Question

Alcoholic KOH is used for:

(A) Dehydration

(B) Dehydrogenation

(C) Dehydrohalogenation

(D) Dehalogenation

Answer 1

Hint:  It is the process of elimination reaction that removes a hydrogen halide from a substrate. The reaction is usually associated with the synthesis of alkenes, but it has various other applications.

Complete step by step solution:

First of all, let us know and define the terms mentioned in the options for better understanding.

The term dehydration refers to the condensation of two molecules by the removal of water. Concentrated hydrochloric acid is a very good example of a dehydrating agent.

Dehydrogenation is just the opposite of hydrogenation. It is the removal of hydrogen from any organic compound.

Dehalogenation is the removal of two halogen atoms from adjacent carbon atoms i.e. vicinal dihalides upon reaction with zinc.

Dehydrohalogenation is basically the removal of hydrogen and a halogen group mainly from the alkyl halides to synthesize alkene or various other compounds. It is generally accomplished by reaction of alkyl halide with strong bases such as sodium ethoxide, alcoholic KOH etc.

The stepwise mechanism is mentioned below –

Firstly, the solvent taken is typical bases like sodium ethoxide , sodium hydroxide or alcoholic KOH.

Then, Alkyl halide is added in this solvent and heated.

B-hydrogen and halide are eliminated from the group.

The relative reactive order of alkyl halide is: $ {{\text{3}}^{\text{o}}}>{{\text{2}}^{\text{o}}}>{{1}^{\text{o}}} $ 

One reaction has been illustrated for an example-

 $ \text{C}{{\text{H}}_{3}}\text{C}{{\text{H}}_{2}}\text{C}{{\text{H}}_{2}}\text{Cl+ alc}\text{.KOH}\to \text{C}{{\text{H}}_{3}}\text{C}{{\text{H}}_{2}}\text{CH=C}{{\text{H}}_{2}} $ 

Therefore, we can conclude that the correct option is (C) Dehydrohalogenation.

Note: While removal of hydrogen, Zaitsev’s rule is followed. It states that the alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms is the desired product. Alexander Zaitsev studied a variety of elimination reactions to get this trend.