During a reaction, a $2^{°}$ (secondary) carbocation is converted into a more stable $3^{°}$ (tertiary) carbocation. What type of carbocation rearrangement has taken place?

• A. A ring expansion
• B. A deprotonation
• C. A hydride shift
• D. A methyl shift

Answer: (C)

Verified step by step guidance

1. Identify the initial and final carbocation types: the problem states a secondary (2°) carbocation is converted into a tertiary (3°) carbocation, indicating an increase in carbocation stability.
2. Recall that carbocation rearrangements occur to form more stable carbocations, often through shifts of atoms or groups adjacent to the positively charged carbon.
3. Understand the common types of carbocation rearrangements: hydride shifts (movement of a hydrogen with its bonding electrons), methyl shifts (movement of a methyl group), and ring expansions (rearrangement involving ring size changes).
4. Since the carbocation changes from secondary to tertiary without mention of ring size change, ring expansion is unlikely; also, deprotonation would remove the positive charge rather than rearrange it.
5. Conclude that the rearrangement involves a hydride shift, where a hydrogen atom adjacent to the carbocation migrates with its electron pair to the positively charged carbon, resulting in a more stable tertiary carbocation.