Question

The decreasing order of hydride affinity for following carbocations is:
Choose the correct answer from the options given below:

• A. C, A, D, B
• B. A, C, D, B
• C. C, A, B, D
• D. A, C, B, D

Answer 1

The Correct Option is C

The given question requires us to determine the decreasing order of hydride affinity for the provided carbocations. This involves understanding the stability of carbocations, as hydride will preferentially add to the least stable one.

Step 1: Understanding Carbocation Stability

• Carbocation C (Tertiary carbocation): This is a stabilized carbocation due to the +I effect and hyperconjugation from three methyl groups making it highly stable.
• Carbocation A (Allylic carbocation): This carbocation is stabilized by resonance, as the positive charge can be delocalized over the π-bond system.
• Carbocation B (Triphenyl carbocation): This is a stable carbocation due to resonance stabilization over three phenyl rings.
• Carbocation D (Cyclic carbocation): This is an unstable carbocation due to ring strain and the lack of resonance stabilization.

Step 2: Determining Hydride Affinity

Hydride will preferentially add to the carbocation with the least stability to stabilize it. The less stable the carbocation, the higher its hydride affinity.

• Carbocation D: Least stable, highest hydride affinity.
• Carbocation B: Stable, but less than C and A.
• Carbocation A: Moderately stable due to resonance.
• Carbocation C: Most stable, lowest hydride affinity.

Based on these observations, the correct order from highest to lowest hydride affinity is:

Order: D, B, A, C

However, the given answer requires the order from least stable to most stable, meaning opposite the hydride affinity. Thus, the answer is:

The correct answer is: C, A, B, D.