Named Rearrangements: Four Reactions, One Pattern

The shared pattern

All four reactions have the same shape. Something creates an electron-deficient atom — nitrogen, carbon or oxygen — and a group on the adjacent atom migrates to it, taking its bonding pair with it. The migration is concerted with the departure of the leaving group, which is why these reactions are stereospecific rather than merely stereoselective.

Beckmann rearrangement

An oxime treated with acid converts to an amide. The hydroxyl is protonated and leaves as water, creating an electron-deficient nitrogen, and a group migrates from carbon to nitrogen.

The reaction is used industrially to make the caprolactam that becomes nylon-6, which is worth knowing as an application question.

Hofmann rearrangement

A primary amide treated with bromine in base gives an amine with one fewer carbon. The nitrogen is brominated, base removes the remaining N–H proton, bromide leaves to give an electron-deficient nitrogen, and the alkyl group migrates from the carbonyl carbon to nitrogen. The resulting isocyanate is hydrolysed to the amine with loss of carbon dioxide.

The carbon count dropping by one is the signature that identifies this reaction in a question, and it comes from the carbonyl carbon leaving as carbon dioxide.

Curtius rearrangement

An acyl azide on heating loses nitrogen gas, generating an electron-deficient nitrogen, and the alkyl group migrates to give an isocyanate. Hydrolysis then gives the amine, again one carbon shorter.

Curtius and Hofmann reach the same isocyanate intermediate by different routes. The practical difference is conditions: Curtius is thermal and neutral, so it suits substrates that would not survive the bromine and base of a Hofmann. If a question specifies a base-sensitive substrate, that is the hint.

Baeyer–Villiger oxidation

A ketone treated with a peroxy acid inserts an oxygen atom next to the carbonyl, giving an ester. The peroxy acid adds to the carbonyl to form the Criegee intermediate, the weak O–O bond breaks, and a group migrates from carbon to the electron-deficient oxygen.

Migratory aptitude decides the product

For an unsymmetrical ketone, which group migrates determines which ester forms. The general order is:

The pattern tracks the ability to stabilise partial positive charge in the transition state, which is why more substituted groups migrate preferentially. Predicting the ester from an unsymmetrical ketone is one of the most common questions in this topic.

What is true of all four

• Migration occurs with retention of configuration at the migrating carbon. The group never fully detaches, so its stereochemistry is preserved — frequently asked and frequently answered incorrectly.
• The migration is concerted with the leaving group's departure. No free nitrene or carbocation intermediate accumulates, which is why the reactions are clean.
• Only one group migrates — the one best able to, or the one geometrically required to, depending on the reaction.

How to identify which reaction a question is asking about

1. Oxime starting material? Beckmann. Look for the anti relationship.
2. Amide plus bromine and base? Hofmann. Expect one carbon fewer.
3. Acyl azide plus heat? Curtius. Nitrogen gas is lost.
4. Ketone plus peroxy acid? Baeyer–Villiger. Decide the product by migratory aptitude.

Frequently asked questions

Why does the anti group migrate in the Beckmann?

Because migration is concerted with the departure of water, and the migrating group must be aligned with the breaking bond on the opposite side. Only the anti group has that alignment.

Do Hofmann and Curtius give the same product?

From equivalent starting materials, yes — both proceed through an isocyanate to the same amine. They differ in conditions and in what the substrate must tolerate.

How is migratory aptitude decided in practice?

By the ability to stabilise developing positive charge in the transition state. More substituted and better electron-donating groups migrate preferentially, which is why methyl is usually last.

Is stereochemistry always retained?

At the migrating carbon, yes, in all four reactions. That retention is a strong piece of evidence for the concerted mechanism, and stating the connection earns more than stating the fact.