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Common Rearrangements
Hydride & Alkyl Shifts

Understand why carbocations rearrange, how 1,2-hydride shifts and 1,2-alkyl shifts work, and when to expect them in reaction mechanisms.

Organic Chemistry 1

What Is a Carbocation Rearrangement?

When a carbocation forms during a reaction, it may rearrange to become more stable. A neighboring group: either a hydrogen atom (\(\text{H}\)) or an alkyl group (\(\text{R}\)): migrates from an adjacent carbon to the positively charged carbon.

Carbocation Stability Review

Recall the stability order. More alkyl groups donate electron density through hyperconjugation and induction, stabilizing the empty \(p\)-orbital.

Methyl
\(\text{CH}_3^+\)
 <
\(1^\circ\)
\(\text{RCH}_2^+\)
 <
\(2^\circ\)
\(\text{R}_2\text{CH}^+\)
 <
\(3^\circ\)
\(\text{R}_3\text{C}^+\)
 Increasing Stability

When Do Rearrangements Occur?

Carbocation rearrangements happen in any mechanism that forms a carbocation intermediate. The three most common reaction types are:

\(\text{S}_\text{N}1\) Reactions
The leaving group departs first, creating a carbocation that can rearrange before the nucleophile attacks.
\(\text{E}1\) Reactions
The carbocation intermediate may shift before a base removes a proton to form the alkene.
Additions to Alkenes
Markovnikov addition of \(\text{HX}\) forms a carbocation that can rearrange before halide attack.

The 1,2-Hydride Shift

A hydrogen atom (with its bonding electrons) migrates from an adjacent carbon to the carbocation center. The \(\text{H}\) moves from the \(\beta\)-carbon to the \(\alpha\)-carbon.

BEFORE C
\(+\)
\(2^\circ\)
 C
\(3^\circ\)
 H R R 1,2-H shift AFTER C H C
\(+\)
\(3^\circ\)
 R R
\(2^\circ\) carbocation \(\to\) \(3^\circ\) carbocation

The 1,2-Alkyl Shift

An entire alkyl group (often a methyl, \(\text{CH}_3\)) migrates with its bonding electrons from the \(\beta\)-carbon to the carbocation center.

BEFORE C
\(+\)
\(2^\circ\)
 C
\(\text{CH}_3\)
 R R 1,2-alkyl shift AFTER C
\(\text{CH}_3\)
 C
\(+\)
\(3^\circ\)
 R R
\(2^\circ\) carbocation \(\to\) \(3^\circ\) carbocation

Hydride vs. Alkyl Shift

Feature 1,2-Hydride Shift 1,2-Alkyl Shift
What migrates \(\text{H}\) with bonding electrons Alkyl group (e.g. \(\text{CH}_3\))
When preferred \(\text{H}\) is available on the \(\beta\)-carbon No \(\beta\)-H, or alkyl shift gives greater stability
Speed Extremely fast (low energy barrier) Slightly slower (larger group migrating)
Result More substituted cation More substituted cation or ring expansion

The Driving Force

The fundamental principle behind every rearrangement: a less stable carbocation will rearrange to a more stable one whenever a pathway exists.

1
\(1^\circ \to 2^\circ\): A primary carbocation is so unstable it almost always rearranges if a neighboring carbon can donate an H or alkyl group.
2
\(2^\circ \to 3^\circ\): A secondary carbocation will rearrange to tertiary if the geometry allows it.
3
\(3^\circ\) stays \(3^\circ\): A tertiary carbocation generally does not rearrange: it is already highly stabilized.

How to Identify a Rearrangement

1
Draw the carbocation. After the leaving group departs or the electrophile adds, identify the carbocation center and its classification (\(1^\circ, 2^\circ, \text{or } 3^\circ\)).
2
Check the neighbor. Look at each \(\beta\)-carbon. Could shifting an H or alkyl group create a more stable cation?
3
Perform the shift. Move the H or alkyl group with its bonding electrons. Redraw the carbocation at the new position.
4
Continue the mechanism. Nucleophilic attack or elimination now occurs at the new cation site.

Example 1: Hydride Shift in \(\text{S}_\text{N}1\)

2-Bromo-3-methylbutane reacts with water. Predict the major substitution product.

Step 1: Ionization
\(\text{CH}_3\)
 CH
\(\text{Br}\)
 CH
\(\text{CH}_3\)
\(\text{CH}_3\)
\(-\text{Br}^-\)
\(2^\circ\) Carbocation
\(\text{CH}_3\)
\(\text{C}^+\)
\(\text{H}\)
 CH
\(\text{CH}_3\)
\(\text{CH}_3\)
 Step 2: 1,2-Hydride Shift H shift
\(3^\circ\) Carbocation (more stable)
\(\text{CH}_3\)
\(\text{CH}_2\)
\(\text{C}^+\)
\(\text{CH}_3\)
\(\text{CH}_3\)
Step 3: \(\text{H}_2\text{O}\) attacks \(3^\circ\) cation \(\to\) 2-Methyl-2-butanol (major product)

Example 2: Methyl Shift in \(\text{S}_\text{N}1\)

Neopentyl bromide (\(\text{(CH}_3\text{)}_3\text{CCH}_2\text{Br}\)) reacts with methanol. Why does the product contain a rearranged skeleton?

\(1^\circ\) Carbocation (initial)
\(\text{C}^+\)
\(1^\circ\)
\(\text{C(CH}_3\text{)}_3\)
\(\text{CH}_3\) shift
\(3^\circ\) Carbocation (after shift)
\(\text{CH}_2\)
(\(\text{CH}_3\))
\(\text{C}^+\)
\(3^\circ\)
\(\text{CH}_3\)
\(\text{CH}_3\)
\(\text{MeOH}\) attacks \(3^\circ\) cation \(\to\) 2-methoxy-2-methylbutane
 (Rearranged skeleton: the carbon chain has changed!)

Example 3: HBr Addition with Rearrangement

3,3-dimethyl-1-butene reacts with \(\text{HBr}\). Predict the major product.

1
Protonation (Markovnikov): \(\text{H}^+\) adds to \(\text{C-1}\), forming a \(2^\circ\) carbocation at \(\text{C-2}\).
2
1,2-Methyl shift: A \(\text{CH}_3\) migrates from \(\text{C-3}\) (quaternary center) to \(\text{C-2}\), converting the \(2^\circ\) cation to a \(3^\circ\) cation at \(\text{C-3}\).
3
\(\text{Br}^-\) attacks: The bromide ion attacks the \(3^\circ\) cation at \(\text{C-3}\), giving 2-bromo-2,3-dimethylbutane as the major product.

Ring Expansion: A Special Alkyl Shift

When a carbocation forms adjacent to a ring, a \(\text{C-C}\) bond within the ring can migrate. This expands the ring by one carbon: a ring expansion.

Cation adjacent to ring
\(\text{C}^+\)
\(\text{C-C}\) shift
 Cyclohexyl cation (more stable) 6-membered ring
\(+\)
 5-membered ring \(\to\) 6-membered ring

Key Rules Summary

1
Only 1,2-shifts occur: The migrating group moves from the \(\beta\)-carbon to the \(\alpha\)-carbon (one carbon over). 1,3-shifts are extremely rare.
2
Shift toward more stable cation: \(1^\circ \to 2^\circ\), \(2^\circ \to 3^\circ\), or to a resonance-stabilized cation.
3
\(\text{H}\) shifts are preferred over alkyl shifts when both produce the same stability gain.
4
Stereochemistry: The migrating group retains its configuration (migrates with its bonding pair via a concerted process).
5
No rearrangement if already stable: \(3^\circ\) and resonance-stabilized cations do not rearrange further.

Common Mistakes

✕ Drawing 1,3-Shifts
Students sometimes migrate a group two carbons away. Only 1,2-shifts (one carbon) are observed for H and alkyl groups.
✕ Forgetting to Check
Any time a carbocation forms, you must check for possible rearrangement. Skipping this step leads to predicting the wrong product.
✕ Shifting to Less Stable
Rearrangements go toward more stable cations, never less stable. A \(3^\circ\) cation does not shift to become \(2^\circ\).

Summary Comparison

Scenario Shift Type Result Example Clue
\(2^\circ \to 3^\circ\) H shift or alkyl shift More stable \(3^\circ\) cation Product has \(\text{-OH/X}\) on unexpected C
\(1^\circ \to 3^\circ\) Alkyl (methyl) shift \(3^\circ\) cation Neopentyl substrates
Ring adjacent \(\text{C-C}\) bond migration Ring expansion 5-membered \(\to\) 6-membered ring
Already \(3^\circ\) None No rearrangement Product at expected position

Question 1 of 5

A secondary carbocation forms during an \(\text{S}_\text{N}1\) reaction. The \(\beta\)-carbon bears two methyl groups and one hydrogen. What type of rearrangement is most likely?

Question 2 of 5

In a 1,2-hydride shift, what migrates along with the hydrogen atom?

Question 3 of 5

Neopentyl bromide undergoes solvolysis in methanol. The major product has a rearranged carbon skeleton. Which rearrangement occurred?

Question 4 of 5

Which reaction mechanism does not involve a carbocation intermediate and therefore cannot undergo a 1,2-shift?

Question 5 of 5

A carbocation adjacent to a cyclopentane ring undergoes ring expansion. What is the ring size of the product?