Preparing aldehydes, ketones and acids

Making aldehydes and ketones.

• Oxidising alcohols: a primary alcohol gives an aldehyde with a mild reagent such as PCC; a secondary alcohol gives a ketone (with CrO₃, for example). Passing the vapour over hot copper at 573 K does the same job by removing hydrogen.
• Rosenmund reduction: an acid chloride with H₂ over Pd on BaSO₄ gives an aldehyde. The H replaces the Cl. The BaSO₄ 'poisons' the catalyst so that the aldehyde is not reduced further.
• Stephen reaction: a nitrile is reduced with SnCl₂ and HCl to an imine, which water then turns into an aldehyde. (Memory aid: the S of Stephen for SnCl₂, the h for HCl.) DIBAL-H (diisobutylaluminium hydride) does the same job: R–CN gives R–CHO after adding water.
• Etard reaction: toluene with chromyl chloride, CrO₂Cl₂, then water, gives benzaldehyde: the CH₃ becomes CHO. Side-chain chlorination of toluene followed by hydrolysis is another route.
• Gattermann-Koch reaction: benzene with CO and HCl, with anhydrous AlCl₃, gives benzaldehyde.

• Ketones from acid chlorides: a dialkylcadmium (R₂Cd) swaps its R group for the Cl: CH₃COCl gives a methyl ketone.
• Ketones from nitriles: a Grignard reagent adds to the C≡N. After hydrolysis, the R group from the Grignard is on the carbon, and the N has been replaced by O: CH₃CN with C₂H₅MgBr gives butan-2-one.
• Friedel-Crafts acylation: benzene with an acid chloride and anhydrous AlCl₃ gives an aromatic ketone such as acetophenone.
• Hydration of alkynes: with dilute H₂SO₄ and Hg²⁺, ethyne gives ethanal and other alkynes give ketones.

Making carboxylic acids.

• Oxidising primary alcohols or aldehydes with a strong oxidising agent (KMnO₄, K₂Cr₂O₇).
• Oxidising a side chain on a benzene ring: with hot alkaline KMnO₄, a whole side chain, however long, becomes –COOH: toluene and ethylbenzene both give benzoic acid.
• Hydrolysing nitriles (R–CN) or amides with acid or alkali.
• Grignard reagent + CO₂ (dry ice), then acid: R–MgX gives R–COOH.

The nitrile route and the Grignard + CO₂ route both add one carbon to the chain.