Give the product and find out which is faster ?
(a) (i) `CH_3 CH_2 I overset (overset (Ө) OH) rarr`
(ii) `D_3 C-CH_2 I overset (overset (Ө) O H) rarr`
(b) Give the product and find out which is faster out of `SN^1` and `E1` and why ?
(i) `(CH_3)_3 C C1 overset (overset (Ө)O H)rarr`
(ii) `(CD_3)_3 C-C1 overset (overset (Ө) O H)rarr`.

Explanation

(a) (i) `CH_2 = CH_2`
(ii) `D_2 C = CH_2`
Elimination of `HI` in (i) is faster than elimination of `DI` from (ii) because `R.D.S.` in `E2` reaction involves the breaking of `(C-H)` or `(C-D)` bond. Further, `(C-D)` bonds are strong and react slowly.
(b) The reaction rates are almost same. So `SN^1` and `E1` involve the formation of the same inermediate `[(CH_3)_3 C^+` or `(CD_3)_3 C^(oplus)]` by the same controlling ionisation step. This step does not involve any breaking of `(C-H)` or `(C-D)` bond (not `R.D.S.`) So no isotope effect is observed.
But (i) is slightly faster due to the secondary isotope effect as `(CH_3)_3 C^+` is more stable than `(CD_3)_3 C^+` (because `CH_3` has more `+ I` effect than `CD_3`). Moreover, `CH_3` has more hyperconjugative effect than `CD_3`.