How can a simple test tube reaction tell an aldehyde apart from a ketone?
Aldehydes are easily oxidised, ketones are not — and two tests exploit this.
Aldehydes are easily oxidised, ketones are not — and two tests exploit this. Tollens' reagent (ammoniacal silver nitrate) is reduced by an aldehyde to metallic silver, coating the tube with a shiny silver mirror. Fehling's solution is reduced by an aldehyde to a red-brown Cu₂O precipitate. A ketone leaves both unchanged. So a silver mirror or red precipitate confirms an aldehyde.
Key point
Aldehydes reduce Tollens' (silver mirror) and Fehling's (red Cu₂O); ketones don't.
Common mistake
expecting ketones to also give a positive Tollens'/Fehling's result.
Memory tip
silver mirror or brick-red = aldehyde; no change = ketone.
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