Choose the correct order of stability for the following carbocations: (I) (II) (III) (IV)

Step-by-step solution

The stability of the given benzyl carbocations is determined by the electron-donating effects of the - CH ₃ and - CD ₃ groups. The - CH ₃ group has a stronger hyperconjugation ( + H ) effect than - CD ₃ because the C - H bond is weaker than the C - D bond. However, the - CD ₃ group has a stronger inductive ( + I ) effect than - CH ₃ . For para-substituted carbocations (I and II), both + H and + I effects operate. Since the + H effect dominates over the + I effect, the stronger + H of - CH ₃ makes (I) more stable th