When the neighbor of a carbocation has no hydrogen to donate for a hydride shift, a whole alkyl group migrates instead — and moving a carbon rearranges the skeleton.
The signature alkyl shift: a 1° neopentyl cation rearranges to a 3° cation by moving a methyl group.
1. An Alkyl Shift Moves a Whole Carbon Group, Not Just a Hydrogen
Same 1,2 migration as a hydride shift — the group travels with its bonding electrons — but the traveler is a carbon group like methyl (CH3−), so the charge lands on the carbon it left.
2. An Alkyl Shift Takes Over When No Hydride Shift Can Reach a Better Cation
The alkyl shift is the backup plan: the textbook trigger is a quaternary carbon next to the cation, which has zero hydrogens to donate, so only a methyl can move.
3. The Driving Force Is Always Carbocation Stability
A shift only happens when it climbs the 3° > 2° > 1° stability ladder — a migration that bridges a 2° cation to a 3° one is strongly downhill.
4. An Alkyl Shift Rewires the Carbon Skeleton, Not Just the Charge
Because a carbon physically relocates, the product's skeleton differs from the starting material's — same formula, different carbon map, which is why solvolysis of neopentyl bromide gives the rearranged tertiary alcohol.
Neopentyl bromide solvolyzes to tert-amyl alcohol — the skeleton rearranges via a 1,2-methyl shift.
5. Rearranged Products Show Up in SN1, E1, and Alkene Additions
Any reaction that forms a free carbocation can rearrange — SN1, E1, and acid-catalyzed alkene additions — so the rearranged cation then traps or eliminates like any tertiary cation.
But a shift must improve stability: the already-tertiary cation below has no reason to move a methyl and simply eliminates.
6. Summary
1,2-migration of a whole carbon group · backup for a hydride shift · triggered by a quaternary carbon next to the cation · must climb the 3° > 2° > 1° ladder · rearranges the skeleton · a wrong-looking skeleton in SN1/E1/alkene additions is the tell.
Quiz yourself
Tap a question to reveal the answer — free, no login.
The carbon adjacent to the positive charge is quaternary — it has four carbons and no hydrogen — so there is no hydride to move. The only way to reach a more stable ion is to migrate a methyl group, which converts the 1° cation into a 3° (tert-amyl) cation.
It has to produce a more stable carbocation (moving up the 3° > 2° > 1° ladder). A shift that keeps the same stability, or lowers it, does not happen — the driving force is always cation stability.
A hydride shift moves only the charge and leaves the carbon skeleton unchanged. An alkyl shift relocates a carbon, so it rearranges the skeleton — the product has the same formula but a different carbon framework (e.g., neopentyl becomes tert-amyl).
2-methyl-2-butanol (tert-amyl alcohol). Neopentyl bromide is too hindered for SN2 and ionizes (SN1) to the neopentyl 1° cation, which immediately does a 1,2-methyl shift to the 3° cation. Water then traps the rearranged tertiary cation, giving the rearranged alcohol rather than neopentyl alcohol.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.