Preparation of aldehydes and ketones

The essentials

Aldehydes and ketones can be made by oxidising or dehydrogenating alcohols, by ozonolysis of alkenes, by hydration of alkynes and by several named reactions that start from acid chlorides, nitriles, esters or aromatic hydrocarbons. The key challenge in making an aldehyde is to stop the reaction before the aldehyde is oxidised further to a carboxylic acid, or to reduce an acid derivative only as far as the aldehyde.

Key points

• Oxidation of alcohols: primary alcohols give aldehydes, secondary alcohols give ketones. Pyridinium chlorochromate (PCC) stops at the aldehyde.
• Dehydrogenation: passing alcohol vapour over heated copper or silver gives aldehydes (from 1° alcohols) or ketones (from 2° alcohols).
• Ozonolysis of alkenes followed by Zn and water gives aldehydes, ketones or both.
• Hydration of ethyne with H₂SO₄ and HgSO₄ gives ethanal; other alkynes give ketones.
• Rosenmund reduction: acyl chloride + H₂ over Pd on BaSO₄ (partly poisoned) → aldehyde.
• Stephen reaction: nitrile + SnCl₂/HCl gives an imine, which is hydrolysed to an aldehyde.
• DIBAL-H reduces nitriles and esters to aldehydes.
• Etard reaction: toluene + chromyl chloride (CrO₂Cl₂) → chromium complex → benzaldehyde on hydrolysis. Toluene can also be converted via benzal chloride (side-chain chlorination then hydrolysis).
• Gattermann–Koch reaction: benzene + CO + HCl with anhydrous AlCl₃ or CuCl → benzaldehyde.
• Ketones: acyl chloride + dialkylcadmium (R₂Cd); nitrile + Grignard reagent then hydrolysis; Friedel–Crafts acylation of benzene with RCOCl and anhydrous AlCl₃.

Exam tips

• Match each named reaction to its reagent; questions often give the reagent and ask for the product.
• Grignard reagents react with acid chlorides beyond the ketone stage, which is why dialkylcadmium is used.

Common mistakes

• Using strong oxidants such as acidified KMnO₄ to make aldehydes; they continue to the carboxylic acid.