Question

why in the presence of alc.KOH alkyl halides undergo elimination whereas in the presence of aq.KOH they undergo substitution?

Answer 1

Aqueous KOHKOH is alkaline in nature i.e. it dissociates to produce a hydroxide ion. These hydroxide ions act as a strong nucleophile and replace the halogen atom in an alkyl halide.

RCl+KOH(aq)⟶ROH+KCl
RCl+KOH(aq)⟶ROH+KCl
This results in the formation of alcohol molecules and the reaction is known as nucleophilic substitution reaction.

Alcoholic, KOHKOH, specially in ethanol, produces C2H5O−CX2HX5OX− ions. The C2H5O−CX2HX5OX− ion is a stronger base than the OH−OHX− ion. Thus,the former abstracts the ß-hydrogen of an alkyl halide to produce alkenes. This reaction is known as elimination reaction.

CH3CH2Br+KOH(alc)⟶H2C=CH2+KBr+H2O