Name reaction · mechanism

Ozonolysis

Ozonolysis cleaves a carbon–carbon double bond into two carbonyl groups. Ozone adds across the C=C (1,3-dipolar cycloaddition) to give the primary ozonide, which breaks into a carbonyl compound and a carbonyl oxide; these recombine to the secondary ozonide, which is reduced (Me₂S, Zn/H₂O) to aldehydes and ketones.

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Ozonolysis mechanism: Ozone adds to but-2-ene (molozonide), which splits to acetaldehyde and a carbonyl oxide; they recombine to the ozonide, and Me₂S reduces it to two acetaldehydes and DMSO. — Ozonolysis: overall reaction, every curly-arrow step and key points. Click to open and edit it in MolDraw.

Overall reaction

CH₃CH=CHCH₃ → (1. O₃, −78 °C; 2. Me₂S or Zn/H₂O) 2 CH₃CHO

Ozone cleaves C=C to two carbonyl groups (Criegee mechanism).

Mechanism step by step

[3+2] Cycloaddition. The primary ozonide (molozonide)
Retro-[3+2]. Carbonyl + carbonyl oxide (Criegee intermediate)
Recombination. The secondary ozonide (1,2,4-trioxolane)
Me₂S attacks. The O–O bond breaks
First aldehyde. The alkoxide re-forms C=O
Second aldehyde. DMSO leaves

Each step is checked electron by electron: the curly arrows give exactly the structures drawn.

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Key points

Reductive work-up (Me₂S or Zn/AcOH) gives aldehydes and ketones.
Oxidative work-up (H₂O₂) turns aldehydes into carboxylic acids.
NaBH₄ work-up gives alcohols.
Locating C=C bonds: the fragments show where the double bond was.

For JEE and NEET

Use the products to locate the C=C: join the two C=O carbons with a double bond.
Reductive work-up keeps aldehydes; oxidative work-up turns them into acids.
A terminal =CH₂ gives formaldehyde.
Cyclic alkenes give one dicarbonyl compound.

FAQ

What is the difference between reductive and oxidative work-up?

Reductive (Zn/H₂O, Me₂S) gives aldehydes and ketones; oxidative (H₂O₂) oxidises any aldehyde to a carboxylic acid.

What is an ozonide?

The 1,2,4-trioxolane formed when the carbonyl oxide and carbonyl fragments recombine; it is explosive and is not isolated.

What does but-2-ene give?

Two molecules of acetaldehyde (ethanal) with a reductive work-up.

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