Oxidation of Aldehydes

The key difference between aldehydes and ketones is that aldehydes are easily oxidised and ketones are not.An aldehyde has a hydrogen atom on its carbonyl carbon. An oxidising agent converts this C-H into C-OH, turning the aldehyde into a carboxylic acid. A ketone has no H on its carbonyl carbon; to oxidise it, a C-C bond would have to break, which does not happen with these oxidising agents.Reagent and conditions: acidified potassium dichromate(VI), K₂Cr₂O₇ with dilute sulfuric acid, heated under reflux. Refluxing lets the reaction go to completion without the volatile aldehyde escaping.Observation: the orange dichromate(VI) ions, Cr₂O₇²⁻, are reduced to green chromium(III) ions, Cr³⁺. With a ketone, the solution stays orange.Equations use [O] to represent the oxidising agent:
RCHO + [O] → RCOOH
Worked example: the oxidation of propanal:
CH₃CH₂CHO + [O] → CH₃CH₂COOH
The product is propanoic acid. To write the equation, add one O atom to the aldehyde group, so CHO becomes COOH. The number of carbon atoms stays the same.Link to alcohols: oxidising a primary alcohol first gives an aldehyde. If the aldehyde is distilled off as it forms, it can be collected. If the mixture is heated under reflux with excess oxidising agent, the aldehyde is oxidised further to the carboxylic acid.Because aldehydes are oxidised so easily, milder oxidising agents also react with them but not with ketones. These are used as tests to tell aldehydes and ketones apart.