The reaction CF3CHCl2alc.KOH−−−−−→CF2=CCl2 is a unimolecular reaction and involves carbanion intermediate.
The addition of HBr to 2-methyl-1-butanol is a unimolecular reaction and involves carbocation intermediate.
In the reaction CH3−CH2−CH2−CH2−Br+alc.KOH→CH3−CH2−CH=CH2 no intermediate is formed.
The reaction of n propyl chloride with KOH to obtain n propanol is a unimolecular reaction and involves carbocation intermediate.