JEE Main 2026 — Hydrocarbons Question with Solution

Addition of $B{r}_2$ to styrene (X) yields 1,2-dibromoethylbenzene.

$C_6{H}_5-CH=C{H}_2\stackrel{B{r}_2/CHC{l}_3}{\to }{C}_6{H}_5-CH(Br)-C{H}_2(Br)$

Reaction with excess $NaN{H}_2$ causes double dehydrohalogenation followed by deprotonation of the terminal alkyne, forming sodium phenylacetylide.

$C_6{H}_5-CH(Br)-C{H}_2(Br)\stackrel{NaN{H}_2\text{ (excess)}}{\to }{C}_6{H}_5-C\equiv C^{-}N{a}^{+}$

Nucleophilic substitution with $C{H}_{3}I$ yields 1-phenylpropyne.

$C_6{H}_5-C\equiv C^{-}N{a}^{+}\stackrel{C{H}_{3}I}{\to }{C}_6{H}_5-C\equiv C-C{H}_3$

Reduction with $Na/N{H}_3(l)$ reduces the internal alkyne to a trans-alkene without affecting the benzene ring.

$C_6{H}_5-C\equiv C-C{H}_3\stackrel{Na/N{H}_3(l)}{\to }{C}_6{H}_5-CH=CH-C{H}_3$

The major product (Y) is trans-1-phenylpropene, which has the molecular formula $C_9{H}_{10}$.

Molar mass of $C_9{H}_{10}=(9\times 12)+(10\times 1)=108+10=118{\text{ g mol}}^{-1}$.

Answer: $118$