Oxidation of Ethanol

Combustion of alcohols is an oxidation process! For organic compounds, only the functional group is oxidized. The carbon chain remains intact. Ethanol can be readily oxidized by the warm acidified aqueous solution of potassium dichromate (VI), K2Cr2O7 (aq), acidified using dilute sulfuric acid.

Wha’ happen? The product is ethanal, an aldehyde. The orange color of K2Cr2O7 (aq) changes to green, owing to the formation of Chromium (III) ions, Cr3+(aq), and the smell of ethanal (a little like apples ) can be detected. Equation:

We really only care about what happens to the alcohol, so we use an easier equation, using [O] to represent the oxygen supplied by the oxidizing agent.

That reaction can keep going, actually, as ethanal is readily oxidized to ethanoic acid: This is what happens when wine is left exposed to the air, and bacteria converts the ethanol into vinegar and the wine goes “bad”.

Back in the day… Police used this to detect the alcohol content in ones breath. Drivers were required to blow into a bag through a tube that contained crystals of potassium dichromate (VI) in an acidic medium. If the crystals turned green, they had been drinking and further blood testing was done!

Getting tested….

Obtaining a better yield Both ethanol and ethanoic acid contain a hydrogen atom bonded directly to an electronegative oxygen atom. This means what? Hydrogen bond!! However, ethanal does not contain a hydrogen atom bonded to the oxygen atom. Therefore, the boiling point of ethanal will be much lower then that of either ethanol or ethanoic acid.

By arranging the apparatus so that the lower boiling ethanal can be distilled from the mixture, a good yield of ethanal can be obtained. If ethanoic acid is required, the apparatus can be arranged for reflux so no ethanal can escape, ensuring it will be further oxidized to the carboxylic acid: