Ozonolysis of Alkenes

Quick answer Ozone (O₃) cleaves a C=C in half, turning each alkene carbon into a carbonyl. Reductive workup (Zn/H₂O or DMS) gives aldehydes and ketones; oxidative workup (H₂O₂) pushes aldehydes on to carboxylic acids.

⚙ Mechanism · Ozonolysis4 steps

Step 1 — ozone adds across the double bond.

A 1,3-dipolar cycloaddition: three arrows round a ring, all at once. Note the connectivity — the two carbons are still bonded to each other, and the three oxygens sit in a row. This is the molozonide, and it is not the ozonide.

Step 2 — the molozonide falls apart.

A retro-cycloaddition snaps the ring at the C–C bond. That single event is what cleaves the original double bond, and it leaves two fragments: a carbonyl oxide and an ordinary carbonyl compound.

Step 3 — the fragments recombine the other way round.

They add back together, but rotated — so this time an oxygen sits between the two carbons. Compare the ring with step 1: same five atoms, different order. That is the difference between a molozonide and an ozonide.

Step 4 — reductive workup cleaves the ozonide.

Zinc (or Me₂S) cleaves the weak O–O bonds and delivers two carbonyl compounds. A reductive workup stops at the aldehyde; an oxidative one (H₂O₂) carries on to the carboxylic acid.

Ozonolysis doesn't add across the C=C — it severs it.

The whole idea in one line: cut the C=C of 2-butene and both halves come out as acetaldehyde.

1. Ozonolysis Cleaves the C=C Completely and Hands You Two Carbonyls

Both σ and π bonds break, capping each cut end with oxygen — one C=C splits into two carbonyl pieces.

2-butene (one C=C)

Acetaldehyde

+ Acetaldehyde

2. It Runs in Two Steps: Ozone First, Then a Workup That Sets the Products

Step 1 is O₃ forming an unstable ozonide; step 2 is the workup, whose reagent decides which carbonyls you actually isolate.

3. Reductive Workup Gives Aldehydes and Ketones; Oxidative Workup Pushes Aldehydes to Acids

Reductive — Zn/H₂O (or DMS): H-bearing carbons give aldehydes, fully substituted carbons give ketones.
Oxidative — H₂O₂: aldehydes are pushed on to carboxylic acids; ketones are untouched.

So a =CHCH₃ end gives acetaldehyde reductively but acetic acid oxidatively:

Reductive: acetaldehyde

Oxidative: acetic acid

4. Each Alkene Carbon Maps to Exactly One Carbonyl

Read each C=C carbon on its own by how many groups it carries:

=CH₂ → formaldehyde (lost as CO₂/formic acid oxidatively).
=CHR → aldehyde reductively, carboxylic acid oxidatively.
=CR₂ → ketone either way.

=CH₂ → formaldehyde

=CHR → aldehyde

=CR₂ → ketone

5. An Unsymmetrical Alkene Gives Two Different Carbonyls

Apply the map to each end separately: 2-methyl-2-butene gives acetone from its =CR₂ end and acetaldehyde from its =CHR end.

2-methyl-2-butene

Acetone (=CR₂ end)

Acetaldehyde (=CHR end)

6. A Double Bond Inside a Ring Gives One Molecule With Two Carbonyls

A ring C=C doesn't split the molecule — it opens the ring, leaving both carbonyls on one chain; cyclohexene opens to hexanedial.

One ring, one C=C, one product: cyclohexene unzips to hexanedial (OHC-(CH₂)₄-CHO).

7. Run the Logic Backward to Find the Starting Alkene

To find an unknown alkene, reverse the map: line up the two carbonyl carbons, delete both oxygens, and join them with a C=C — two acetones recover 2,3-dimethyl-2-butene.

Acetone

+ Acetone

→ 2,3-dimethyl-2-butene

8. Summary

Each C=C carbon → a carbonyl · Step 1 O₃, step 2 workup · Zn/H₂O or DMS → aldehydes + ketones · H₂O₂ → acids · =CHR → aldehyde, =CR₂ → ketone · ring opens to one difunctional product · reverse the map to find the alkene.

Worked example

Problem. What do you get from but-2-ene with 1) O₃ 2) Zn, H₂O?

Ozonolysis cleaves the C=C entirely, splitting the molecule at the double bond.
Each alkene carbon becomes a carbonyl carbon.
The reductive workup (Zn) stops at aldehydes/ketones (oxidative workup would give acids).

Answer. Two equivalents of acetaldehyde (ethanal).

How each reagent works — the arrow pushing

Electron flow only. Follow the arrows; the structures do the talking.

O₃; then Zn / Me₂S — ozonolysis2 steps

Why it works · Ozone is a 1,3-dipole — electron-poor at the central O (δ+), so it reacts fast with the electron-rich alkene. It adds and rearranges to a five-membered ozonide, which completely breaks the C=C. A mild reductant (Zn/AcOH or Me₂S) then cleaves the ozonide to two carbonyls (aldehydes/ketones); an oxidative workup (H₂O₂) would instead give acids.

Quiz yourself

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Two molecules of acetaldehyde (CH₃CHO). The alkene is symmetrical, so both cut ends are identical =CHCH₃ groups, and reductive workup stops each at the aldehyde.

2-methyl-2-butene, (CH₃)₂C=CHCH₃. Join the two carbonyl carbons (the acetone C and the acetaldehyde C), delete the oxygens, and connect them with a double bond.

Under reductive workup (Zn/H₂O or DMS) it becomes an aldehyde; under oxidative workup (H₂O₂) that aldehyde is oxidized further to a carboxylic acid.

The double bond is inside the ring, so cleaving it just opens the ring rather than splitting the molecule. Both new carbonyls stay connected on one chain, giving hexanedial.

Draw this on the whiteboard

Open the OChem Board whiteboard — benzene rings, curved arrows, wedge/dash bonds and a clickable periodic table built in. No account needed.

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