Ozonolysis of Alkenes

A representative example — structures drawn live.

What ozonolysis does

Ozonolysis takes a carbon–carbon double bond and breaks it apart entirely, snapping both the σ and π bonds. Where the two carbons were joined by a double bond, you end up with two separate carbonyl groups (C=O). Think of it as taking scissors to the alkene: cut through the middle of the double bond, and cap each cut end with an oxygen.

The reaction runs in two operations. First, the alkene is treated with ozone (O₃), typically at low temperature, forming an unstable cyclic intermediate called an ozonide. Second, that ozonide is broken down in a workup step, and the choice of workup decides exactly which carbonyl products you isolate.

Reductive vs. oxidative workup

This is the detail most exam questions hinge on:

• Reductive workup — Zn/AcOH or dimethyl sulfide, (CH₃)₂S (DMS). Carbons that carried at least one hydrogen become aldehydes; fully substituted carbons become ketones. The reducing agent stops the products from being over-oxidized.
• Oxidative workup — H₂O₂. Any carbon that would have become an aldehyde is instead oxidized further to a carboxylic acid. Ketones are unaffected because they have no C–H bond on the carbonyl carbon to oxidize.

So the rule of thumb: a carbon with a hydrogen gives an aldehyde under reductive workup but a carboxylic acid under oxidative workup. A disubstituted carbon gives a ketone either way.

Mapping the double bond to the products

To predict products, look at each carbon of the C=C and count its non-double-bond substituents:

• A =CH₂ terminal carbon becomes formaldehyde (H₂C=O) reductively, or is lost as CO₂/formic acid oxidatively.
• A =CHR carbon (one H, one R) becomes an aldehyde reductively, a carboxylic acid oxidatively.
• A =CR₂ carbon (two R groups) becomes a ketone in both workups.

For example, ozonolysis of 2-methyl-2-butene [(CH₃)₂C=CHCH₃] with reductive workup gives acetone [(CH₃)₂C=O] from the disubstituted carbon and acetaldehyde [CH₃CHO] from the monosubstituted carbon.

Retrosynthesis: finding the original alkene

Ozonolysis is a favorite structure-determination and retrosynthesis tool. If you know the carbonyl products, you can reconstruct the starting alkene by doing the reverse operation: take the two carbonyl carbons and join them with a double bond, deleting both oxygens.

Suppose ozonolysis of an unknown gives two molecules of acetone. Line up the two carbonyl carbons, connect them with a C=C, and you recover 2,3-dimethyl-2-butene. If a single ring-containing molecule yields one product with two carbonyl groups (a dial or diketone), that tells you the double bond was inside a ring — cleaving it opens the ring rather than splitting the molecule into two pieces.

Common pitfalls

Watch for these traps. A ring alkene gives one difunctional product, not two molecules. Under oxidative workup, remember that any =CH₂ is destroyed (lost as CO₂/formic acid), so it won't show up as an isolable fragment. And ozonolysis cleaves only C=C double bonds — it does not touch alkynes the same way, single bonds, or most other functional groups, which makes it a clean, selective diagnostic for where the double bond was.