Aldehydes can be oxidised further to carboxylic acids, but ketones cannot. Two tests use this difference. In each, the aldehyde acts as a reducing agent and is itself oxidised:RCHO + [O] → RCOOH Tollens' reagent
Tollens' reagent is a colourless solution containing the complex ion [Ag(NH3)2]+. Warm the sample with it in a water bath (not over a flame, as organic compounds are flammable).
Aldehyde: a silver mirror forms on the inside of the test tube, as silver(I) ions are reduced to silver atoms:
Ag+ + e− → Ag
Ketone: no change; the solution stays colourless.Fehling's solution
Fehling's solution is a blue solution containing copper(II) complex ions. Warm the sample with it.
Aldehyde: the blue solution gives a brick-red precipitate of copper(I) oxide, Cu2O, as Cu2+ ions are reduced to Cu+.
Ketone: no change; the solution stays blue.Worked example: a compound C3H6O gives a silver mirror with Tollens' reagent. Identify it.
C3H6O could be propanal, CH3CH2CHO, or propanone, CH3COCH3. Only the aldehyde gives a silver mirror, so the compound is propanal. It is oxidised to propanoic acid.These tests also help identify the alcohol that was oxidised: a primary alcohol gives an aldehyde (positive test), while a secondary alcohol gives a ketone (negative test).
Tollens' reagent is a colourless solution containing the complex ion [Ag(NH3)2]+. Warm the sample with it in a water bath (not over a flame, as organic compounds are flammable).
Aldehyde: a silver mirror forms on the inside of the test tube, as silver(I) ions are reduced to silver atoms:
Ketone: no change; the solution stays colourless.Fehling's solution
Fehling's solution is a blue solution containing copper(II) complex ions. Warm the sample with it.
Aldehyde: the blue solution gives a brick-red precipitate of copper(I) oxide, Cu2O, as Cu2+ ions are reduced to Cu+.
Ketone: no change; the solution stays blue.Worked example: a compound C3H6O gives a silver mirror with Tollens' reagent. Identify it.
C3H6O could be propanal, CH3CH2CHO, or propanone, CH3COCH3. Only the aldehyde gives a silver mirror, so the compound is propanal. It is oxidised to propanoic acid.These tests also help identify the alcohol that was oxidised: a primary alcohol gives an aldehyde (positive test), while a secondary alcohol gives a ketone (negative test).