Alcohols are prepared by the following methods :

(i) `color{green}(text(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. See fig.1 for overall mechanism.


(ii) `color{green}(text(By hydroboration–oxidation ))` : Diborane `color{red}(BH_3)_2` reacts with alkenes to give trialkyl boranes as addition product. This is oxidised to alcohol by hydrogen peroxide in the presence of aqueous sodium hydroxide. See fig.2.

● The addition of borane to the double bond takes place in such a manner that the boron atom gets attached to the `color{red}(sp^2)` carbon carrying greater number of hydrogen atoms.

● The alcohol so formed looks as if it has been formed by the addition of water to the alkene in a way opposite to the Markovnikov’s rule. In this reaction, alcohol is obtained in excellent yield.

(i) `color{green}(text(By reduction of aldehydes and ketones ))` : Aldehydes and ketones are reduced to the corresponding alcohols by addition of hydrogen in the presence of catalysts (catalytic hydrogenation).

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

● It is also prepared by treating aldehydes and ketones with sodium borohydride `color{red}(NaBH_4)` or lithium aluminium hydride `color{red}(LiAlH_4)`.

● Aldehydes yield primary alcohols whereas ketones give secondary alcohols.

`color{red}(RCHO +H_2 overset(Pd)→ R CH_2OH)`

`color{red}(RCOR overset(NaBH_4)→ R - underset (underset (OH) (|))CH - R)`

(ii) `color{green}(text(By reduction of carboxylic acids and esters ))` : Carboxylic acids are reduced to primary alcohols in excellent yields by lithium aluminium hydride, a strong reducing agent.

`color{red}(RCOOH underset ((ii) H_2O) overset((i) LiAlH_4)→ RCH_2OH)`

● However, `color{red}(LiAlH_4)` is an expensive reagent, and therefore, used for preparing special chemicals only.

● Commercially, acids are reduced to alcohols by converting them to the esters, followed by their reduction using hydrogen in the presence of catalyst (catalytic hydrogenation).

`color{red}(RCOOH underset(H^+) overset(R'OH)→ RCOOR undersettext(Catalyst) overset(H_2)→ RCH_2OH +R'OH)`

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

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

`=>` Hydrolysis of the adduct yields an alcohol. See fig.1.

The overall reactions using different aldehydes and ketones are as shown in fig.2.

`color{red}(text(Note ))` : The reaction produces a primary alcohol with methanal, a secondary alcohol with other aldehydes and tertiary alcohol with ketones.