Why do non-polar solvents favour Sn2 reactions and why do polar solvents favour Sn1 reactions? Please simple ways. Thanks a lot

Question

Christian Anderson · Accepted Answer

Well, #S_N1# reactions feature a reactive intermediate...a carbocation....

In an #S_N1# reaction, say of isopropyl halide, the reaction is presumed to proceed thru a carbocationic intermediate....i.e. a bond-breaking rate determining step. #H_3C-CHXCH_3stackrel"polar solvent, slow step"rarrH_3C-stackrel(+)CHCH_3+X^-# And the presumed intermediate #H_3C-stackrel(+)CHCH_3# is certainly stabilized by interaction with the solvent.... On the other hand, should isopropyl react with a NUCLEOPHILE, the reaction is presumed to occur by a bond-making mechanism that involves reaction of (say) hydroxide anion at the ipso carbon... #H_3C-CHXCH_3+HO^(-)stackrel"fast step"rarrH_3C-CH(OH)CH_3+X^-# And this is a concerted mechanism, that involves no reactive intermediates. Less polar solvents magnify the reactivity of the intermediate...hydroxide, for instance is a more powerful nucleophile in ethanol or ether than it is in water, and the nucleophile is more reactive in the alcoholic/ethereal solvent....of course there is a trade-off in solubility versus reactivity... I would look in your text for bond making versus bond-breaking reactivity....