Questions: Carbocation Stability and Rearrangements

Question 1

Which of the following carbocations is most stable?

• A. CH₃⁺ (methyl carbocation)
• B. CH₃CH₂⁺ (primary, ethyl)
• C. (CH₃)₂CH⁺ (secondary, isopropyl)
• D. (CH₃)₃C⁺ (tertiary, tert-butyl)

Answer: (D)

Carbocation stability increases with substitution: tertiary > secondary > primary > methyl. Each additional alkyl group donates electron density toward the electron-deficient carbon through hyperconjugation and inductive effects, spreading and stabilizing the positive charge. The tert-butyl cation has three methyl groups doing this work, making it the most stable of the four.

Question 2

In a 1,2-hydride shift, the hydrogen atom first breaks away from the adjacent carbon as a free H⁻ ion, then rebonds to the carbocation.

• T. True
• F. False

Answer: False

A 1,2-hydride shift is a concerted process: the hydrogen migrates with its bonding electron pair in a single step, directly from the adjacent carbon to the carbocation center. There is no free H⁻ intermediate and no bond-breaking before bond-making. This distinguishes rearrangements from radical or ionic chain reactions — the migrating group is always bonded to something throughout the shift.

Question 3

Why do SN1 reactions sometimes produce a product with a different carbon skeleton than the starting material?

Model answer: After the leaving group departs and a carbocation forms, a 1,2-hydride or methyl shift can occur if migration produces a more stable (more substituted) carbocation. The rearranged carbocation then captures the nucleophile, giving a product with a different connectivity.

SN1 proceeds through a free carbocation intermediate, and any carbocation that can rearrange to a more stable one will do so before the nucleophile attacks. Because the rearrangement moves a carbon–hydrogen or carbon–carbon bond to a new position, the carbon skeleton itself changes. This is why neopentyl substrates — which would form an unstable primary carbocation — rearrange to give products derived from a tertiary carbocation with a different framework.