Oxidation of Primary Alcohols to Aldehydes and Carboxylic Acids

A primary alcohol is oxidised by acidified potassium dichromate(VI) in two stages: first to an aldehyde, then to a carboxylic acid. The product you get depends on the conditions.Making the aldehyde: distil it off as it forms
Use an excess of the alcohol and gently warm it with the oxidising agent. Aldehydes have lower boiling points than their alcohols, because they cannot form hydrogen bonds with each other. The aldehyde vaporises and is distilled off immediately as it forms, before it can be oxidised further.CH₃CH₂OH + [O] → CH₃CHO + H₂Oethanol → ethanalMaking the carboxylic acid: heat under reflux
Use an excess of the oxidising agent and heat under reflux. The aldehyde that forms cannot escape, so it is oxidised further to the carboxylic acid:CH₃CHO + [O] → CH₃COOH
Overall, using 2[O]:
CH₃CH₂OH + 2[O] → CH₃COOH + H₂Oethanol → ethanoic acidWorked example: give the products of oxidising propan-1-ol under each set of conditions.
Distilling off the product as it forms gives propanal, CH₃CH₂CHO.
Heating under reflux with excess oxidising agent gives propanoic acid, CH₃CH₂COOH:
CH₃CH₂CH₂OH + 2[O] → CH₃CH₂COOH + H₂OIn both cases the orange dichromate(VI) turns green.