Alcohols and

(ii) By hydroboration–oxidation : Diborane (BH ) 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. The addition of borane to the double bond takes place in such a manner that the boron atom gets attached to the sp 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. . From carbonyl compounds (i) 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 (NaBH ) or lithium aluminium hydride (LiAlH ). Aldehydes yield primary alcohols whereas ketones give secondary alcohols. (ii) 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.