Question

For the preparation of chloroethane
A. HCl gas is passed through ethanol.
B. ethanol is treated with thionyl chloride in the presence of dimethyl amine or pyridine.
C. Ethyl sulphide is treated with hydrogen chloride.
D. Any of the above methods can be employed.

Answer 1

Hint:To prepare chloroethane from ethanol, reduction of alcohol group is required. Ethanol gives nucleophilic substitution with hydrochloric acid. But this reaction requires a catalyst. Thionyl forms ethoxide ion and dimethyl amine or pyridine helps to generate a chloride nucleophile.

Complete answer:
Alcohol gives nucleophilic substitution with hydrochloric acid. But this reaction requires a catalyst such as zinc chloride. The tertiary alcohol is more reactive for this reaction than the primary alcohol. Ethanol is the primary alcohol and the catalyst is also not present, so chloroethane cannot be prepared by passing the hydrochloric gas through ethanol.
Reaction, when ethanol is treated with thionyl chloride in the presence of dimethyl amine or pyridine, is as follows:

Ethyl sulphide on treating with hydrogen chloride does not give chloroethane.
So, for the preparation of chloroethane, ethanol is treated with thionyl chloride in the presence of dimethyl amine or pyridine.

Therefore, option (B) ethanol is treated with thionyl chloride in the presence of dimethyl amine or pyridine, is correct.

Note:
The reaction of an alcohol with thionyl chloride to generate alkyl halide is known as the Darzens process. The reaction of an alcohol with hydrochloric acid in presence of a catalyst such as zinc chloride is known as the Lucas test. This test is used to differentiate alcohols. This is a nucleophilic substitution reaction, so a carbocation generates during the reaction so, the rate of reaction depends upon the stability of the carbocation. Thus Lucas tests differentiate primary, secondary and tertiary alcohols.