Ring contraction
Favorskii rearrangement
How the Favorskii rearrangement works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: α-Halo ketone with an α′-H; NaOH, NaOMe or amine. Conditions: MeOH / H₂O, 0–60 °C. Bonds formed: cyclopropanone; ring-contracted acid / ester; bonds broken: α′ C–H; C–Cl; ring C–C. Stereochemistry: Ring contraction by one carbon. Rate and selectivity: Enolate formation and cyclisation are fast; ring opening follows.
At a glance
Step 1: Enolate closes a cyclopropanone
Base deprotonates the carbon opposite an α-halo ketone's halogen; the resulting enolate displaces the halide intramolecularly, closing a strained cyclopropanone ring.
Step 2: Hydroxide adds to the strained C=O
A nucleophile (like hydroxide or an alkoxide) attacks the strained, highly electrophilic cyclopropanone carbonyl carbon.
Step 3: Ring opens: ring-contracted acid
The ring opens at the weaker of its two C–C bonds, relieving strain and giving a ring-contracted carboxylic acid or ester — one carbon smaller than the starting ketone's ring.
Abbreviations and terms
- MeOH
- Methanol. Polar protic solvent (also a nucleophile in solvolysis).
- NaOMe
- Sodium methoxide. Alkoxide base matched to methyl esters.
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
- SN2
- Bimolecular nucleophilic substitution. One concerted step with backside attack and inversion; rate = k[RX][Nu].
How it compares with related reactions
| Favorskii rearrangement | Acetic acid transfers a proton to water | Acid-catalyzed amide hydrolysis | |
|---|---|---|---|
| Reagents | α-Halo ketone with an α′-H; NaOH, NaOMe or amine | Acetic acid (pKa 4.76) + water | Amide, H₃O⁺ (HCl or H₂SO₄), H₂O |
| Conditions | MeOH / H₂O, 0–60 °C | Aqueous solution, 25 °C | 6 M acid, reflux (hours) |
| Stereochemistry | Ring contraction by one carbon | None (achiral species) | No stereocentre created; α-stereocentres retained |
| Rate / selectivity | Enolate formation and cyclisation are fast; ring opening follows | Equilibrium: Ka = 1.8 × 10⁻⁵, lies mostly left | Water attack / C–N cleavage rate-limiting; far slower than ester hydrolysis |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Favorskii rearrangement?
α-Halo ketone with an α′-H; NaOH, NaOMe or amine. Conditions: MeOH / H₂O, 0–60 °C.
Which bonds are formed and which are broken?
Formed: cyclopropanone; ring-contracted acid / ester. Broken: α′ C–H; C–Cl; ring C–C.
What is the stereochemical outcome?
Ring contraction by one carbon
What controls the rate or the selectivity?
Enolate formation and cyclisation are fast; ring opening follows
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go