Which one of the pairs undergoes hydrolysis by aqueous KOH readily and Why?
(ii) `(CH_3)_(3)C Cl and (CD_3)_(3)C Cl`.

Explanation

These `3^(@)` alkyl halides undergo alkaline hydrolysis by `S_(N)1` mechanism because the `3^(@)` carbocation obtained on their ionisation are stabilised by inductive as well as hyperconjugative effect. Also, water which is highly polar and a protic solvent, makes the ionisation favourable by stabilising the resulting ions. The high dielectric constant of water also favours ionisation by decreasing the electrostatic force between the incipient ions.

Since D is more electron-releasing than H, therefore, `-CD_(3)` is expected to be a better electron-donating (+I) group than `-CH_(3)`. Thus, `(CH_3)_(3)overset(o+)(C)` is expected to be more than `(CH_3)_(3)overset(o+)(C)`. But actually `(CH_3)_(3)overset(o+)(C)` is relatively more stable than `(CH_3)_(3)overset(o+)(C)`. It is because the hyperconjugative stabilisation of `(CH_3)_(3)overset(o+)(C)` involving relatively weaker C-H bond is much greater than the hyperconjugative stabilisation `(CH_3)_(3)overset(o+)(C)` involving relatively stronger C-D bond. Hence, due to formation of more stable carbocation, the hydrolysis of `(CH_3)_(3)C Cl` (solvolysis) is much faster than that of `(CD_3)_(3)C Cl`.
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