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. 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:
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.
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.
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.
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
- 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).
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.