Lesson 3 of 13 · 5 min
Aromatic aldehydes and ketones
NCERT §8.2.2–8.2.3
Benzaldehyde, the almond-smelling flavour, is on Tara's stock list. A junior chemist suggests making it by boiling toluene with KMnO₄. The senior chemist shakes her head.
The lesson in notes
In short
Strong oxidising agents take toluene all the way to benzoic acid. To stop at benzaldehyde the methyl group is first turned into an intermediate that resists further oxidation.
Etard reaction: chromyl chloride, CrO₂Cl₂, converts the methyl group of toluene into a chromium complex, and hydrolysis of the complex gives benzaldehyde.
With chromic oxide, CrO₃, in acetic anhydride, toluene gives benzylidene diacetate, which aqueous acid hydrolyses to benzaldehyde.
Side-chain chlorination of toluene gives benzal chloride, C₆H₅CHCl₂, and hydrolysing it gives benzaldehyde. This is a commercial method for benzaldehyde.
Gatterman-Koch reaction: benzene or a derivative treated with carbon monoxide and hydrogen chloride, with anhydrous aluminium chloride or cuprous chloride, gives benzaldehyde or a substituted benzaldehyde.
Ketones from acyl chlorides: a Grignard reagent and cadmium chloride give a dialkylcadmium, R₂Cd, which turns an acyl chloride into a ketone.
Ketones from nitriles: a nitrile treated with a Grignard reagent and then hydrolysed gives a ketone.
Friedel-Crafts acylation: benzene or a substituted benzene with an acid chloride and anhydrous AlCl₃ gives the aromatic ketone, for example acetophenone from benzene and ethanoyl chloride.