Phthalic acid is treated with ammonia and the product is strongly heated. What is the final product?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Phthalimide
A carboxylic acid and ammonia first give an ammonium salt; heating then removes water to give an amide. Phthalic acid (benzene-1,2-dicarboxylic acid) has two –COOH groups, so it goes one step further:
With ammonia: ammonium phthalate is formed.
\( \displaystyle \ce{C6H4(COOH)2 + 2NH3} \rightleftharpoons \ce{C6H4(COONH4)2} \)
On heating: loss of two water molecules gives phthalamide.
\( \displaystyle \ce{C6H4(COONH4)2} \xrightarrow{\Delta} \ce{C6H4(CONH2)2 + 2H2O} \)
On strong heating: the two amide groups lose one molecule of ammonia and close a ring, giving phthalimide.
\( \displaystyle \ce{C6H4(CONH2)2} \xrightarrow{\text{strong heating}} \ce{C6H4(CO)2NH + NH3} \)
Why the other options are incorrect
Option (1) Ammonium phthalate — Incorrect. It is the first product, formed before any heating; strong heating changes it further.
Option (2) Acetamide — Incorrect. Acetamide comes from ethanoic acid and ammonia (through ammonium acetate), not from phthalic acid.
Option (4) Benzamide — Incorrect. Benzamide comes from benzoic acid, which has only one –COOH group.
NCERT (2026-27 edition):
8.9.2 Reactions Involving Cleavage of C–OH Bond – “4. Reaction with ammonia
Carboxylic acids react with ammonia to give ammonium salt which on further heating at high temperature give amides.”
