Although C-D bond is stronger than C-H bond, yet (CH_(3))_(3)C^(+)(i) is more stable than (CD_(3))_(3)C^(+) (ii) Why so ?

Although C-D bond is stronger than C-H bond, yet (CH_(3))_(3)C^(+)(i)

1.	Although C-D bond is stronger than C-H bond, yet (CH_(3))_(3)C^(+)(i) is more stable than (CD_(3))_(3)C^(+) (ii) Why so ?
Answer» Solution :Both carbocations (i) and (ii) are stabilized by hyperconjugation as shown below: Due to stronger C-D bond, CONTRIBUTION of STRUCTURES (V-VIII) towards stability of carbocations, `(CD_(3))_(3)C^(+)` is less than those of structures (I-IV) for carbocation, `(CH_(3))_(3)C^(+)`, therefore, carbocation (i) is more stable than carbocation (ii). This effec is also sometimes called as `beta-` or secondary isotope EFFECT.