why alcoholic.KOH gives elimination reaction but aqueous.KOH gives substitution reaction?

In aqueous medium that water KOH will be completely dissociated to give OH negative ions. They being strong nucleophile,will bring about the substitution of alkyl halides to form alcohols. At the same time the OH negative i o n s will be hydrated also they will not be able to abstract a Proton from the beta carbon atom to form alkenes. In other words, in aqueous medium oh negative i o n s will behave as weak base and elimination leading T0 alkenes will not be feasible.In alcoholic KOH, the solution will also contain ethoxide ions in addition to OH negative ions. They being a stronger base then OH negative ions,will abstract a H positive ion from the beta carbon atom giving alkene as the product as a result of dehydrohalogenation.

Hello Student


Aqueous KOH is alkaline in naturei.e.It gives OH^-ions.

These hydroxyde ions ions which act as strong nucleophile and replace halogen atom from alkyl halide.
RCl + KOH (aq.) → ROH.

This results in the formation of alcohol molecules and the reaction is known as nucleophil substitution reaction.

Alcoholic, KOH , specially in ethylene alcohol, produce C2H5O^-ions. These ions are stronger base than OH^-ion. Thus they abstracts the ß-hydrogen of alkyl halide to produce alkene. This reaction is known as elemenation reaction.
CH3CH2Br + KOH (alc.) → H2C=CH2+ KBr + H2O

I hope this Answer will help you.