For carbon compounds oxidation is an increase in the oxygen to hydrogen ratio. For carbon compounds, reduction is a decrease in the oxygen to hydrogen ratio.

Hot copper(II) oxide or acidified dichromate(VI) solutions can be used to oxidise primary and secondary alcohols.

Hot copper(II) oxide or acidified dichromate(VI) solutions are oxidising agents and are themselves reduced.

Primary alcohols oxidise to aldehydes and then to carboxylic acids. Secondary alcohols oxidise to ketones. Tertiary alcohols cannot be oxidised using oxidising agents.

During oxidation reactions of alcohols, black copper(II) oxide forms a brown solid. During oxidation reactions of alcohols orange dichromate solution turns green.

Aldehydes, but not ketones, can be oxidised to carboxylic acids.

Oxidising agents can be used to differentiate between an aldehyde and a ketone — Fehling’s solution, Tollens’ reagent, and acidified dichromate.

With the oxidation of an aldehyde blue Fehling’s solution forms a brick red precipitate; clear, colourless Tollens’ reagent forms a silver mirror; and orange acidified dichromate solution turns green.

Aldehydes and ketones are molecules containing a carbonyl functional group, $-CO$.

aldehyde

ketones

Many flavour and aroma molecules are aldehydes.