Questions: Carbocation Rearrangement: Hydride and Alkyl Shifts

Question 1

An SN1 reaction begins with 2-bromo-3-methylbutane. The leaving group departs from C2, generating an initial secondary carbocation. What product do you expect?

• A. Substitution at C2 — the nucleophile attacks the initial secondary carbocation directly
• B. Substitution at C3 — a hydride shift from C3 to C2 produces a tertiary carbocation at C3, and the nucleophile attacks there
• C. No reaction — secondary carbocations are too unstable to form
• D. Elimination only — secondary carbocations always eliminate rather than rearrange

Answer: (B)

After the leaving group departs from C2, the adjacent C3 bears a hydrogen and is part of a tertiary arrangement. A 1,2-hydride shift from C3 to C2 converts the secondary carbocation at C2 into a more stable tertiary carbocation at C3. The nucleophile then attacks C3, giving a product that a student predicting from the original leaving group position would get wrong. This is exactly why checking for possible rearrangements is mandatory before predicting SN1 products.

Question 2

What is the fundamental driving force behind 1,2-hydride and 1,2-alkyl shifts in carbocation intermediates?

• A. Relief of ring strain in cyclic systems
• B. Formation of a more stable (more highly substituted) carbocation
• C. Charge neutralization — the positive carbon becomes neutral after the shift
• D. Faster reaction kinetics — shifted carbocations react more quickly with nucleophiles

Answer: (B)

Rearrangements are driven by the thermodynamic gain from moving from a less stable to a more stable carbocation. Tertiary carbocations are more stable than secondary due to greater hyperconjugation and inductive stabilization from surrounding alkyl groups. The shift occurs because the transition state leading to the more stable carbocation is lower in energy. Charge is not neutralized — it simply relocates to a position that is better stabilized.

Question 3

In a 1,2-hydride shift, the hydrogen migrates from the positively charged carbocation center to an adjacent neutral carbon.

• T. True
• F. False

Answer: False

This reverses the direction. A 1,2-hydride shift moves a hydrogen (with its bonding electrons) FROM a neutral adjacent carbon TO the positively charged carbocation center. The positive charge then moves to the carbon that donated the hydrogen. The migration always flows toward the positive center, because it is the carbocation's empty orbital that accepts the electron pair from the migrating group.

Question 4

A 1,2-methyl shift can occur when the carbon adjacent to a secondary carbocation is a quaternary carbon bearing no hydrogens.

• T. True
• F. False

Answer: True

This is precisely the scenario where alkyl shifts occur rather than hydride shifts. If the adjacent carbon has no H atoms available to migrate (as in a quaternary carbon with four C substituents), a methyl or other alkyl group can migrate instead. The driving force is the same: the rearrangement produces a more stable (often tertiary) carbocation. In fact, a 1,2-methyl shift from a quaternary carbon to an adjacent secondary carbocation is one of the clearest examples of carbocation rearrangement.

Question 5

Why do carbocation rearrangements cause SN1 reactions to produce unexpected products, and what should you check before predicting any SN1 product?

Model answer: Rearrangements cause unexpected products because the nucleophile attacks the rearranged carbocation, not the one initially formed when the leaving group departs. If the initial carbocation rearranges (via a hydride or alkyl shift) to a more stable carbocation at a different carbon, the product will have the nucleophile attached at that new position — not where the leaving group was. Before predicting any SN1 product, you should examine the carbons adjacent to the initially formed carbocation and check whether a hydride or alkyl shift would produce a more stable (more substituted) carbocation. If it would, assume the rearrangement occurs first.

The key habit is never predicting the SN1 product purely from where the leaving group departs. Rearrangement is a competing process that occurs before the nucleophile arrives, and it is favored whenever it yields a stability gain. The practical consequence is that SN1 reactions adjacent to quaternary or tertiary carbons should always be examined for possible rearrangements.