THE CONCEPT
Migrating One Bond to Reach a More Stable Cation
Whenever a carbocation forms that isn't already at the most stable possible position, it has a strong thermodynamic incentive to rearrange into a more stable one — and it can do this without needing any external reagent, simply by having a neighboring group migrate over with its bonding electron pair. This migration always moves exactly one carbon over (a 1,2-shift), from the carbon directly adjacent to the cation, onto the cation itself.
Two kinds of groups commonly migrate this way: a hydride shift moves a hydrogen (along with its bonding electron pair) from the adjacent carbon onto the cationic carbon, while a methyl shift moves an entire CH₃ group (along with its bonding electron pair) the same way. Either shift leaves a brand-new carbocation sitting on the carbon the migrating group departed from — and critically, that new cation is only formed if it's genuinely more stable than the one that existed before the shift.
💡 Memory Trick
The hub's trick names both shift types directly: 1,2-hydride or 1,2-methyl shift to a more stable carbocation. The hub also gives a second, less obvious rearrangement pathway worth remembering: a ring can expand or contract to reach a more stable ring system, using the same underlying migration logic just applied within a cyclic structure. Most usefully, the hub gives you a standing habit to apply on every relevant problem: predict rearrangements on every SN1 or electrophilic addition question — always ask: could this carbocation rearrange?
WHEN TO EXPECT A REARRANGEMENT
Two Specific Trigger Conditions
Rearrangements happen specifically under two conditions, both boiling down to 'a more stable cation is reachable one bond over.' Condition one: the initially formed carbocation is secondary, and an adjacent carbon is tertiary (or would become tertiary after the shift) — a hydride or methyl shift from that adjacent carbon converts the original secondary cation into a new, more stable tertiary one. Condition two: a ring system can expand or contract into a more stable ring size or substitution pattern through the same migration logic.
The practical exam-taking habit this creates is worth stating explicitly: any time you draw an SN1 mechanism, or an electrophilic addition to an alkene that proceeds through a carbocation intermediate (like acid-catalyzed hydration, or HX addition), pause immediately after drawing that first carbocation and check its neighboring carbons for a more stable option one shift away — before continuing on to predict the final product. Skipping this check is the single most common reason a mechanism prediction ends up wrong.
🧪 Lab Application
You're predicting the product of 3-methyl-2-butanol reacting with HBr via an SN1 mechanism, and need to check for a possible carbocation rearrangement before finalizing your answer.
1
Draw the initially formed carbocation. Protonation of the alcohol followed by loss of water generates a secondary carbocation at C2.
2
Check the adjacent carbon for a more stable option. C3, directly adjacent, bears a methyl group and would become a tertiary carbocation if a hydride shifted over from it.
3
Perform the hydride shift. A hydrogen migrates from C3 to C2 (bringing its bonding electrons along), converting the original secondary cation into a new, more stable tertiary cation now sitting at C3.
4
Predict the final product from the rearranged cation. Bromide attacks the new tertiary carbocation, giving a rearranged product with bromine at C3 — not the straightforward, unrearranged product you'd get by naively attacking the original secondary cation at C2.
📌 Exam Application
Rearrangement questions are a favorite way to test whether a mechanism prediction is genuinely mechanistic or just pattern-matched — always explicitly check the carbon(s) adjacent to any carbocation intermediate for a more stable rearrangement option before finalizing a predicted product, even when the question doesn't explicitly hint that a rearrangement is involved.
⚠️ Most Common Carbocation Rearrangements Mistakes
The most common mistake is forgetting to check for a possible rearrangement at all, especially when a mechanism seems to already 'make sense' without one. The other frequent trap is performing a rearrangement that actually leads to a LESS stable carbocation — always confirm the post-shift cation is genuinely more stable than the pre-shift one before accepting the rearranged pathway as favorable.
✓ Quick Self-Test
1) What is a 1,2-hydride shift? 2) What is a 1,2-methyl shift? 3) Under what general condition does a carbocation rearrangement occur? 4) Besides hydride and methyl shifts, what other structural change can a carbocation undergo to reach a more stable arrangement? 5) On which two types of mechanism should you routinely check for a possible carbocation rearrangement?
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