JEE Main · 2021 · 26 Feb 2021 · Shift 2 · Aldehydes Ketones and Carboxylic Acids

The substrate is 4-ethylhexan-3-one. Zinc and hydrochloric acid reduce the carbonyl group all the way to a methylene group, the Clemmensen reduction, giving 3-ethylhexane, ${\mathrm{C}}_8{\mathrm{H}}_{18}$. Over $\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_3$ at $773$ K and $10$-$20$ atm this alkane undergoes dehydrocyclisation to an arene of the same carbon count, ${\mathrm{C}}_8{\mathrm{H}}_{10}$. A benzene ring needs six carbons in one continuous chain, and in 3-ethylhexane every six-carbon chain that can be found leaves a single two-carbon branch outside the ring. The ring therefore closes with one ethyl group attached to it, and the aromatic product is ethylbenzene.