Alcohols

Properties of Alcohols

Alcohols are organic compounds containing a hydroxyl (-OH) group. Their properties are significantly influenced by this functional group.

Physical Properties

• Alcohols have relatively high boiling points compared to alkanes of similar molecular weight. This is due to hydrogen bonding between the -OH groups.
• They are generally soluble in water due to hydrogen bonding. The solubility decreases as the size of the alkyl group increases.
• Lower alcohols have characteristic sweet smells.

Chemical Properties

• Alcohols can act as both acids and bases, although they are much weaker acids and bases than water.
• They undergo various reactions, including oxidation, dehydration, and reactions with acids.

Reactions of Alcohols

Oxidation

Alcohols can be oxidized to aldehydes or ketones, depending on the degree of oxidation and the oxidizing agent used.

Alcohol	Mild Oxidizing Agent (e.g., PCC, Swern)	Product
Primary Alcohol	PCC (Pyridinium Chlorochromate)	Aldehyde
Secondary Alcohol	PCC (Pyridinium Chlorochromate)	Ketone
Tertiary Alcohol	Requires strong oxidizing agents (e.g., KMnO4, CrO3)	Carboxylic Acid (after further oxidation)

Dehydration

Alcohols can be dehydrated to form alkenes. This reaction is typically catalyzed by a strong acid (e.g., H₂SO₄, H₃PO₄) at high temperatures.

The reaction follows Zaitsev's rule, meaning the major product is the more substituted alkene.

$$R-CH_2-CH_2-OH \xrightarrow{H_2SO_4, \Delta} R-CH=CH_2 + H_2O$$

Reactions with Acids

Alcohols react with strong acids to form alkyl halides. This is a substitution reaction where the -OH group is replaced by a halogen.

$$R-OH + HX \xrightarrow{H_2SO_4} R-X + H_2O$$

Esterification

Alcohols react with carboxylic acids in the presence of an acid catalyst to form esters and water. This is a reversible reaction.

$$R-OH + R'-COOH \xrightarrow{H_2SO_4} R-O-C(O)-R' + H_2O$$

Reaction with Sodium Ethoxide

A primary alcohol reacts with sodium ethoxide (NaOEt) to form an ethyl ether and water. This is an elimination reaction.

$$R-OH + NaOEt \rightarrow R-O-CH_2-CH_3 + NaOH$$

Preparation of Alcohols

From Alkenes

Alcohols can be prepared from alkenes by hydroboration-oxidation. This is a regioselective reaction that adds the -OH group to the less substituted carbon of the alkene.

Step 1: Hydroboration - Addition of borane (BH₃) to the alkene.

Step 2: Oxidation - Reaction with hydrogen peroxide (H₂O₂) in alkaline conditions.

From Oleum and Alkali

Alcohols can be prepared by the reaction of oleum (fuming sulfuric acid) with an alkene followed by hydrolysis with alkali.

This reaction proceeds via the formation of a vicinal diol, which is then dehydrated to form an alkene. Subsequent hydroboration-oxidation yields the alcohol.

From Ethylene and Water (Industrial Process)

Methanol is industrially produced by the catalytic hydrogenation of carbon monoxide and carbon dioxide.

$$CO + 2H_2 \rightarrow CH_3OH$$

$$CO_2 + 3H_2 \rightarrow CH_3OH + H_2O$$