Oxidation of Aldehydes

Aldehydes are easily oxidised to carboxylic acids. The usual oxidising agent is acidified potassium dichromate(VI), K₂Cr₂O₇ with dilute sulfuric acid, and the mixture is heated under reflux so that oxidation is complete.In equations the oxidising agent is written as [O]:RCHO + [O] → RCOOH
CH₃CHO + [O] → CH₃COOHAs the aldehyde is oxidised, the dichromate(VI) ions are reduced to chromium(III) ions, so the colour changes from orange to green.Ketones are not oxidised by acidified dichromate. In an aldehyde the carbonyl carbon has a hydrogen atom that can be replaced by OH. In a ketone the carbonyl carbon has no hydrogen atom, and oxidation would mean breaking a strong C-C bond. A ketone therefore leaves the dichromate orange.This difference lets you tell aldehydes and ketones apart: warm the compound with acidified potassium dichromate(VI). Orange to green means an aldehyde; staying orange means a ketone (tertiary alcohols also give no change, so you must know the compound is a carbonyl).Worked example: 2.90 g of propanal (M_r = 58.0) is fully oxidised. Moles = 2.90 ÷ 58.0 = 0.0500 mol. CH₃CH₂CHO + [O] → CH₃CH₂COOH is 1 : 1, so 0.0500 mol of propanoic acid (M_r = 74.0) forms, a maximum mass of 0.0500 × 74.0 = 3.70 g.