The reactions in this section all oxidize a carbon–carbon double bond, and they differ in how far they go. One adds a single oxygen, one adds two hydroxyls, and one cuts the molecule in half. Choosing between them is choosing how much of the alkene you want left.
Epoxidation: one oxygen, ring intact
mCPBA (meta-chloroperoxybenzoic acid) delivers a single oxygen atom across the double bond to give an epoxide. The reaction is concerted — the oxygen is transferred in one step through a cyclic transition state — so it is stereospecific: a cis alkene gives the cis-substituted epoxide and a trans alkene gives the trans one, cleanly.
Epoxides are useful precisely because they are strained and therefore reactive, which is the subject of their own section in the alcohols chapter. Here the point is what happens when you open one with water.
The two diols, and how to choose between them
A 1,2-diol (a glycol) can be made from an alkene in two ways, and they give opposite stereochemistry. This is the most examinable fact in the section.
- OsO₄ (often with NMO as a co-oxidant to make it catalytic) adds both oxygens to the same face through a cyclic osmate ester. The result is a syn (cis) diol. Cold, dilute, basic KMnO₄ does the same job more cheaply and less cleanly.
- mCPBA, then aqueous acid makes the epoxide first and then opens it. The nucleophile attacks the protonated epoxide from the face opposite the C–O bond being broken, so the two oxygens end up on opposite faces: an anti (trans) diol.
Ozonolysis: cutting the double bond in half
Ozone (O₃) cleaves a C=C completely, breaking both the σ and the π bond and leaving two separate carbonyl fragments. The workup then decides what those fragments are:
- Reductive workup — dimethyl sulfide (Me₂S) or zinc in acetic acid. Stops at the carbonyls: a carbon that bore two substituents becomes a ketone, and one that bore a hydrogen becomes an aldehyde.
- Oxidative workup — hydrogen peroxide. Anything that would have been an aldehyde is oxidized on to a carboxylic acid. Ketones, having no hydrogen to lose, are unaffected.
Hot, concentrated, acidic KMnO₄ does the same cleavage with the oxidative outcome, and is sometimes written in place of ozonolysis with peroxide workup.
An unknown alkene C₆H₁₂ is ozonolyzed with Me₂S workup, giving propanal and propanone (acetone).
Rejoin at the carbonyl carbons. Propanal is CH₃CH₂CHO, so that carbon carried one hydrogen and an ethyl group. Propanone is (CH₃)₂CO, so that carbon carried two methyls.
Rebuild the double bond between those two carbons: CH₃CH₂–CH=C(CH₃)₂, which is 2-methylpent-2-ene. Check the formula: C₆H₁₂. ✓
The tell in the question is that one fragment is an aldehyde and one a ketone, which immediately says the original alkene was trisubstituted — one carbon bore a hydrogen, the other did not.
The whole section as one decision
| Reagent | What happens to the C=C | Product |
|---|---|---|
| mCPBA | One O added, ring kept | Epoxide (stereospecific) |
| OsO₄ / NMO | Two OH, same face | syn diol |
| mCPBA then H₃O⁺ | Two OH, opposite faces | anti diol |
| O₃ then Me₂S | Cleaved | Aldehydes and/or ketones |
| O₃ then H₂O₂ | Cleaved | Carboxylic acids and/or ketones |
| KMnO₄, hot/conc. | Cleaved | Carboxylic acids and/or ketones |
| KMnO₄, cold/dilute/basic | Two OH, same face | syn diol |
KMnO₄ appears twice on purpose, and the conditions are doing all the work: cold and dilute it stops at the diol, hot and concentrated it cleaves right through. A question that specifies KMnO₄ without conditions is incomplete, and one that specifies them is telling you which row to read.
What carries forward
Two threads. The syn/anti diol pair is a stereochemical choice you make by reagent, which is the same kind of control as Lindlar against dissolving metal in the last section — and both are the sort of thing a synthesis question is built around. And ozonolysis run backwards is a structure-determination technique, which puts it alongside the spectroscopy chapter rather than only alongside the reactions: given fragments, find the parent.