GUJCET 2026 Chemistry Paper

Concept:Find the longest carbon chain that includes the ketone group, then number it so the ketone carbon gets the lowest possible number.Explanation:Mesityl oxide has the structure $(\mathrm{CH}_3)_2\mathrm{C}=\mathrm{CH}-\mathrm{CO}-\mathrm{CH}_3$.The longest chain containing the $\mathrm{C=O}$ group has $5$ carbon atoms, so the parent is pentane.Numbering starts from the ketone end, so the $\mathrm{C=O}$ is at position $2$.Thus the base name becomes pent-$2$-one.A double bond lies at position $3$, giving pent-$3$-en-$2$-one.A methyl group is attached at position $4$.Combining all parts gives $4$-methylpent-$3$-en-$2$-one.Answer:The $IUPAC$ name of mesityl oxide is $4$-methylpent-$3$-en-$2$-one.