Oxidative cleavage
Ozonolysis: cleaving an alkene to two carbonyls
How the Ozonolysis 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: 1) O₃ 2) Me₂S or Zn/AcOH (reductive workup). Conditions: CH₂Cl₂ or MeOH, –78 °C. Bonds formed: two C=O; bonds broken: C=C σ and π; O–O. Stereochemistry: Alkene geometry is lost. Rate and selectivity: Fast, even at –78 °C; ozonide is explosive, never isolated.
At a glance
Step 1: [3+2] cycloaddition of O₃
Ozone's terminal oxygen adds across the alkene in a concerted [3+2] cycloaddition, forming a five-membered ring called a molozonide.
Step 2: Retro-[3+2] and recombination
The strained molozonide fragments and recombines — a retro-[3+2] followed by a new [3+2] — into the more stable isomer, the ozonide.
Step 3: Reductive workup
Reductive workup (Zn/AcOH or dimethyl sulfide) cleaves the ozonide's O–O bonds, turning each half of the original double bond into a separate carbonyl.
Abbreviations and terms
- DMSO
- Dimethyl sulfoxide. Polar aprotic solvent; also the oxidant in Swern/Kornblum-type reactions.
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- MeOH
- Methanol. Polar protic solvent (also a nucleophile in solvolysis).
- AcOH
- Acetic acid. Weak acid and polar protic solvent.
How it compares with related reactions
| Ozonolysis | Lemieux-Johnson oxidation | KMnO₄ oxidation of an alkene | |
|---|---|---|---|
| Reagents | 1) O₃ 2) Me₂S or Zn/AcOH (reductive workup) | Alkene, OsO₄ (cat., 1–5 mol%), NaIO₄ (≥ 4 equiv.), 2,6-lutidine (optional) | cold dilute KMnO₄, OH⁻ (diol); hot conc. KMnO₄ (cleavage) |
| Conditions | CH₂Cl₂ or MeOH, –78 °C | Dioxane/H₂O or THF/H₂O, 25 °C | 0–5 °C, basic, vs. heat, acid |
| Stereochemistry | Alkene geometry is lost | Alkene geometry lost; α-stereocentres retained | Syn addition (cis diol) in cold conditions |
| Rate / selectivity | Fast, even at –78 °C; ozonide is explosive, never isolated | Osmylation is fast; NaIO₄ turns over Os(VI) → Os(VIII) | Cycloaddition fast; Mn(VII) → Mn(V) → MnO₂ |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Ozonolysis?
1) O₃ 2) Me₂S or Zn/AcOH (reductive workup). Conditions: CH₂Cl₂ or MeOH, –78 °C.
Which bonds are formed and which are broken?
Formed: two C=O. Broken: C=C σ and π; O–O.
What is the stereochemical outcome?
Alkene geometry is lost
What controls the rate or the selectivity?
Fast, even at –78 °C; ozonide is explosive, never isolated
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go