Ozonolysis and Oxidation of Alkenes

Ozonolysis. Passing ozone gas into a solution of an alkene forms an unstable cyclic

intermediate called an ozonide, which is not usually isolated but is immediately decomposed by a

reductive work-up -- classically zinc dust and water, or dimethyl sulfide -- to cleave the

original carbon-carbon double bond completely and give two separate carbonyl compounds. Each

double-bond carbon becomes the carbon of a new carbonyl group: a carbon that carried two alkyl (or

one alkyl and one H) groups becomes a ketone or an aldehyde carbon respectively, while a carbon

that carried two hydrogens (a terminal $\text{=CH}_2$) becomes formaldehyde, $\text{HCHO}$. For

example, 2-methylbut-2-ene ozonolyses to give acetone, $(\text{CH}_3)_2\text{C=O}$, and

acetaldehyde, $\text{CH}_3\text{CHO}$. Because the reductive work-up prevents over-oxidation of

any aldehyde formed to a carboxylic acid, ozonolysis products can be read backwards, unambiguously,

to deduce exactly which carbons were joined by the original double bond -- an important tool for

determining the structure of an unknown alkene before spectroscopic methods became routine.

Oxidation with cold, dilute alkaline $\text{KMnO}_4$ (Baeyer's reagent). A cold, dilute,

alkaline solution of potassium permanganate adds two $-\text{OH}$ groups across the double bond,

one to each carbon, on the same face of the alkene (syn addition), converting it to a vicinal diol

(a 1,2-diol): $\text{RCH=CHR}' + [\text{O}] + \text{H}_2\text{O} \rightarrow \text{RCH(OH)CH(OH)R}'$. The reaction is accompanied by the rapid disappearance of

$\text{KMnO}_4$'s characteristic purple/pink colour, so it is used as a simple, quick chemical

test -- Baeyer's test -- to distinguish an alkene (or alkyne) from an alkane, which does not

decolourise the reagent under these mild conditions.

Oxidation with hot, concentrated $\text{KMnO}_4$. Under harsher, hot and concentrated

conditions, the same reagent goes further and oxidatively cleaves the double bond completely,

much like ozonolysis but with a different outcome depending on substitution: a $\text{=CH}_2$ or

$\text{=CHR}$ end is oxidised all the way to carbon dioxide (for a terminal $\text{=CH}_2$) or to

a carboxylic acid, $\text{RCOOH}$ (for a $\text{=CHR}$ end), whereas a fully substituted …