivatives of hydrocarbon in which one hydrogen atom is replaced by an alkoxy (- OR) or an aryloxy (- OAr) group. The general formula of aliphatic ether is C_nH_2n+2O.

Classification:

Structure of Functional Group

The structure of ethereal oxygen which is attached to two alkyl groups is similar to the structure of -O-H group of alcohol. The oxygen atom is sp³ hybridized. Two sp³ hybridized orbitals of oxygen linearly overlap with two sp³ hybrid orbitals of the carbon which are directly attached to the oxygen forming two C – O ‘σ’ bonds. The C-O-C bond angle is slightly greater than the tetrahedral bond angle due to the repulsive interaction between the two bulkier alkyl groups.

IUPAC System:

Let us recall the naming of ethers according to IUPAC nomenclature.

Preparation of Ethers:

1. Inter Molecular Dehydration of Alcohol.

We have already learnt that when ethanol is treated with con. H₂SO₄ at 443K, elimination takes place to form ethene. If the same reaction is carried out at 413K, substitution competes over elimination to form ethers.

Mechanism:

This method is useful for the preparation of simple ethers and not suitable for preparing mixed ethers. If a mixture of two different alcohols is used, mixture of different ethers will be formed and they are difficult to separate.

2. Williamsons Synthesis:

When an alkyl halide is heated with an alcoholic solution of sodium alkoxide, the corresponding ethers are obtained. The reaction involves SN² mechanism.

Mechanism:

We know that primary alkyl halides are more susceptible for SN² reaction. Hence for the preparation of mixed ether having primary and tertiary alkyl group, primary alkyl halide and tertiary alkoxide are used. On the other hand, if we use tertiary alkyl halide and primary alkoxide, elimination dominates and succeeds over substitution to form an an alkene.

Methylation of Alcohol

Methyl ethers can be prepared by treating an alcohol with diazomethane in presence of catalyst, floroboric acid.

Physical Properties:

Ethers are polar in nature. The dipolemoment of ether is the vector sum of two polar C-O bonds with significant contribution from two lone pairs of electrons. For example, the dipole moment of diethyl ether is 1.18D. Boiling point of ethers are slightly higher than that of alkanes and lower than that of alcohols of comparable masses.

Oxygen of ether can also form Hydrogen bond with water and hence they are miscible with water. Ethers dissolve wide range of polar and non-polar substances.

Chemical Properties of Ethers:

1. Nucleophilic Substitution Reactions of Ethers.

Ethers can undergo nucleophilic substitution reactions with HBr or HI . HI is more reactive than HBr.

Mechanism:

Ethers having primary alkyl group undergo SN² reaction while tertiary alkyl ether undergo SN¹ reaction.
Protonation of ether is followed by the attack of halide ion. The halide ion preferentially attacks the less sterically hindered of the two alkyl groups which are attached to etherial oxygen.

When excess HBr or HI is used, the alcohol formed will further react with HBr or HI to form alkyl halides.

Autooxidation of Ethers:

When ethers are stored in the presence of atmospheric oxygen, they slowly oxidise to form hydroperoxides and dialkylperoxides. These are explosive in nature. Such a spontaneous oxidation by atmospheric oxygen is called autooxidation.

Some of the Reaction of Diethyl Ether.

Aromatic Electrophilic Substitution Reactions:

The alkoxy group (- OR) is an ortho, para directing group as well as activating group. It activates the aromatic ring towards electrophilic substitution.

(i) Halogenation:

Anisole undergoes bromination with bromine in acetic acid even in the absence of a catalyst, para isomer is obtained as the major product.

(ii) Nitration:

Anisole reacts with a mixture of conc. H₂SO₄ Conc. HNO₃ to yield a mixture of ortho nitro anisole and
para nitro anisole.

(iii) Friedel Crafts Reaction:

Anisole undergoes Fridel Craft’s reaction in presence of anhydrous AlCl₃ as a catalyst.

Uses of Ethers

Uses of Diethyl Ether

1. Diethyl ether is used as a surgical anaesthetic agent in surgery.
2. It is a good solvent for organic reactions and extraction.
3. It is used as a volatile starting fluid for diesel and gasoline engine.
4. It is used as a refrigerant.

Uses of Anisole

1. Anisole is a precursor to the synthesis of perfumes and insecticide pheromones,
2. It is used as a pharmaceutical agent.

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Classification of Alcohols

Alcohols can be classified based on the number of hydroxyl groups and the nature of the carbon to which the functional group (- OH) is attached.

Alcohols may be classified as primary, secondary, or tertiary, according to which carbon of the alkyl group is bonded to the hydroxyl group. Most alcohols are colourless liquids or solids at room temperature.

Types of Alcohols:

Primary Alcohols:

A primary alcohol is one in which the hydroxyl group (- OH) is attached to a carbon atom with at least two hydrogen atoms.

Secondary Alcohols:

A secondary alcohol is one in which the hydroxyl group (- OH) is attached to a carbon with only one hydrogen atom attached.

Tertiary Alcohols:

One way of classifying alcohols is based on which carbon atom is bonded to the hydroxyl group. If this carbon is primary (1°, bonded to only one other carbon atom), the compound is a primary alcohol. A secondary alcohol has the hydroxyl group on a secondary (2°) carbon atom, which is bonded to two other carbon atoms.

The four types of alcohol are ethyl, denatured, isopropyl and rubbing. The one that we know and love the best is ethyl alcohol, also called ethanol or grain alcohol. It’s made by fermenting sugar and yeast, and is used in beer, wine, and liquor. Ethyl alcohol is also produced synthetically.

2 Types of Alcohols:

Distilled and Undistilled Alcohol:

There are two categories of alcoholic beverages:
Distilled and Undistilled.

The functional group in the alcohols is the hydroxyl group, – OH.

Primary alcohols are those alcohols where the carbon atom of the hydroxyl group (OH) is attached to only one single alkyl group. Some examples of these primary alcohols include Methanol (propanol), ethanol, etc.

Most believe the word “alcohol” originated in the Middle East since the prefix al is a definite article in Arabic the debate is about which word it stems from, either alcohol. “Alcohol” was later used specifically to mean ethanol, with the essence or spirit released through the distillation process.

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Various Application of Colloids

In every path of life, colloids play a great role. Human body contains the numerous colloidal solutions. The blood in our body, protoplasma of plant and animal cell, and fats in our intestines are in the form of emulsions. Synthetic polymers like polystyrene silicones and PVC are colloids.

Food

Food stuff like milk cream, butter, etc are present in colloidal form.

Medicines

Antibodies such as penicillin and streptomycin are produced in colloidal form for suitable injections. Colloidal gold and colloidal calcium are used as tonics. Milk of magnesia is used for stomach troubles. Silver sol protected by gelatine known as Argyrol is used as eye lotion.

In Industry

Colloids find many applications in industries.

(i) Water Purification:

Purification of drinking water is activated by coagulation of suspended impurities in water using alums containing Al³⁺

(ii) In washing:

The cleansing action of soap is due to the formation of emulsion of soap molecules with dirt and grease.

(iii) Tanning of Leather

Skin and hides are protein containing positively charged particles which are coagulated by adding tannin to give hardened leather for further application. Chromium salts are used for the purpose. Chrome tanning can produce sof and polishable leather.

(iv) Rubber Industry:

Latex is the emulsion of natural rubber with negative particles. By heating rubber with sulphur, vulcanized rubbers are produced for tyres, tubes, etc.

(v) Sewage Disposal

Sewage contains dirt, mud and wastes dispersed in water. The passage of electric current deposits the wastes materials which can be used as a manure.

(vi) Cortrell’s Precipitator

Carbon dust in air is solidified by cortrell’s precipitator. In it, a high potential difference of about 50,000V is used. The charge on carbon is neutralized and solidified. This the air is free from carbon particles.

(vii) The Blue Colour

The blue colour  of the sky in nature is due to Tyndall effect of air particles.

(viii) Formation of Delta:

The electrolyte in sea and river water coagulates the solid particles in river water at their intersection. So, the earth becomes a fertile land.

(ix) Analytical Application

Qualitative and quantitative analysis are based on the various properties of colloids. Hence we can conclude that in our life, there is hardly any field which is not including the applications of colloids.

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Emulsions

Emulsions are colloidal solution in which a liquid is dispersed in an another liquid. Generally there are two types of emulsions.

1. Oil in water (O/W)
2. Water in oil (W/O)

Example:

• Milk is example of the oil in water type emulsion.
• Stif greases are emulsion of water in oil i.e. water dispersed in lubricating oil.
• The process of preparation of emulsion by the dispersal of one liquid in another liquid is called Emulsification.
• A colloid mill can be used as a homogeniser to mix the two liquid. To have a stable emulsion a small amount of emulsifier or emulsification agent is added.

Several Types of Emulsifiers are known.

1. Most of the lyophillic colloids also act as emulsifiers. Example: glue, gelatine.
2. Long chain compounds with polar groups like soap and sulphonic acids.
3. Insoluble powders like clay and lamp black also act as emulsifiers.

Identification of Types of Emulsion

The two types of emulsions can be identified by the following tests.

(i) Dye Test

A small amount of dye soluble in oil is added to the emulsion. The emulsion is shaken well. The aqueous emulsion will not take the colour whereas oily emulsion will take up the colour of the dye.

(ii) Viscosity Test

Viscosity of the emulsion is determined by experiments. Oily emulsions will have higher value than aqueous emulsion.

(iii) Conductivity Test

Conductivity of aqueous emulsions are always higher than oily emulsions.

(iv) Spreading Test

Oily emulsions spread readily than aqueous emulsion when spread on an oily surface.

Deemulsification:

Emulsion can be separated into two separate layers. The process is called Deemulsification.

Various Deemulsification Techniques are Given Below

1. Distilling of one component
2. Adding an electrolyte to destroy the charge
3. Destroying the emulsifir using chemical methods
4. Using solvent extraction to remove one component
5. By freezing one of the components
6. By applying centrifugal force
7. Adding dehydrating agents for water in oil (W/O) type
8. Using ultrasonic waves.
9. Heating at high pressures.

Inversion of Phase:

The change of W/O emulsion into O/W emulsion is called inversion of phases.

For example:

An oil in water emulsion containing potassium soap as emulsifying agent can be converted into water in oil emulsion by adding CaCl₂ or AlCl₃. The mechanism of inversion is in the recent developments of research.

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Colloid, Dispersion Phase and Dispersion Medium

Origin of study of colloid starts with Thomas Graham who observed diffusion of that a solution of sugar, urea or sodium chloride through a membrane but not glue, gelatine or gum. He called the former substances as crystalloids and the latter as colloids (In Greek, kola as gum, eidos-like).

Later it was realised that any substance can be converted into a colloid by reducing its particle size to 1-200nm.

Hence, colloid is a homogeneous mixture of two substances in which one substance (smaller proportion) is dispersed in another substance (large proportion).

In a colloid, the substance present in larger amount is called dispersing medium and the substance present in less amount is called dispersed phase.

Classifications of Colloidal Solution

Probably the most important colloidal systems have dispersed phase as solid and the dispersion medium as liquid. If the dispersion medium considered is water, then the colloids are referred as hydrosols or aquasols.

If the dispersion medium is an alcohol, the colloid is termed as alcosol, and if benzene is the dispersion medium, it is called as benzosol.

One more type of classification is based on the forces acting between the dispersal phase and dispersion medium.

In lyophillic colloids definite attractive force or affinity exists between dispersion medium and dispersed phase. Examples: sols of protein and starch. They are more stable and will not get precipitated easily. They can be brought back to colloidal solution even after the precipitation by addition of the dispersion medium.

In a lyophobic colloids, no attractive force exists between the dispersed phase and dispersion medium. They are less stable and precipitated readily, but can not be produced again by just adding the dispersion medium. They themselves undergo coagulation after a span of characteristic life time.

They are called irreversible sols
Examples: sols of gold, silver, platinum and copper.

The following table lists the types of colloids based on the physical states of dispersed phase and dispersion medium.

Classification of Colloids Based on the Physical State of Dispersed Phase and Dispersion Medium.

Dipersion Medium	Dispersed Phase	Name of the Colloid	Examples
1. Gas	Liquid	Liquid Aerosol	Fog Aerosol spray
2. Gas	Solid	Solid Aerosol	Smoke, Air pollutants likes fumes, dust
3. Liquid	Gas	Foam	Whipped cream, Shaving cream, Soda water, Froth
4. Liquid	Liquid	Emulsion	Milk, Cream, Mayonnaise
5. Liquid	Solid	Sol	Inks, Paints, Collodial gold
6. Solid	Gas	Solid foam	Pumice stone, Foam rubber bread
7. Solid	Liquid	Gel	Butter, Cheese
8. Solid	Solid	Solid sol	Pearls, opals, coloured glass alloys colloidal dispersed eutics

Preparation of Colloids

Many lyophillic substances are made in their colloidal form by warming with water. Rubber forms colloidal solution with benzene. Soap spontaneously forms a colloidal solution by just mixing with water.

In general, colloidal are prepared by the following methods.

1. Dispersion Methods:
In this method larger particles are broken to colloidal dimension.

2. Condensation Methods:
In this method, smaller atom or molecules are converted into larger colloidal sized particles.

1. Dispersion Methods

(i) Mechanical Dispersion:

Using a colloid mill, the solid is ground to colloidal dimension. The colloid mill consists of two metal plates rotating in opposite direction at very high speed of nearly 7000 revolution/minute.

The colloidal particles of required colloidal size is obtained by adjusting the distance between two plates. By this method, colloidal solutions of ink and graphite are prepared.

(ii) Electro Dispersion:

A brown colloidal solution of platinum was first prepared by George Bredig in 1898. An electrical arc is struck between electrodes dispersed in water surrounded by ice. When a current of 1 amp/100 V is passed an arc produced forms vapours of metal which immediately condense to form colloidal solution.

By this method colloidal solution of many metals like copper, silver, gold, platinum, etc. can be prepared Alkali hydroxide is added as an stabilising agent for the colloidal solution.

Svedberg modified this method for the preparation of non aqueous inflammable liquids like pentane, ether and benzene, etc using high frequency alternating current which prevents the decomposition of liquid.

(iii) Ultrasonic Dispersion

Sound waves of frequency more than 20kHz (audible limit) could cause transformation of coarse suspension to colloidal dimensions.

Claus obtained mercury sol by subjecting mercury to sufficiently high frequency ultrasonic vibrations.

The ultrasonic vibrations produced by generator spread the oil and transfer the vibration to the vessel with mercury in water.

(iv) Peptisation:

By addition of suitable electrolytes, precipitated particles can be brought into colloidal state. The process is termed as peptisation and the electrolyte added is called peptising or dispersing agent

2. Condensation Methods:

When the substance for colloidal particle is present as small sized particle, molecule or ion, they are brought to the colloidal dimension by condensation methods. Here care should be taken to produce the particle with colloidal size otherwise precipitation will occur. Various chemical methods for the formation of colloidal particles.

(i) Oxidation

Sols of some non metals are prepared by this method.

(a) When hydroiodic acid is treated with iodic acid, I₂ sol is obtained.
HIO₃ + 5HI → 3H₂O + I₂ (Sol)

(b) When O₂ is passed through H₂Se, a sol of selenium is obtained.
H₂Se + O₂ → 2H₂O + Se (sol)

(ii) Reduction

Many organic reagents like phenyl hydrazine, formaldehyde, etc are used for the formation of sols. For example: Gold sol is prepared by reduction of auric chloride using formaldehyde.

2 AuCl₃ + 3HCHO + 3H₂O → Au(sol) + 6HCl + 3HCOOH

(iii) Hydrolysis

Sols of hydroxides of metals like chromium and aluminium can be produced by this method.

For Example,
FeCl₃+3H₂O → Fe(OH)₃+3HCl

(iv) Double Decomposition

For the preparation of water insoluble sols this method can be used. When hydrogen sulphide gas is passed through a solution of arsenic oxide, a yellow coloured arsenic sulphide is obtained as a colloidal solution.
As₂O₃ +3H₂S → As₂S₃ + 3H₂O

(v) Decomposition

When few drops of an acid is added to a dilute solution of sodium thiosulphate, the insoluble free sulphur produced by decomposition of sodium thiosulphate accumulates into small, clusters which impart various colours blue, yellow and even red to the system depending on their growth within the size of colloidal dimensions.

3. By Exchange of Solvent:

Colloidal solution of few substances like phosphorous or sulphur is obtained by preparing the solutions in alcohol and pouring them into water. As they are insoluble in water, they form colloidal solution.

P in alcohol + water → Psol.

Purification of Colloids

The colloidal solutions due to their different methods of preparation may contain impurities. If they are not removed, they may destablise and precipitate the colloidal solution. This is called coagulation. Hence the impurities mainly electrolytes should be removed to increase the stabilisation of colloid. Purification of colloidal solution can be done by the following methods.

1. Dialysis
2. Electrodialysis
3. Ultrafilteration.

1. Dialysis

In 1861, T. Graham separated the electrolyte from a colloid using a semipermeable membrane (dialyser). In this method, the colloidal solution is taken in a bag made up of semipermeable membrane. It is suspended in a trough of flowing water, the electrolytes diffuse out of the membrane and they are carried away by water.

2. Electrodialysis

The presence of electric field increases the speed of removal of electrolytes from colloidal solution. The colloidal solution containing an electrolyte as impurity is placed between two dialysing membranes enclosed into two compartments filled with water.

When current is passed, the impurities pass into water compartment and get removed periodically. This process is faster than dialysis, as the rate of diffusion of electrolytes is increased by the application of electricity.

3. Ultrafiltration

The pores of ordinary filter papers permit the passage of colloidal solutions. In ultra filtrations, the membranes are made by using collodion cellophane or visiking. When a colloidal solution is filtered using such a filter, colloidal particles are separated on the filter and the impurities are removed as washings.

This process is quickened by application of pressure. The separation of sol particles from electrolyte by filteration through an ultrafilter is called ultrafiltration. Collodion is 4% solution of nitrocellulose in a mixture of alcohol and water.

Properties of Colloids

1. Colour

The colour of a sol is not always the same as the colour of the substance in the bulk. For example bluish tinge is given by diluted milk in reflected light and reddish tinge in transmitted light.

Colour of the sol, generally depends on the following factors.

• Method of preparation
• Wavelength of source of light.
• Size and shape of colloidal particle
• Whether the observer views the reflected light or transmitted light.

2. Size

The size of colloidal particles ranges from 1nm (10^-9m) to 1000 nm (10^-6m) diameter.

3. Colloidal Solutions are Heterogeneous in Nature Having two Distinct Phases

Though experiments like dialysis, ultrafiltration and ultracentrifuging clearly show the heterogeneous nature in the recent times colloidal solution are considered as border line cases.

4. Filtrability

As the size of pores in ordinary filter paper are large the colloidal particles easily pass through the ordinary filter papers.

5. Non-Setting Nature

Colloidal solutions are quite stable i.e. they are not affcted by gravity.

6. Concentration and Density

When the colloidal solution is dilute, it is stable. When the volume of medium is decreased coagulation occurs. Generally, density of sol decreases with decrease in the concentration.

7. Diffusability

Unlike true solution, colloids diffuse less readily through membranes.

8. Colligative Properties

The colloidal solutions show colligative properties i.e. elevation of boiling point, depression in freezing point and osmotic pressure. Measurements of osmotic pressure is used to find molecular weight of colloidal particle.

9. Shape of Colloidal Particles

It is very interesting to know the various shapes of colloidal particles. Here are some examples

Colloidal Particles	Shapes
As2S3	Spherical
Fe(OH)3sol (blue gold sol)	Disc or plate like
W2O5sol (tungstic acid sol)	Rod like

10. Optical Property

Colloids have optical property. When a homogeneous solution is seen in the direction of light, it appears clear but it appears dark, in a perpendicular direction.

But when light passes through colloidal solution, it is scattered in all directions. This effect was first observed by Faraday, but investigations are made by Tyndall in detail, hence called as Tyndall effect.

The colloidal particles absorb a portion of light and the remaining portion is scattered from the surface of the colloid. Hence the path of light is made clear.

11. Kinetic Property

Robert Brown observed that when the pollen grains suspended in water were viewed through ultra microscope, they showed a random, zigzag ceaseless motion.

This is called Brownian movement of colloidal particles.

This can be explained as follows

The colloidal sol particles are continuously bombarded with the molecules of the dispersion medium and hence they follow a zigzag, random, continuous movement.

Brownian Movement Enables Us,

I. To calculate Avogadro number.

II. To confirm kinetic theory which considers the ceaseless rapid movement of molecules that increases with increase in temperature.

III. To understand the stability of colloids:

As the particles in continuous rapid movement they do not come close and hence not get condensed. That is Brownian movement does not allow the particles to be acted on by force of gravity.

12. Electrical Property

(i) Helmholtz Double Layer

The surface of colloidal particle adsorbs one type of ion due to preferential adsorption. This layer attracts the oppositely charged ions in the medium and hence at the boundary separating the two electrical double layers are setup. This is called as Helmholtz electrical double layer.

As the particles nearby are having similar charges, they cannot come close and condense. Hence this helps to explain the stability of a colloid.

(ii) Electrophoresis:

When electric potential is applied across two platinum electrodes dipped in a hydrophilic sol, the dispersed particles move toward one or other electrode. This migration of sol particles under the influence of electric field is called electrophoresis or cataphoresis.

If the sol particles migrate to the cathode, then they posses positive (+) charges, and if the sol particles migrate to the anode then they have negative charges(-). This from the direction of migration of sol particles we can determine the charge of the sol particles. Hence electrophoresis is used for detection of presence of charges on the sol particles.

Few Examples of Charges of Sols Detected by Electrophoresis are Given Below:

Positively charge colloids	Negatively charge colloids
Ferric hydroxide	Ag, Au & Pt
Aluminium hydroxide	Arsenic sulphide
Basic dyes	Clay
Haemoglobin	Starch

(iii) Electro Osmosis

A sol is electrically neutral. Hence the medium carries an equal but opposite charge to that of dispersed particles. When sol particles are prevented from moving, under the influence of electric field the medium moves in a direction opposite to that of the sol particles. This movement of dispersion medium under the influence of electric potential is called electro osmosis.

13. Coagulation or Precipitation

The flocculation and settling down of the sol particles is called coagulation.
Various method of coagulation are given below:

• Addition of electrolytes
• Electrophoresis
• Mixing oppositively charged sols.
• Boiling

Addition of Electrolytes

A negative ion causes the precipitation of positively charged sol and vice versa. When the valency of ion is high, the precipitation power is increased. For example, the precipitation power of some cations and anions varies in the following order

Al³⁺ > Ba²⁺ > Na⁺, Similarly [Fe(CN)₆]^3- > SO₄^2- > Cl^–

The precipitation power of electrolyte is determined by finding the minimum concentration (millimoles/lit) required to cause precipitation of a sol in 2 hours. This value is called flocculation value. The smaller the flocculation value greater will be precipitation.

Electrophoresis

In the electrophoresis, charged particles migrate to the electrode of opposite sign. It is due to neutralization of the charge of the colloids. The particles are discharged and so they get precipitated.

By Mixing two Oppositively Charged Sols

When colloidal sols with opposite charges are mixed mutual coagulation takes place. It is due to migration of ions from the surface of the particles.

By Boiling

When boiled due to increased collisions, the sol particles combine and settle down.

14. Protective Action

Generally, lyophobic sols are precipitated readily even with small amount of electrolytes. But they are stabilised by addition of a small amount of lyophillic colloid.

A small amount of gelatine sol is added to gold sol to protect the gold sol.

Zsigmondy introduced the term ‘gold number’ as a measure of protecting power of a colloid. Gold number is defined as the number of milligrams of hydrophilic colloid that will just prevent the precipitation of 10ml of gold sol on the addition of 1ml of 10% NaCl solution. Smaller the gold number greater the protective power.

Colloid	Gold number
Gelatin	0.005-0.01
Egg albumin	0.08-0.10
Gum Arabic	0.1-0.15
Potato starch	25

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Zeolite Catalysis

The details of heterogeneous catalysis will be in complete, if zeolites are not discussed. Zeolites are microporous, crystalline, hydrated, alumino silicates, made of silicon and aluminium tetrahedron. There are about 50 natural zeolites and 150 synthetic zeolites. As silicon is tetravalent and aluminium is trivalent, the zeolite matrix carries extra negative charge.

To balance the negative charge, there are extra framework cations for example, H⁺ or Na⁺ ions. Zeolites carrying protons are used as solid acid catalysts and they are extensively used in the petrochemical industry for cracking heavy hydrocarbon fractions into gasoline, diesel, etc., Zeolites carrying Na⁺ ions are used as basic catalysts.

One of the most important applications of zeolites is their shape selectivity. In zeolites, the active sites namely protons are lying inside their pores. So, reactions occur only inside the pores of zeolites.

Reactant Selectivity:

When bulkier molecules in a reactant mixture are prevented from reaching the active sites within the zeolite crystal, this selectivity is called reactant shape selectivity.

Transition State Selectivity:

If the transition state of a reaction is large compared to the pore size of the zeolite, then no product will be formed.

Product Selectivity:

It is encountered when certain product molecules are too big to diffuse out of the zeolite pores.

Phase Transfer Catalysis:

Suppose the reactant of a reaction is present in one solvent and the other reactant is present in an another solvent. The reaction between them is very slow, if the solvents are immiscible. As the solvents form separate phases, the reactants have to migrate across the boundary to react. But migration of reactants across the boundary is not easy.

For such situations a third solvent is added which is miscible with both. So, the phase boundary is eliminated, reactants freely mix and react fast. But for large scale production of any product, use of a third solvent is not convenient as it may be expensive.

For such problems phase transfer catalysis provides a simple solution, which avoids the use of solvents. It directs the use a phase transfer catalyst (a phase transfer reagent) to facilitate transport of a reactant in one solvent to the other solvent where the second reactant is present. As the reactants are now brought together, they rapidly react and form the product.

Example:

Substitution of Cl^– and CN^– in the following reaction.
R-Cl + NaCN → R-CN + NaCl

organic phase aqueous phase organic phase aqueous phase

R – C l = 1 – chlorooctane
R – C N = 1 – cyanooctane

By direct heating of two phase mixture of organic 1-chlorooctane with aqueous sodium cyanide for several days, 1-cyanooctane is not obtained. However, if a small amount of quaternary ammonium salt like tetraalkylammoniumchloride is added, a rapid transition of 1-cyanooctane occurs in about 100% yield after 1 or 2 hours.

In this reaction, the tetraalkylammonium cation, which has hydrophobic and hydrophilic ends, transports CN^– from the aqueous phase to the organic phase using its hydrophilic end and facilitates the reaction with 1-chloroocatne as shown below:

So phase transfer catalyst, speeds up the reaction by transporting one reactant from one phase to another.

Nano Catalysis:

Nano materials such a metallic nano particles, metal oxides, etc., are used as catalyst in many chemical transformation, Nanocatalysts carry the advantages of both homogeneous and heterogeneous catalyses.

Like homogeneous catalysts, the nanocatalysts give 100% selective transformations and excellent yield and show extremely high activity. Like the heterogeneous catalysts, nanocatalysts can be recovered and recycled. Nanocatalysts are actually soluble heterogeneous catalysts. An example for nanoparticles catalysed reaction is given below.