(A) When polyhalogenated alkanes are heated with alc. KOH, a polyhalogenated alkene is obtained. This is a dehydrohalogenation reaction.
A carbanion is formed as the reaction intermediate.
(B) Tert− butyl alcohol on dehydration forms 2-methyl-2-butene.
A carbocation is formed as the reaction intermediate.
(C) Dehydrobromination of ethyl bromide (on heating with alcoholic KOH) forms ethylene.
A transition state is present but no intermediate is formed.
(D) Dehydrobromination of tert butyl bromide (on heating with alcoholic KOH) forms 2-methyl-2-butene.
A carbocation is formed as the reaction intermediate.