Name: Acetaldehyde and Acetone
Uploaded: 2019-07-19
Duration: 3 min 15 s
Description: Do you know carbonyl compounds are used in medicines? In this video, you will be able to understand the important uses of the two most common compounds, acet...

Question

Give chemical tests to distinguish between the following pair of compounds:

Propanal and propanone.

Accepted Answer

Propanal and propanone can be distinguished by the following tests.

(i)Tollen's test:

Propanal is an aldehyde. Thus, it reduces Tollen's reagent. But, propanone being a ketone does not reduce Tollen's reagent.

$\underset{Propanal}{{CH}_{3} {CH}_{2} CHO} + 2 {[Ag {({NH}_{3})}_{2}]}^{+} + 3 {OH}^{-} \overset{}{\to} {CH}_{3} {CH}_{2} {COO}^{-} + Ag + 4 {NH}_{3} + 2 H_{2} O$

$\underset{Propanone}{{CH}_{3} {COCH}_{3}} + 2 {[Ag {({NH}_{3})}_{2}]}^{+} + 3 {OH}^{-} \overset{}{\to} No reaction$

(ii) Aldehydes respond to Fehling's test, but ketones do not.

Propanal being an aldehyde reduces Fehling's solution to a red-brown precipitate of ${Cu}_{2} O$ , but propanone being a ketone does not.

$\underset{Propanal}{{CH}_{3} {CH}_{2} CHO} + 2 {Cu}^{2 +} + 5 {OH}^{-} \overset{}{\to} {CH}_{3} {CH}_{2} {COO}^{-} + {Cu}_{2} O + 3 H_{2} O$

$\underset{Propanone}{{CH}_{3} {COCH}_{3}} + 2 {Cu}^{2 +} + 5 {OH}^{-} \overset{}{\to} No reaction$

(iii) Iodoform test:

Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom respond to iodoform test. They are oxidised by sodium hypoiodite ( $NaOI$ ) to give iodoforms. Propanone being a methyl ketone responds to this test, but propanal does not.

$\underset{Propanone}{{CH}_{3} {COCH}_{3}} + 3 NaOI \overset{}{\to} {CH}_{3} COONa + {CHI}_{3} + 2 NaOH$

$\underset{Propanal}{{CH}_{3} {CH}_{2} CHO} + 3 NaOI \overset{}{\to} No reaction$

Give chemical tests to distinguish between the following pair of compounds:

Propanal and propanone.

Important Questions on Aldehydes, Ketones and Carboxylic Acids

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