Ketones are most commonly prepared by the oxidation of secondary alcohols. Many Cr6+ reagents will acheive this transformation, including pyridinium chlorochromate (PCC) or chromic acid (H2CrO4). Chromic acid is unstable and so must be made in the reaction by a combination of aqueous sulfuric acid with a Cr6+ source such as chromium trioxide (CrO3), sodium chromate (Na2CrO4) or potassium dichromate (K2Cr2O7). Primary alcohols and aldehydes may be oxidized at the same time, while tertiary alchols are inert to oxidation to a carbonyl compound (Section 8.4F).
Aryl ketones can be prepared from the reaction of an acid chloride and an aromatic compound by Freidel-Crafts acylation. This is an electrophilic aromatic substitution reaction, so substitutients on the ring determine the position of the new ketone group. The reaction fails if the aromatic ring has a strong electron-withdrawing (deactivating) substituent, such as a nitro group. Lewis acid catalysis is required; AlCl3 is usually used (Section 9.7C).
Decarboxylation (loss of CO2) from a b-ketocarboxylic acid yields a ketone. The reaction proceeds via the enol form of the ketone product (Sections 12.8A and 14.3D).
The synthesis of b-ketoesters
(Claisen reaction) and b-hydroxyketones
(aldol reaction) are discussed separately.