Question

Alcoholic potash is used to bring about
A. Dehydrogenation
B. Dehydration
C. Dehydrohalogenation
D. Dehalogenation

Answer 1

Hint: Potash means potassium hydroxide and alcoholic potash means the alcoholic solution of potassium hydroxide. It helps in the formation of alkene.

Complete Step by Step Solution:
The alkene is formed from alkyl halide by reacting with alcoholic potassium hydroxide. The reaction involved is the elimination reaction. Alkene is the hydrocarbon containing a carbon-carbon double bond. Mono Haloalkanes are compounds where only one halogen group is attached to the alkyl group.

In dehydrogenation reaction, a hydrogen atom is removed from the reactant whereas, in dehalogenation, the halogen group is removed from the reactant. Both reactions taking place are known as dehydrohalogenation reactions.

The dehydrohalogenation reaction means the removal of hydrogen and halogen atoms.
Dehydrohalogenation is the method used for the preparation of alkenes from alkyl halides. In this reaction, hydrogen halide is removed. This reaction takes place by heating in presence of alcoholic potassium hydroxide or alcoholic potash.

The reaction of dehydrohalogenation of n-butyl chloride is shown below:
\[C{H_3}C{H_2}C{H_2}C{H_2}Cl + alc.KOH \to C{H_3}C{H_2}CH = C{H_2} + HCl\]
In the above reaction n-butyl chloride is heated with alcoholic potassium hydroxide to form but-1-ene as the main product by eliminating hydrochloric acid.
Therefore, Alcoholic potash is used to bring about dehydrohalogenation.
Hence, the correct option is C.

Note: The alcoholic potassium hydroxide act as a base so the hydroxide ion abstracts hydrogen from the beta carbon of butyl chloride to form a carbanion which further attack the alpha carbon to form a double bond by removing the bromide ion.