On the Basis of the Position of the -OH Group:

(a) Primary Alcohols

(b) Secondary Alcohols

(c) Tertiary Alcohols

Based on the Number of -OH Groups Present:

The alcohols may be monohydric (i.e., have one -OH group), dihydric (have two -OH groups), trihydric (have three -OH groups):

(i) By acid catalysed hydration: Alkenes react with water in the presence of acid as catalyst to form alcohols. In case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markovnikov’s rule.

Mechanism - 

(ii) By hydroboration-oxidation:

Diborane reacts with alkenes to give trialkyl boranes as an addition product. This is oxidised to alcohol by hydrogen peroxide in the presence of aqueous sodium hydroxide.

The addition of borane to the double bond takes place in such a manner that the boron atom gets attached to the sp2 carbon carrying a greater number of hydrogen atoms. The product formed seems like the reaction has undergone an anti-markonikoff's reaction and this reaction gives excellent yield.

(i) By reduction of aldehydes and ketones: Aldehydes and ketones are reduced to the corresponding alcohols through catalytic hydrogenation.

The usual catalyst is a finely divided metal such as platinum, palladium or nickel.

(ii) By reduction of carboxylic acids and esters:

It is also prepared by treating aldehydes and ketones with sodium borohydride or lithium aluminium hydride.

Aldehydes yield primary alcohols whereas ketones give secondary alcohols.

Alcohols are produced by the reaction of Grignard reagents with aldehydes and ketones.

The first step of the reaction is the nucleophilic addition of the Grignard reagent to the carbonyl group to form an adduct.

Hydrolysis of the adduct yields an alcohol.

1. From haloarenes

Chlorobenzene is fused with NaOH at 623K and 320 atmospheric pressure. Phenol is obtained by acidification of sodium phenoxide.

2. From benzenesulphonic acid

Benzene is sulphonated with oleum, and the resulting benzene sulphonic acid is then converted to sodium phenoxide by heating with molten sodium hydroxide. Acidification of the sodium salt gives phenol.

3. From diazonium salts

A diazonium salt is formed by treating an aromatic primary amine with nitrous acid at 273-278 K. Diazonium salts are hydrolysed to phenols by warming with water or by treating with dilute acids.

4. From cumene

Phenol is manufactured from the hydrocarbon, cumene. Cumene (isopropylbenzene) is oxidised in the presence of air to cumene hydroperoxide. It is converted to phenol and acetone by treating it with dilute acid. 

(i) Boiling Points

Alcohols have a higher boiling point than haloalkanes of comparable molecular mass because alcohols have intermolecular hydrogen bonding. As the number of carbon atoms increases, the boiling point increases. The boiling point decreases with an increase in branching in the carbon chain.

(ii) Solubility

Alcohols are soluble in water due to their ability to form hydrogen bonds with water. As the number of carbon atoms increases, solubility decreases.

1. Acidity of alcohols and phenols

Alcohols and phenols react with active metals such as sodium, potassium and aluminium to yield corresponding alkoxides/phenoxides and hydrogen. In addition to this, phenols react with aqueous sodium hydroxide to form sodium phenoxides and water

2. Esterification

Alcohols and phenols react with carboxylic acids, acid chlorides and acid anhydrides to form esters.

3. Dehydration

Alcohols undergo dehydration (the removal of a molecule of water) to form alkenes when treated with a protic acid, such as concentrated sulfuric acid or phosphoric acid, or with catalysts like anhydrous zinc chloride or alumina.

Ethanol undergoes dehydration by heating it with concentrated sulphuric acid at 443 K.

Secondary and tertiary alcohols are dehydrated under milder conditions - 

Thus, the relative ease of dehydration of alcohols follows the following order: Tertiary > Secondary > Primary

4. Oxidation 

Oxidation of alcohols involves the formation of a carbon-oxygen double bond with cleavage of an O-H and C-H bond. Such a cleavage and formation of bonds occur in oxidation reactions.

These are also known as dehydrogenation reactions as these involve loss of dihydrogen from an alcohol molecule. Depending on the oxidising agent used, a primary alcohol is oxidised to an aldehyde which in turn is oxidised to a carboxylic acid. Some of these agents include - PCC, CrO3, Cu at 573K, etc.

5. Electrophilic aromatic substitution

(i) Nitration: With dilute nitric acid at low temperature (298 K), phenol yields a mixture of ortho and para nitrophenols.

With concentrated nitric acid, phenol is converted to 2,4,6-trinitrophenol. The product is commonly known as picric acid. The yield of the reaction product is poor.

(ii) Halogenation

On treating phenol with bromine, different reaction products are formed under different experimental conditions.

(a) When the reaction is carried out in solvents of low polarity such as CHCl3 or CS2 and at low temperature, monobromophenols are formed.

(b) When phenol is treated with bromine water, 2,4,6-tribromophenol is formed as white precipitate.

6. Kolbe’s reaction

Phenoxide ion generated by treating phenol with sodium hydroxide is even more reactive than phenol towards electrophilic aromatic substitution.

Hence, it undergoes electrophilic substitution with carbon dioxide, a weak electrophile. Ortho-hydroxybenzoic acid is formed as the main reaction product.

7. Oxidation

Oxidation of phenol with chromic acid produces a conjugated diketone known as benzoquinone. In the presence of air, phenols are slowly oxidised to dark coloured mixtures containing quinones.

8. Reimer-Tiemann reaction

On treating phenol with chloroform in the presence of sodium hydroxide, a –CHO group is introduced at ortho position of benzene ring. The intermediate substituted benzal chloride is hydrolysed in the presence of alkali to produce salicylaldehyde.

9. Reaction of phenol with zinc dust

Phenol is converted to benzene on heating with zinc dust.