Oxidation, reduction and tests

Reduction.

• NaBH₄ or LiAlH₄ (or H₂ with a catalyst) reduce aldehydes to primary alcohols and ketones to secondary alcohols.
• Two reactions remove the oxygen completely, turning C=O into CH₂: the Clemmensen reduction (zinc amalgam and concentrated HCl) and the Wolff-Kishner reduction (hydrazine, then KOH in ethylene glycol, heated).

Oxidation. This is where aldehydes and ketones differ most. An aldehyde has an H on its carbonyl carbon, so it is easily oxidised to an acid, even by mild reagents. Ketones are not, except by strong oxidising agents that break the carbon chain. Mild reagents therefore make good tests for aldehydes:

• Tollens' test: warm with ammoniacal silver nitrate. An aldehyde reduces the silver ions to metal, which coats the tube as a silver mirror.
• Fehling's test: warm with Fehling's solution (alkaline copper(II) tartrate, deep blue). Aliphatic aldehydes give a red-brown precipitate of Cu₂O. Aromatic aldehydes do not give this test.

The iodoform test. Warm with iodine and sodium hydroxide. Compounds with a CH₃CO– group (methyl ketones, and any aldehyde with that group) give a yellow precipitate of iodoform, CHI₃. Alcohols with a CH₃CH(OH)– group give it too, because the reagent first oxidises them to CH₃CO– compounds.

Acids and NaHCO₃. Carboxylic acids are strong enough to release CO₂ (fizzing) from sodium hydrogencarbonate. Phenols and esters are not, so this test tells an acid apart from them.

To identify an unknown, combine tests. A compound that gives an orange 2,4-DNP precipitate has a C=O group; if it gives a silver mirror it is an aldehyde, if not it is a ketone; and a yellow iodoform precipitate shows a CH₃CO– group. Acetophenone, for instance, is 2,4-DNP positive, Tollens' negative and iodoform positive.