Aldehydes and Ketones – Distinguish Tests Class 12

Aldehydes and Ketones

Que. Give simple chemical tests to distinguish between the following pairs of compounds.

(i) Propanal and propanone

(ii) Acetophenone and benzophenone

(iii) Phenol and benzoic acid

(iv) Benzoic acid and ethyl benzene

(v) Ethanal and propanal

(vi) Benzaldehyde and acetophenone

Answers:

(i) Propanal and Propanone: These two are distinguished by the iodoform test. propanal does not give an iodoform test when it reacts with I₂ in the presence of NaOH while propanone gives an iodoform test when reacts with I₂ in the presence of NaOH.
CH₃COCH₃ + NaOI → CH₃COONa + CHI₃ + NaOH

(ii) Acetophenone and Benzophenone: Acetophenone gives a positive iodoform test whereas benzophenone (C₆H₅COC₆H₅) does not.
C₆H₅COCH₃ + NaOI → C₆H₅COONa + CHI₃ + NaOH
Yellow ppts
C₆H₅COC₆H₅ + NaOI → No Yellow ppts

(iii) Phenol and Benzoic acid: Phenol gives a violet coloration with neutral FeCl₃ solution while benzoic acid gives a buff coloured precipitate of ferric benzoate.

(iv) Benzoic acid and Ethylbenzene: Benzoic acid on warming with sodium hydrogen carbonate gives brisk effervescence of CO₂ gas while ethyl benzoate does not respond to this test.

(v) Ethanal and propanal: Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom respond to the iodoform test. Ethanal having one methyl group linked to the carbonyl carbon atom responds to this test. But propanal does not have a methyl group linked to the carbonyl carbon atom and thus, it does not respond to this state.
CH₃CHO + 4 NaOH + 3 I₂ → CHI₃ (Yellow ppt.) + HCOONa + 3 NaI + 3 H₂O
Propanal → Does not give this test

(vi) Benzaldehyde and Acetophenone: Benzaldehyde is an aromatic aldehyde, while acetophenone is an aromatic ketone compound. Therefore, these can be distinguished by using Tollen’s reagent because the aldehyde functional group can form a precipitate with this reagent.  Benzaldehyde can reduce Tollen’s reagent, giving a red-brown precipitate of Cu₂O, whereas acetophenone does not show any reaction towards Tollen’s reagent.

Aldehydes and Ketones