Carbocation Rearrangement: 1,2-Hydride and 1,2-Alkyl Shifts

Core Idea

Carbocations can undergo 1,2-hydride or 1,2-alkyl shifts from an adjacent carbon to form more stable carbocations. These rearrangements occur when a secondary carbocation can rearrange to a tertiary (more stable) one. The migrating group moves with its bonding electrons toward the carbocation center.

How It's Best Learned

Identify secondary vs. tertiary carbocations and predict which rearrangements increase stability. Draw electron flow diagrams for hydride and alkyl shifts.

Common Misconceptions

• Assuming all carbocations rearrange; rearrangement only occurs if it leads to a more stable carbocation.
• Misunderstanding the electron flow; the migrating group's bonding electrons move toward the positive charge, not away.

Explainer

You already know that carbocations are classified by substitution — primary, secondary, tertiary — and that more substituted carbocations are more stable due to hyperconjugation and inductive effects. Carbocation rearrangement is the direct consequence of this stability hierarchy: if a reaction generates a less stable carbocation and a more stable one is just one bond-shift away, the rearrangement will happen, often faster than any competing reaction. This is not an optional side reaction — it is a thermodynamic imperative that the mechanism follows automatically.

A 1,2-hydride shift is the most common rearrangement. Imagine a secondary carbocation on carbon-2 of a chain, with a tertiary carbon adjacent at carbon-3 bearing a hydrogen. The hydrogen on carbon-3 migrates *with its bonding electrons* to the positively charged carbon-2. The result: the positive charge has moved from carbon-2 (secondary) to carbon-3 (now tertiary, because the hydrogen left). Crucially, what migrates is not a bare proton (H⁺) — it is a hydride (H:⁻), carrying the bonding pair. The electron flow arrow points from the C–H bond toward the empty p orbital of the carbocation. This is why the shift is drawn as a curved arrow from the adjacent C–H bond to the cation center.

A 1,2-alkyl shift (also called a 1,2-methyl shift when the migrating group is –CH₃) works identically, except an entire alkyl group migrates with its bonding electrons instead of a hydrogen. This occurs when no hydride shift can improve stability, but moving an alkyl group can. For example, a secondary carbocation adjacent to a quaternary carbon (which has no hydrogen to shift) can rearrange via methyl migration to form a tertiary carbocation. The principle is the same: the group moves toward the positive charge, carrying its electrons with it.

Not every carbocation rearranges. The shift only occurs if it leads to a *more stable* carbocation — secondary to tertiary, or secondary to a resonance-stabilized cation. A tertiary carbocation adjacent to another tertiary carbon has no driving force to rearrange and will proceed directly to product. When predicting products, always check: does the initially formed carbocation have a neighboring carbon that could donate a hydride or alkyl group to produce a more substituted cation? If yes, draw the rearranged intermediate *before* predicting the final product. Failing to check for rearrangement is one of the most common mistakes in organic mechanism problems, leading to incorrect regiochemistry in addition, substitution, and elimination products.