Preparation of Alcohols

Alcohols are compounds which contain one or more hydroxyl (-OH) groups directly attached to a carbon chain. Alcohols in the free form are not a common occurrence in nature; they are found mainly in the essential or volatile oils obtained from flowers, leaves, and stems of the plants.

General Methods of Preparation of Alcohols

1. Hydrolysis of Halides

Alkyl halides when boiled with an aqueous solution of an alkali hydroxide give alcohol through nucleophilic substitution mechanism.

R-X + KOH → R-OH + KX

This general procedure produces primary and secondary alcohols. Glycerol can be synthesized from propylene by a series of reactions including the hydrolysis of a halide as one step in the process.

2. Hydration of Alkanes

Direct hydration takes place by adding water in the presence of a catalyst.

Hydration of Alkanes

Indirect hydration is achieved by the addition of sulphuric acid to alkane followed by hydrolysis of the alkyl hydrogen sulfate.

Hydration of Alkanes

3. Hydroformylation of Alkenes

Lower molecular weight olefins react with carbon monoxide and hydrogen in the presence of a catalyst in a reaction called hydroformylation or the oxo reaction.

The resulting aldehyde is subsequently hydrogenated to form an alcohol.

Hydroformylation of Alkenes

Hydroformylation of Alkenes

4. Hydroboration of Alkenes

Alkenes, when treated with diborane, give alkyl boranes, R3B. Alkylboranes on oxidation with alkaline hydrogen peroxide give alcohol.

Hydroboration of Alkenes

Hydroboration of Alkenes

It is significant to note that this method always leads to anti-Markovnikov’s addition of water to alkenes.

5. Grignard Synthesis

All three types of alcohol (primary, secondary and tertiary) can be prepared from the Grignard reagents by interaction with suitable carbonyl compounds.

Grignard Synthesis

Grignard Synthesis

The reaction of the Grignard reagent with formaldehyde leads to primary alcohols, that with aldehydes other than formaldehyde yield secondary alcohols and that with ketones give tertiary alcohols.

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