An organic compound (P) on treatment with aqueous ammonia under hot condition forms compound (Q) which on heating with and forms compound (R) having molecular formula . Names of P, Q and R respectively are.
Phenylethanoic acid, phenylethanamide, benzamine
Benzoic acid, 4-methylbenzamide, 4-methylaniline
Benzoic acid, benzamide, aniline
Toluic acid, methylbenzamide, 2-methylaniline
Correct answer:C
Standard Method
Given: Compound P on treatment with aqueous ammonia under hot condition gives Q. Compound Q on heating with and gives R having molecular formula .
Find: The names of P, Q and R.
The final product R has molecular formula . This corresponds to aniline, i.e. .
Now analyze the reaction Q → R. Heating an amide with and is the Hofmann bromamide degradation reaction. In this reaction, a primary amide gives a primary amine with one less carbon atom.
So, to obtain aniline , the amide Q must be benzamide .
The reaction is:
Working Backward from the Product
Next analyze P → Q. Benzamide is formed from the corresponding carboxylic acid on treatment with aqueous ammonia followed by heating. The acid first forms the ammonium salt, which on heating dehydrates to the amide.
Thus P must be benzoic acid.
The sequence is:
Named Reaction Shortcut
A quick route is to start from the given formula and identify R as aniline. Then recall that Hofmann bromamide degradation converts benzamide to aniline by removing one carbon from the amide carbonyl carbon. Therefore Q = benzamide and the corresponding acid P = benzoic acid.
Therefore, the compounds are P = benzoic acid, Q = benzamide, and R = aniline. The correct option is C.
Mistake: Identifying as toluidine or another substituted aromatic amine. Why it is wrong: toluidines have formula . What to do instead: match the molecular formula carefully; corresponds to aniline.
Mistake: Forgetting that Hofmann bromamide degradation gives an amine with one less carbon atom than the amide. Why it is wrong: this leads to choosing the wrong intermediate Q. What to do instead: move from the amine product back to the amide by adding one carbonyl carbon.
Mistake: Assuming aqueous ammonia directly converts any aromatic compound into an amine. Why it is wrong: here ammonia with a carboxylic acid under heating gives an amide, not a direct aromatic amine. What to do instead: identify P as the corresponding carboxylic acid that forms benzamide on heating with ammonia.
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