Preparation of monohydric alcohols

1. From aldehyde and ketones

Aldehydes and ketones are reduced to the corresponding alcohols by

a) Addition of hydrogen in the presence of catalysts like finely divided platinum, palladium, nickel and ruthenium.

This method is called catalytic hydrogenation.

b) Treatment with chemical reagents such as sodium borohydride (NaBH₄) or Lithium aluminium hydride (LiAlH₄).

Aldehydes yield primary alcohols while ketones give secondary alcohols.

2. From carboxylic acids and esters

Carboxylic acids are reduced to primary alcohols in the presence of strong reducing agent like lithium aluminium hydride.

The yield of alcohol here is high but LiAlH₄ being an expensive reagent, this method is not commonly used.

Commercially acids are reduced to alcohols by converting them to the esters followed by their reduction using either:

a) Hydrogen in the presence of a catalyst (Catalytic hydrogenation) or

b) Sodium and alcohol.

From alkenes

a) Hydration

Alkenes undergo hydration (addition of water across C=C bond) in the presence of dilute H₂SO₄ to produce alcohols. The alkyl hydrogen sulphate is formed which on hydrolysis with hot water gives alcohol.

The addition of water to the double bond is in accordance with Markownikov's rule. The alkene is obtained by cracking of hydrocarbons. The alkene is then absorbed by passing it into sulphuric acid at 353 K and 30 atmosphere pressure. The acid is diluted and treated with stream to release the alcohol.

The preparation of ethyl alcohol is done starting with ethane.

Similarly isopropyl alcohol is prepared from Propene.

b) Oxymercuration - demercuration

Alkenes react with mercuric acetate in presence of water to yield hydroxy mercurial compounds. These are reduced to alcohols by sodium borohydride.

This reaction gives a good yield of alcohol.

The alcohol obtained corresponds to Markownikov's addition of water to an alkene.

c) Hydroboration

Alkenes react with diborane (B₂H₆), which is an electron deficient molecule to yield alkylboranes (R₃B). These are oxidised to alcohols on reaction with hydrogen peroxide in presence of alkali.

In each addition step, the boron atom is attached to the sp² carbon atom that is bonded to greater number of hydrogen atoms. The hydrogen atom of the boron atom attaches to the other carbon of the double bond. Thus this is anti-Markovnikov's addition.

During the oxidation of trialkyl borane, boron is replaced by -OH group.

The yield of alcohol in this method is good and the product is easy to isolate.

d) From Grignard's reagent

Grignard reagents (R MgX) are alkyl or aryl magnesium halides.

The C Mg bondin Grignard reagent is a highly polar bond as carbon is electronegative relative to electropositive magnesium. Due to this polar nature of C-Mg bond, Grignard reagents are very versatile magnets in organic synthesis.

Grignard reagents regents react with aldehydes and ketones to form products, which decompose with dil HCl or dil H₂SO₄ to give primary secondary and tertiary alcohols.

The overall result is to bind the alkyl group of Grignard reagent to carbon of the carbonyl group and hydrogen to oxygen. Formaldehyde gives primary alcohol where as all other aldehydes give secondary alcohols and ketones furnish tertiary alcohols.