Physical and Chemical Properties of alcohols

Properties of alcohols

Physical properties

i. Lower alcohols are colourless liquids and the higher members are waxy solids.

ii. 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.

iii. Among isomeric alcohols primary alcohols have higher boiling point and the tertiary alcohols have lower boiling points.

iv. 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 _−⁺OH₂ group by adding an acid. The −⁺OH₂ 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 S_N 2 mechanism

Example

Nucleophilic attack of Br^− and leaving of H₂O takes place simultaneously.

Alkyl halide formation of tertiary alcohols follow S_N1mechanism.

Example

Here, the carbocation formed can undergo elimination to give an alkene. However the alkene can again undergo addition reaction with HBr to give the substituted product.

Conversion of alcohol into alkyl halides: Other methods

Alcohols can also be converted into an alkyl halides using PCl₃ , PBr₃ .

Mechanism : S_N 2 reaction on phosphorous tri chloride

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

This reaction also follows theS_N i 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 carbocation.

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

Evaluate yourself

Identify the products in the following reactions. Write their IUPAC names and mention the mechanism involved in the reactions.

Saytzeff’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.

Evaluate yourself : What is the major product obtained when 2,3 – dimethyl pentan -3 – ol is heated in the presence of H₂SO₄ .

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.

Example

Tertiary alcohols do not undergo oxidation reaction under normal conditions, but at elevated temperatures, under strong oxidising agent cleavage of C –C bond takes place to give a mixture of carboxylic acid.

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. H₂ SO₄ , 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.

a) Oxidation of glycerol with dil. HNO₃ gives glyceric acid and tartronic acid.

b) Oxidation of glycerol with Conc. HNO₃ gives mainly glyceric acid.

c) Oxidation of glycerol with bismuth nitrate gives as meso oxalic acid.

d) Oxidation of glycerol with Br₂/H₂O (or) NaOBr (or) Fenton's reagent (FeSO₄ + H₂O₂) gives a mixture of glyceraldehyde and dihydroxy acetone(This mixture is named as glycerose).

e) On oxidation with HIO₄ or Lead tetra acetate (LTA) it gives formaldehyde and formic acid.

f) Acidified KMnO₄ oxidises glycerol into oxalicacid.