For a given secondary halide the SN2 rxn is favoured by aprotic solvents why?
1Aprotic = no donation of H+.
for ex. DMF (dimethyl fluoride)
nucleophile gets weaker in presence of water
because solvation weakens the nucleophile...
its donating ability reduces.
but aprotic solvents dun solvate nu-.
so SN2 is favoured here.
DMF is a popular aprotic solvent..
(there was question on it in iit-2008 as well)
1in case of protic solvelnts the activation energy of SN1 rxn decreases so SN2 reactions are favored in weakly polar medium or aprotic medium
1also, solvents with high high dielectric constant helps in making more stable carbocation...
so in presence of water, SN2 tendency is more than SN1.
1if the nu- weakens, its lone pair donating ability reduces,
so, it will not have enuf force to attack the molecule from backwards.
also,
since i told above dat C+ is more stable in H2O, so C+ forms quickly...
resulting in SN1.
