Ozonolysis of Alkenes (Oxidative Workup)
Ozonolysis of Alkenes (Oxidative Workup): Ozonolysis cuts a C=C double bond into two separate carbonyl-containing products.
What happens
Ozonolysis cuts a C=C double bond into two separate carbonyl-containing products. First the alkene reacts with ozone (O₃); then an oxidative workup, commonly H₂O₂, is used. Cut the C=C in half and turn each alkene carbon into a carbonyl carbon: the carbon with no hydrogen attached becomes a ketone, and the carbon that does have a hydrogen attached is oxidized further, during the workup, into a carboxylic acid.
Reaction summary
Ethylidenecyclohexane + O₃, then H₂O₂ → Cyclohexanone + Acetic acid
Reagents and conditions
- Starting material: Ethylidenecyclohexane (alkene)
- Reagents: O₃; H₂O₂
- Conditions: 1. O₃; 2. H₂O₂ (oxidative workup)
- Products: Cyclohexanone (ketone); Acetic acid (carboxylic acid)
Mechanism
- Ozone Addition: Ozone reacts with the alkene's C=C double bond in a concerted cycloaddition, forming a five-membered ring (the molozonide, which rapidly rearranges to the ozonide).
- Ozonide Intermediate: The addition forms a five-membered ozonide ring bridging both former alkene carbons, each still carrying its original substituents.
- Oxidative Workup: H₂O₂ breaks the ozonide apart into the final products: the carbon that had no hydrogen becomes a ketone; the carbon that had a hydrogen is oxidized all the way to a carboxylic acid.
Common mistakes
- Do not confuse oxidative ozonolysis with reductive ozonolysis. With a reductive workup, an alkene carbon bearing H can survive as an aldehyde; with an oxidative workup (as here), that aldehyde is further oxidized all the way to a carboxylic acid.
Memory trick
O₃ = scissors for double bonds ✂️. Then ask each carbon: do you have an H? Yes → carboxylic acid. No → ketone.