JEE Main · 2026 · 04 Apr 2026 · Shift 2 · Hydrocarbons

Styrene first undergoes electrophilic addition of bromine to give (1,2-dibromoethyl)benzene, ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CHBr}-\mathrm{C}{\mathrm{H}}_2\mathrm{Br}$. Excess sodamide eliminates two molecules of HBr to form phenylacetylene and then deprotonates the terminal alkyne to its acetylide, ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{C}\equiv {\mathrm{C}}^{-}$. Alkylation with $\mathrm{C}{\mathrm{H}}_3\mathrm{I}$ gives 1-phenylpropyne, ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{C}\equiv \mathrm{C}-\mathrm{C}{\mathrm{H}}_3$. Dissolving-metal reduction with sodium in liquid ammonia converts the internal alkyne to the trans alkene, ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CH}=\mathrm{CH}-\mathrm{C}{\mathrm{H}}_3$ (1-phenylpropene, ${\mathrm{C}}_9{\mathrm{H}}_{10}$). Its molar mass is $9(12)+10(1)=118\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$.