ozonolysis
/ oh-ZON-oh-ly-sis /
Ozonolysis is a reaction that snips a carbon-carbon double bond clean in two, replacing the C=C with two separate C=O (carbonyl) groups. Where there was one molecule with a double bond in the middle, you end up with two smaller pieces, each capped with an oxygen. The reagent that does the cutting is ozone, O3, followed by a workup that determines what the cut ends become.
Mechanistically, ozone adds across the double bond and, through a short rearrangement, forms an unstable cyclic species called an ozonide. This ozonide is then broken open in the workup. The choice of workup decides the oxidation level of the products: a reductive workup (zinc and acetic acid, or dimethyl sulfide) stops at aldehydes and ketones, while an oxidative workup (hydrogen peroxide) pushes any aldehydes on to carboxylic acids. A carbon that bore a hydrogen becomes an aldehyde (or acid); a carbon bearing two carbon groups becomes a ketone.
Ozonolysis matters as both a synthesis tool and a structure-detective tool. Synthetically it is a reliable way to convert an alkene into two carbonyl compounds. Diagnostically it is a classic trick from before modern spectroscopy: by cleaving an unknown alkene and identifying the carbonyl fragments, you can work backward to pinpoint exactly where the double bond was in the original molecule, because each fragment marks one carbon of the former C=C.
Ozonolysis of 2-methyl-2-butene ((CH3)2C=CH-CH3) with a reductive workup gives two carbonyls: acetone ((CH3)2C=O) from the disubstituted carbon and acetaldehyde (CH3-CHO) from the carbon that bore a hydrogen.
The C=C is cut in two; each former alkene carbon becomes a carbonyl.
The workup is not a footnote: the same ozonide gives aldehydes/ketones with a reductive workup but acids/ketones with an oxidative one, so always state which workup you mean.