Alcohols can be oxidised by warming them with acidified potassium dichromate(VI), K2Cr2O7 in dilute sulfuric acid. In equations the oxidising agent is written as [O]. As the alcohol is oxidised, the orange dichromate(VI) ions, Cr2O72-, are reduced to green chromium(III) ions, Cr3+.Primary alcohols are oxidised first to an aldehyde and then to a carboxylic acid. To obtain the aldehyde, use excess alcohol and distil the aldehyde off as soon as it forms, so it cannot be oxidised further.CH3CH2OH + [O] → CH3CHO + H2O To obtain the carboxylic acid, use excess oxidising agent and heat under reflux. The vapour condenses and drips back into the flask, so the aldehyde stays in the mixture until it is fully oxidised.
CH3CH2OH + 2[O] → CH3COOH + H2O Secondary alcohols are oxidised to ketones. Ketones are not oxidised further, so the mixture is heated under reflux.CH3CH(OH)CH3 + [O] → CH3COCH3 + H2O Tertiary alcohols are not oxidised by acidified dichromate(VI), because the carbon atom bonded to the OH group has no hydrogen atom attached to it. The solution stays orange.The colour change can therefore be used to tell tertiary alcohols apart from primary and secondary alcohols.