Question

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

Answer 1

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

Explanation:

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....