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Questions
Propanal and Propanone
Give a chemical test to distinguish between propanal and propanone.
Give a simple chemical test to distinguish between propanal and propanone.
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Solution 1
Propanal and propanone can be distinguished by the following tests:
- Tollen’s test: Propanal is an aldehyde. Thus, it reduces Tollen’s reagent. But, propanone being a ketone does not reduce Tollen’s reagent.
\[\ce{\underset{Propanal}{CH3CH2CHO} + \underset{Tollen's reagent}{2[Ag(NH3)2]+} + 3OH- -> \underset{Propanoate ion}{CH3CH2COO-} + \underset{Silver mirror}{Ag\downarrow} + NH3 + 2H2O}\] - 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.
\[\ce{\underset{Propanal}{CH3CH2CHO} + 2Cu^2+ + 5OH- -> \underset{Propanoate ion}{CH3CH2COO-} + \underset{(Red-brown ppt)}{\underset{Cuprous oxide}{Cu2O\downarrow}}+ 2H2O}\] - 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.
\[\ce{\underset{Propanone}{CH3COCH3} + 3NaOI -> \underset{Sodium acetate}{CH3COONa} + \underset{(yellow ppt)}{\underset{Iodoform}{CHI3}}+ 2NaOH}\]
Solution 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. | On heating in a water bath with Tollen’s reagent does not show any reaction. |
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