Question

Give a simple chemical test to distinguish between the following pair of compounds:

Propanal and Propanone

Answer 1

Propanal and propanone can be distinguished by the following tests:

1. Tollen’s test: Propanal is an aldehyde. Thus, it reduces Tollen’s reagent. But, propanone being a ketone does not reduce Tollen’s reagent.
   \[\ce{\underset{Propanal}{CH3CH2CHO} + \underset{Tollen's reagent}{2[Ag(NH3)2]+} + 3OH- -> \underset{Propanoate ion}{CH3CH2COO-} + \underset{Silver mirror}{Ag\downarrow} + NH3 + 2H2O}\]
2. Fehling’s test: Aldehydes respond to Fehling’s test, but ketones do not. Propanal being an aldehyde reduces Fehling’s solution to a red-brown precipitate of Cu₂O, but propanone being a ketone does not.
   \[\ce{\underset{Propanal}{CH3CH2CHO} + 2Cu^2+ + 5OH- -> \underset{Propanaoate ion}{CH3CH2COO-} + \underset{(Red-brown ppt)}{\underset{Cuprous oxide}{Cu2O\downarrow}}+ 2H2O}\]
3. Iodoform test: Aldehydes and ketones with at least one methyl group linked to the carbonyl carbon atom respond to the iodoform test. They are oxidized by sodium hypoiodite (NaOI) to give iodoforms. Propanone being a methyl ketone responds to this test, but propanal does not.
   \[\ce{\underset{Propanone}{CH3COCH3} + 3NaOI -> \underset{Sodium acetate}{CH3COONa} + \underset{(yellow ppt)}{\underset{Iodoform}{CHI3}}+  2NaOH}\]

Answer 2

Reagent	Propanal	Propanone
Tollen’s Reagent	On heating with Tollen’s reagent, a silver mirror is formed on the inner walls of the test tube. Aldehyde groups reduce the Tollen's reagent.	No change in propanone.