Oxidation of alcohols: Tertiary alcohols are not easily oxidised.

The idea

In a tertiary alcohol the carbon carrying the hydroxyl group is bonded to three alkyl groups and no hydrogen. Oxidation by acidified dichromate works by removing hydrogen from that carbon and adding oxygen to it; with no hydrogen available, the only route to a carbonyl would be to break a strong carbon–carbon bond, which this reagent cannot do. So nothing happens: the mixture stays orange because the dichromate is not reduced to chromium(III). That negative result is itself a useful test, since primary and secondary alcohols both turn the solution green on warming. AQA writes "not easily oxidised" rather than never, because vigorous conditions or simple combustion will still destroy the molecule.

What earns the marks

• Stating that the carbon bearing the hydroxyl group has no hydrogen atom attached.
• Saying oxidation would require a carbon–carbon bond to break.
• The observation that the solution remains orange.
• Using that observation to identify the alcohol as tertiary.

Common mistakes and how to put them right

• Claiming a tertiary alcohol can never be oxidised at all, when it still burns.

• Explaining the lack of reaction by the bulk of the three alkyl groups instead of the missing hydrogen.

• Expecting a green solution, or recording no observation at all rather than "stays orange".

• Confusing the negative dichromate result with a negative Tollens' or Fehling's result.

Exam tips

• Record "remains orange" as a positive piece of evidence; leaving the box blank scores nothing.
• Say which hydrogen is missing, the one on the carbon holding the hydroxyl group.
• Pair this test with the classification question, since the two are usually asked together.