CBSE Class 12 Chemistry Aldehyde Ketones And Acids Worksheet Set B

Question. Give simple chemical tests to distinguish between the following pairs of compounds :
(i) Benzaldehyde and benzoic acid
(ii) Propanal and propanone.
Answer. (i) Benzaldehyde and benzoic acid can be distinguished by sodium bicarbonate test.
Benzoic acid will give effervescence with NaHCO₃ but benzaldehyde will not react.
(ii) Propanal and propanone can be distinguished by their reactions with Tollens’ reagent.
Propanal will form the silver mirror, but propanone does not react.

Question. Account for the following :
CH₃CHO is more reactive than CH₃COCH₃
towards reaction with HCN.
Answer. It is a nucleophilic addition reaction, in which CN – acts as a nucleophile. CH₃CHO undergoes nucleophilic addition reactions faster than CH₃COCH₃ as in CH₃COCH₃ there are two electron releasing methyl groups attached to the carbonyl carbon that hinders the approach of nucleophile to carbonyl carbon and reduce the electrophilicity of the carbonyl group while in CH₃CHO, there is only one methyl group attached to carbonyl carbon.

Question. Distinguish between the following :
(i) C₆H₅—COCH₃ and C₆H₅—CHO
Answer. Benzaldehyde and acetophenone can be distinguished by Tollens’ test.
Benzaldehyde will form silver mirror, on treatment with Tollens’ reagent whereas acetophenone will not show Tollens’ Test.

Question. Aldehydes and Ketones have lower boiling points than corresponding alcohols. Why?
Answer. The boiling points of aldehydes and ketones are lower than that of corresponding alcohols and acids due to absence of intermolecular H–bonding in aldehydes and ketones.

Question. Arrange the following in the increasing order of their boiling points.
CH₃CHO, CH₃COOH, CH₃CH₂OH
Answer. Increasing order of boiling point :
CH₃ — CHO < C₂H₅OH < CH₃ — COOH

Question. Give simple chemical tests to distinguish between the following pair of compounds.
Propanal and Butan-2-one.
Answer. Propanal and Butan-2-one can be distinguised by their reactions with tollen’s reagent. Propanal will form the silver mirror, but Butan-2-one does not react.

Question. Give simple chemical tests to distinguish between the following pair of compounds :
Ethanal and Propanal
Answer.Ethanal and propanal can be distinguished by iodoform test.
Yellow precipitate of iodoform will be formed from ethanal on heating with iodine and sodium hydroxide solution.

Question. Given reason :
pH of reaction should be carefully controlled while preparing ammonia derivatives of carbonyl compounds.
Answer. In strongly acidic medium ammonia derivatives being basic will react with acids and will not react with carbonyl compound. In basic medium, OH– will attack carbonyl group. Therefore, pH of a reaction should be carefully controlled.

Question. An organic compound with molecular formula C₉H₁₀O forms 2,4-DNP derivative, reduces Tollen’s reagent and undergoes Cannizzaro’s reaction. On vigorous oxidation it gives 1,2-benzenedicarboxylic acid. Identify the compound.
Answer. The compound forms 2,4-DNP derivative. It shows that it is a carbonyl compound. Further it reduces Tollens’ reagent which shows that it contains aldehydic group. It undergoes Cannizzaro reaction indicating that aldehyde group is without any a-hydrogen. On vigorous oxidation, it gives 1,2-benzenedicarboxylic acid which shows that there are two carbon residues on benzene ring. Since the molecular formula is C₉H₁₀O, it fits into the structure, 2-ethylbenzaldehyde.