In the case of a primary or secondary alcohol, the orange solution turns green. With a tertiary alcohol there is no colour change.
|Orange Acidified Potassium Dichromate|
|Green Chromium ions|
Primary alcohols can be oxidised to either aldehydes or carboxylic acids depending on the reaction conditions. In the case of the formation of carboxylic acids, the alcohol is first oxidised to an aldehyde which is then oxidised further to the acid.
Partial oxidation to aldehydes
You get an aldehyde if you use an excess of the alcohol, and distil off the aldehyde as soon as it forms. The excess of the alcohol means that there isn't enough oxidising agent present to carry out the second stage.
Ethanol a primary alcohol would produce the aldehyde ethanal, CH3CHO.
Full oxidation to carboxylic acids
You need to use an excess of the oxidising agent and make sure that the aldehyde formed as the half-way product stays in the mixture. The alcohol is heated under reflux with an excess of the oxidising agent. When the reaction is complete, the carboxylic acid is distilled off.
The full equation for the oxidation of ethanol to ethanoic acid is:
Secondary alcohols are oxidised to ketones
Heating the secondary alcohol propan-2-ol with acidified potassium dichromate(VI) solution produces propanone.
Tertiary alcohols are not oxidised by acidified potassium dichromate(VI) solution. There is no reaction.
|Dr. Paul Bowyer chatting to students about the oxidation of alcohols.|
|Paul looking at a students result.|