What is the IUPAC name of the ketone \( \ce{(CH3)2CHCH2COCH3} \)?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 4-Methylpentan-2-one
The IUPAC name of a ketone comes from the alkane of its longest carbon chain, with the ending –e replaced by –one. The chain is numbered from the end nearer to the carbonyl group, and substituents are prefixed with numerals showing their positions.
The longest chain has five carbon atoms (pentane):
\( \displaystyle \ce{CH3-CO-CH2-CH(CH3)-CH3} \)
Numbered from the left, the end nearer to C=O, the carbonyl carbon is C-2 and the methyl group is on C-4: 4-methylpentan-2-one.
Why the other options are incorrect
Option (1) 2-Methylpentan-4-one — Incorrect. This name numbers the same chain from the wrong end; the carbonyl carbon must get the lower number, 2, not 4.
Option (2) 3-Methylpentan-2-one — Incorrect. This compound is \( \ce{CH3COCH(CH3)CH2CH3} \), with the methyl group on the carbon next to the carbonyl group.
Option (4) 2-Methylpentan-3-one — Incorrect. This compound is \( \ce{(CH3)2CHCOCH2CH3} \), with the carbonyl group at C-3 of the chain.
NCERT (2026-27 edition):
8.1.1 Nomenclature – “The IUPAC names of open chain aliphatic aldehydes and ketones are derived from the names of the corresponding alkanes by replacing the ending –e with –al and –one respectively. In case of aldehydes the longest carbon chain is numbered starting from the carbon of the aldehyde group while in case of ketones the numbering begins from the end nearer to the carbonyl group. The substituents are prefixed in alphabetical order along with numerals indicating their positions in the carbon chain.”