The structure representing semicarbazone of cyclopentanone correctly is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → cyclopentanone semicarbazone, (C5H8)C=N–NH–CO–NH2
Semicarbazide, \( \ce{H2N-NH-CO-NH2} \), reacts with a ketone through its free –\( \ce{NH2} \) attached to NH (the other \( \ce{NH2} \) is part of an amide and is not nucleophilic). Water is lost and the C=O becomes C=N:
cyclopentanone + \( \ce{H2N-NH-CO-NH2} \) → \( \ce{(C5H8)C=N-NH-CO-NH2} \) + \( \ce{H2O} \)
Option (1) shows the cyclopentane ring with =N–NH–CO–NH2, the semicarbazone.
Why the other options are incorrect
Option (2) — Incorrect. =\( \ce{N-NH-C6H5} \), the phenylhydrazone.
Option (3) — Incorrect. =\( \ce{N-NH-C6H3(NO2)2} \), the 2,4-dinitrophenylhydrazone.
Option (4) — Incorrect. =N–OH, the oxime.
NCERT (2026-27 edition):
Addition of Ammonia and Its Derivatives – “Nucleophiles, such as ammonia and its derivatives H2N-Z add to the carbonyl group of aldehydes and ketones. The reaction is reversible and catalysed by acid. The equilibrium favours the product formation due to rapid dehydration of the intermediate to form >C=N-Z. Z = Alkyl, aryl, OH, NH2, C6H5NH, NHCONH2, etc.”