Question

Choose the correct statement regarding the formation of carbocations A and B given.

• A. Carbocation A is more stable and formed relatively at faster rate
• B. Carbocation B is more stable and formed relatively at slow rate
• C. Carbocation B is more stable and formed relatively at faster rate
• D. Carbocation A is more stable and formed relatively at slow rate

Hint:

Compare a primary (A) with a secondary (B) carbocation; the more stable one has the lower-energy transition state and forms faster.

Answer 1

The Correct Option is C

1-Butene adds \(\mathrm{H^+}\) from HBr. Adding \(\mathrm{H^+}\) to C-1 gives the secondary carbocation \(\mathrm{CH_3CH_2\overset{+}{C}HCH_3}\) (B); adding to C-2 gives the primary carbocation \(\mathrm{CH_3CH_2CH_2\overset{+}{C}H_2}\) (A).
A secondary carbocation is stabilised by more hyperconjugation and +I effect of alkyl groups than a primary one, so B is more stable than A.
The more stable carbocation has a lower-energy transition state (lower activation energy), so it is formed at a faster rate (Markovnikov addition).
Answer: (3) Carbocation B is more stable and formed relatively at faster rate