Question

(A),(B) and (C) are three non-cyclic functional isomers of a carbonyl compound with molecular formula $C_4{H}_{8}O.$ Isomers (A) and (C) give positive Tollens' test whereas isomer (B) does not give Tollens' test but gives positive Iodoform test. Isomers (A) and (B) on reduction with Zn(Hg)/conc. HCI give the same product (D).

(a) Write the structures of (A),(B),(C) and (D).

(b) Out of (A), (B) and (C) isomers, which one is least reactive towards addition of HCN?

Answer 1

Part (a): Structure of A, B, C, and D

Analyzing the given tests:

• Tollen’s test is given by aldehydes and ketones do not give Tollen's test.
• The iodoform test is given by compounds that have a methyl group attached to a carbonyl carbon.
• The reagent $Zn(Hg)/HCl$ is Clemmensen’s reagent and reduces carbonyl carbon into methylene.

Identifying the compound B:

• Compound B does not give Tollen's test but gives the iodoform test.
• So, compound B is a methyl ketone.
• The only structure possible with the molecular formula C4H8OC_4H_8OC4​H8​O is

Identifying the compound A, C, and D:

• Compound A gives the Tollen's test. So, it is an aldehyde and it also gave the same product on reaction with Clemmenson reagent as given by compound B.
• So, the possible structure of compound A is

• The compounds B and A gave the same product D on reacting with Clemmensen reagent.
• So, the possible structure of D is

• Compound C is also an aldehyde.​
• So, the only possible structure of compound C is

Part b: Which is less reactive towards $HCN$

• $HCN$ addition reaction to a carbonyl compound is a bimolecular reaction.
• The rate of bimolecular reaction depends upon the steric hindrance.
• So, the compound containing large substituents is less reactive towards $HCN$ addition reaction.​
• Hence, ketones are less reactive as compared to aldehyde.
• So, compound B is least reactive towards $HCN$ addition reaction.