Question

Why polarportic directly propotional to nucleophiles?

Answer 1

Answer:

Since the hydrogen atom in a polar protic solvent is highly positively charged, it can interact with the anionic nucleophile which would negatively affect an SN2, but it does not affect an SN1 reaction because the nucleophile is not a part of the rate-determining step (See SN2 Nucleophile).

Answer 2

ANSWER :

For this reason, the concentrations of

both the nucleophile and the alkyl halide are proportional to the observed SN2 reaction rate. ... Polar protic solvents (e.g. alcohols, water) are much better at solvating anions, so nucleophiles are highly solvated in these solvents.