Explain the effect of `SN^2` rates for the following reactions :
(a) Increasing the polarity of solvent.
(b) Show the `T.S` with all partial charges for each reaction type.
(I) `Me_2 overset (oplus) S - Me + NH_3 rarr Me overset (oplus)N H_3 + Me_2 S`
(II) `Me I + NH_3 rarr Me overset (oplus)N H_3 + I^(Ө)`
(III) `Me I + overset (Ө) O H rarr Me OH + I^(Ө)`
(IV) `Me_2 S^(oplus) - Me + overset (Ө) OH rarr Me OH + Me_2 S`.

Explanation

(I)
The charge in the `T.S` is more diffused than the charge in the reactant `(Me_2 S^(oplus) Me)`, hence the increase in solvent polarity stabilises the reactant more than the `T.S` So the rate decrease.
(II)
There is no charge either on the reactant or nucleophile, but `T.S` has `delta` -charges. Hence, an increase in solvent polarity stabilises the `T.S`., lowers `E_(act)`, and increase the rate.
(III)
This case is similar to (I). Therefore is a changed nucleophile and diffused charge `T.S`., and the rate is decreased.
(IV)
Both the reactant and nucleophile are charge. These are more stabilised by increasing solvent polarity than the `T.S` whose charge is much more diffused and hence there is a sharp decrease in the rate of `SN^2` reaction.