Which one of the following will be named as a carbaldehyde?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → cyclohexanecarbaldehyde
In the IUPAC system, an aldehyde whose –CHO carbon is part of a chain is named with the suffix -al (e.g. butanal). When the –CHO group is attached to a ring, the suffix carbaldehyde is added to the full name of the ring (e.g. benzenecarbaldehyde for benzaldehyde). Structure B, a –CHO group on a cyclohexane ring, is cyclohexanecarbaldehyde.
Why the other options are incorrect
Option (1) — Incorrect. \( \ce{C6H5COCH2CH3} \) is a ketone, 1-phenylpropan-1-one (propiophenone).
Option (3) — Incorrect. A methylcyclopentanone, a cyclic ketone; the C=O is part of the ring.
Option (4) — Incorrect. \( \ce{CH3CH(Br)CH2CHO} \) has its –CHO on a chain, so it is named 3-bromobutanal.
NCERT (2026-27 edition):
Nomenclature – “… The same applies to cyclic ketones, where the carbonyl carbon is numbered one. When the aldehyde group is attached to a ring, the suffix carbaldehyde is added after the full name of the cycloalkane. The numbering of the ring carbon atoms start from the carbon atom attached to the aldehyde group.”