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`.