17.1.5—The nature (aldehyde or ketone) of an unknown

Aldehydes are readily oxidised to carboxylic acids; ketones are not readily oxidised under these mild tests. Fehling’s gives a brick-red Cu₂O precipitate and Tollens’ gives a silver mirror for an aldehyde.

A positive oxidation test supports aldehyde identification; a negative result under valid conditions supports ketone, but controls, freshness and heating conditions matter.

An unknown is positive with 2,4-DNPH and gives a silver mirror. The combined evidence identifies an aldehyde rather than a ketone.

A negative Tollens’ test alone is not proof of a ketone if the reagent or conditions were wrong. Combine independent observations.