Correct IUPAC name of is:

The correct answer is Option (1) → pent-2-enal

In naming an aldehyde, the longest chain that contains the –CHO group is chosen and numbered from the carbon of the aldehyde group (C1). A C=C double bond in the chain is shown by changing “-ane” to “-ene” with its locant, and the suffix “-al” is added.

For \( \ce{CH3-CH2-CH=CH-CHO} \):

• The chain has five carbons including the CHO carbon: pent-.

• Numbering from CHO (C1), the double bond lies between C2 and C3: -2-en-.

• The aldehyde group gives the suffix -al.

So the name is pent-2-enal.

Why the other options are incorrect

• Option (2) pent-2-enol — Incorrect. The suffix “-ol” means an –OH group; the compound has a –CHO group.

• Option (3) pent-3-enal — Incorrect. This locant comes from numbering the double bond from the wrong carbon; with C1 = CHO, the double bond starts at C2.

• Option (4) pent-3-enol — Incorrect. The suffix “-ol” would mean an –OH compound, and the locant 3 comes from numbering from the \( \ce{CH3} \) end; with C1 = CHO the name is pent-2-enal.

NCERT (2026-27 edition):
IUPAC Names