---
title: "Claisen Rearrangement | Organic Chemistry"
description: "Claisen rearrangement is a thermal [3,3]-sigmatropic shift that turns an allyl vinyl ether into a new C-C bond and a cyclohexenone in Organic Chemistry."
canonical: "https://fiveable.me/organic-chem/key-terms/claisen-rearrangement"
type: "key-term"
subject: "Organic Chemistry"
unit: "Unit 30"
---

# Claisen Rearrangement | Organic Chemistry

## Definition

The Claisen rearrangement is a [3,3]-sigmatropic rearrangement in Organic Chemistry that converts an allyl vinyl ether into a carbon-carbon bonded product, often a cyclohexenone. It is a concerted way to build rings and new carbon skeletons.

## What It Is

The Claisen rearrangement is a concerted Organic Chemistry reaction where an allyl vinyl ether rearranges to form a new carbon-carbon bond and, after tautomerization, a carbonyl-containing product such as a cyclohexenone. It is one of the classic examples of a [3,3]-sigmatropic rearrangement, so the atoms move in a single step rather than through a carbocation or radical intermediate.

The key idea is that the allyl group shifts from oxygen to carbon. If you start with an allyl vinyl ether, the bond between oxygen and the allyl fragment breaks while a new bond forms between the allyl end and the carbonyl-side carbon of the vinyl portion. That swap is why the reaction is so useful for making carbon-carbon bonds in one step instead of building the chain piece by piece.

Mechanistically, the rearrangement goes through a cyclic, chair-like transition state. You can picture the six atoms involved lining up in a loop so their orbitals overlap smoothly. That geometry matters because it controls both the speed of the reaction and the stereochemistry of the product. The chair-like transition state usually gives a predictable relative arrangement of substituents, which is why this reaction shows up in synthesis problems where you need to track how groups end up on a ring.

The product is not always a cyclohexenone immediately from the rearrangement step itself. In many cases, the first product is an enol that quickly tautomerizes to the more stable carbonyl form, such as a cyclohexenone or related 1,5-dicarbonyl-type framework depending on the substrate. That means when you see a Claisen rearrangement in a reaction sequence, you should think about both the rearrangement and the follow-up shift to the carbonyl product.

This reaction usually happens under thermal conditions, which fits the rules for many pericyclic reactions. Lewis acids can sometimes help by activating the substrate or changing the reaction pathway, but the standard version is driven by heat. In organic chemistry problems, the most important recognition step is spotting the allyl vinyl ether motif and predicting that the allyl group will migrate to carbon in a concerted rearrangement.

## Why It Matters

The Claisen rearrangement matters because it gives you a clean way to make a new carbon-carbon bond while also reorganizing a molecule into a ring or a more functionalized skeleton. In Organic Chemistry, that makes it a high-value synthesis tool, especially when a problem asks how to turn a relatively simple starting material into a more complex cyclic product.

It also connects several big ideas from the course at once: rearrangement reactions, stereochemistry, orbital overlap, and reaction mechanism. If you can recognize why the rearrangement is concerted and why the chair-like transition state matters, you can predict products instead of memorizing them case by case.

This reaction shows up when instructors want you to think beyond substitution and elimination. It is a good example of how a molecule can change its connectivity without adding or removing atoms overall. That makes it useful for comparing reaction categories and for reading multi-step syntheses where a rearrangement sets up the final structure.

It also reinforces the idea that product stability matters after the rearrangement. A lot of Claisen problems end with an enol that rapidly tautomerizes to a ketone or enone, so you need to recognize the final isolable product, not just the immediate rearrangement step. That skill shows up again in spectroscopy and synthesis questions, where the carbonyl product gives strong clues about what happened.

## Connections

### Sigmatropic Rearrangement

The Claisen rearrangement is a specific example of a sigmatropic rearrangement, so it fits into the broader family of reactions where a sigma bond shifts across a pi system in a concerted way. If you know the general sigmatropic pattern, the Claisen reaction becomes easier to recognize because the atom movement and cyclic transition state follow the same logic.

### Allyl Vinyl Ether

This is the starting material you look for when identifying a Claisen rearrangement. The allyl group attached to oxygen is the part that migrates, and the vinyl ether portion provides the carbon framework that ends up rearranged. If the substrate does not have this pattern, it is probably not a Claisen rearrangement.

### Cyclohexenone

Many Claisen rearrangement products are cyclohexenones or closely related carbonyl compounds after tautomerization. That makes cyclohexenone a common product pattern to recognize in synthesis problems. When you see a six-membered ring with an enone, it may be the rearrangement product of an allyl vinyl ether starting material.

### [Concerted Mechanism](/organic-chem/key-terms/concerted-mechanism)

The Claisen rearrangement happens in one coordinated step, not through separate bond-breaking and bond-making intermediates. That concerted mechanism explains why the reaction has strong stereochemical predictability. It also helps distinguish the Claisen rearrangement from reactions that go through carbocations, which behave very differently.

## On the AP Exam

A quiz question or mechanism problem usually asks you to identify the starting material, trace where the allyl group moves, or draw the product after the rearrangement and tautomerization. The move is to spot the allyl vinyl ether and follow the [3,3]-sigmatropic shift through the chair-like transition state.

If the question gives you a product such as a cyclohexenone, you may need to work backward and reconstruct the allyl vinyl ether precursor. That means paying attention to where the new C-C bond formed and which atoms came from oxygen-containing parts of the molecule. In synthesis or lab-based questions, the reaction often shows up as a named step that creates a ring or sets up a carbonyl compound for later transformations.

## Key Takeaways

- The Claisen rearrangement is a thermal [3,3]-sigmatropic rearrangement that turns an allyl vinyl ether into a new carbon-carbon bonded product.
- It is concerted, so the bonds change in one step through a cyclic transition state instead of separate intermediates.
- The chair-like transition state helps explain the stereochemistry you see in the product.
- Many products end up as cyclohexenones or related carbonyl compounds after tautomerization.
- In Organic Chemistry, this reaction is a classic example of how rearrangements build complexity without adding new atoms.

## FAQs

### What is Claisen rearrangement in Organic Chemistry?

It is a [3,3]-sigmatropic rearrangement where an allyl vinyl ether shifts its connectivity and forms a new carbon-carbon bond. The reaction is concerted and usually heated, which makes it a classic pericyclic transformation in Organic Chemistry. The final product often ends up as a cyclohexenone after tautomerization.

### How do you recognize a Claisen rearrangement problem?

Look for an allyl group attached to oxygen in an allyl vinyl ether. If the product shows a new C-C bond and a reorganized ring or carbonyl system, the reaction may be a Claisen rearrangement. The biggest clue is the allyl fragment moving from oxygen onto carbon in one step.

### Why does the Claisen rearrangement give a specific stereochemical outcome?

Because it passes through a cyclic, chair-like transition state. That geometry controls how the atoms line up while the bond migration happens, so the product arrangement is usually predictable. When you draw the mechanism, the 3D shape matters as much as the atom count.

### Is the product always a cyclohexenone?

No, but cyclohexenone is a very common final product when the substrate can form that ring and then tautomerize. The immediate rearrangement product may be an enol, which quickly shifts to the more stable carbonyl form. So on a problem set, make sure you draw the stable end product, not just the first shifted structure.

## Related Study Guides

- [30.8 Some Examples of Sigmatropic Rearrangements](/organic-chem/unit-30/examples-sigmatropic-rearrangements/study-guide/NCoA9OarcaWGKVPk)
- [6.1 Kinds of Organic Reactions](/organic-chem/unit-6/kinds-organic-reactions/study-guide/UeXVn5VzIi3LY9kU)

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