Carbocation Rearrangements

Introduction

Carbocations can change their carbon skeleton or charge position before a nucleophile traps them. A neighbouring C–H or C–C bond can migrate with its electron pair to the electron-deficient centre. The positive charge then appears at the atom that lost the migrating bond. These 1,2-shifts are important in SN1, E1, and acid-catalysed additions, but they should be drawn only when a plausible cation and favourable shift exist.

Core explanation

In a 1,2-hydride shift, a hydrogen attached to a carbon next to the carbocation moves together with the electrons of its C–H bond. Draw a curved arrow from that C–H bond toward the carbocation carbon. The formerly positive carbon gains a C–H bond and becomes neutral; the neighbouring carbon that lost H becomes positively charged. This is not a free H⁻ ion traveling through solution. The word hydride describes the bond pair's allocation during migration.

In a 1,2-alkyl shift, a neighbouring C–C bond migrates toward the cationic carbon. The carbon group attached through that bond changes its attachment point, potentially altering the skeleton. The positive charge relocates to the carbon that lost the migrating group. Draw the old and new C–C bonds explicitly and recalculate each carbon's valence. An alkyl shift can expand a strained small ring or create a more stable tertiary cation, but exact preference depends on barriers as well as final stability.

Why might a shift occur? A secondary cation next to a carbon able to produce a tertiary cation after hydride migration may lower its energy. A ring expansion can relieve strain, adding another driving force. But a possible “more stable” drawing is not sufficient proof. The shifted cation must be reachable by an adjacent bond in a suitable geometry, and nucleophile attack or deprotonation may happen faster than rearrangement. Product mixtures can contain both rearranged and unrearranged structures.

The timing matters in product prediction. In SN1, draw leaving-group ionisation, check rearrangement, then let nucleophile attack the final cation. In E1, check rearrangement before choosing the β-H that forms an alkene. In alkene electrophilic addition, protonation can create a cation that shifts before halide or water capture. A student who directly attaches the nucleophile where the leaving group was can miss a rearranged product even if the initial cation was correctly drawn.

Concerted SN2 and E2 pathways do not contain a free carbocation intermediate, so ordinary carbocation shifts should not be inserted into them. If a rearranged product appears, it may be evidence for a cationic pathway or another skeletal reaction, but mechanism assignment still needs kinetics and conditions. A rearrangement is an electron-pair and atom-movement event, unlike resonance, where atoms stay fixed.

Step-by-step reasoning

1. Draw the initial cation formed by a justified mechanistic step. 2. Inspect only directly adjacent C–H and C–C bonds for a 1,2-shift. 3. Move the chosen bond pair to the cation centre and relocate + charge. 4. Compare starting and shifted cation stability and possible ring strain. 5. Only then draw nucleophile capture or β-H elimination products.

Visual explanation

Draw a secondary cation adjacent to a tertiary carbon with one H. Show the C–H bond arrow toward C⁺, then mark the positive charge on the former H-bearing carbon.

Real-world analogy

One worker shifts into an adjacent vacancy, leaving a new vacancy where that worker stood. The positive charge moves to the atom that lost the migrating bond.

Real-world example

An acid-catalysed alcohol reaction unexpectedly yields a rearranged carbon skeleton. Drawing the intermediate cation and an adjacent alkyl shift can explain the product connectivity and location of substitution.

Why?

Why must the positive charge relocate after a hydride shift? The migrated C–H bond supplies electrons to the old cation centre, while the carbon losing that bond becomes electron deficient.

Common misconception

“A hydride shift moves H alone and leaves its bond electrons behind.” The C–H bond electron pair migrates with H to form the new bond.

Worked example

Suppose a secondary carbocation R–CH⁺–C(H)(CH₃)₂ forms beside a carbon bearing H and two methyl groups. Move the adjacent C–H bond pair from C(H)(CH₃)₂ toward the cation carbon. The first carbon gains H and becomes neutral; the neighbouring carbon loses H and becomes a tertiary carbocation bonded to three carbon groups. A nucleophile arriving afterward attacks this new cation centre, potentially giving a product with Nu at a different carbon than the original leaving group. Draw both stages before naming product.

Quick check

1. Which carbon bears positive charge after a 1,2-hydride shift? Answer: The carbon that lost the migrating C–H bond.

Exam focus

Show the migrating bond as the arrow source, never a free hydride. Recalculate charge and carbon valence before drawing final products.

Advanced insight

Rearrangement competition can be probed by product ratios and isotope labeling. A hydride carrying a deuterium label can reveal whether and where a shift occurred.

Summary

Carbocation 1,2-shifts migrate a neighbouring C–H or C–C bond pair to an electron-deficient carbon and relocate positive charge. They can alter later products.

Practice questions

1. Can a nonadjacent hydrogen shift directly in a 1,2-hydride shift? Answer: No. The migrating bond starts on a carbon immediately adjacent to the cation. 2. Does ordinary SN2 allow a discrete carbocation shift? Answer: No. Its concerted pathway contains no free cation intermediate. 3. What feature can favour a ring-expansion alkyl shift? Answer: Relief of ring strain, sometimes combined with cation stabilisation.