See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Chemoselective hydrogenation with PtO2 (Adams catalyst) and one mole of H2 preferentially reduces the more substituted (more reactive toward catalytic hydrogenation) or more accessible double bond. In catalytic heterogeneous hydrogenation, more substituted alkenes are generally hydrogenated faster because they adsorb more strongly on the metal surface. Step 1: Identify the two double bonds in the starting material. The starting material is limonene: a cyclohexene ring with a methyl group at C1 (trisubstituted endocyclic double bond) and an isopropenyl group at C4 (terminal exocyclic CH2= double bond, less substituted, 1,1-disubstituted vinyl). Step 2: Compare substitution of the two double bonds. - Endocyclic double bond (ring C=C with methyl): trisubstituted (three carbon substituents around the double bond). - Exocyclic/side chain double bond (CH2=C<): 1,1-disubstituted (two carbons on one end, terminal =CH2 on other end). Step 3: Under catalytic hydrogenation (PtO2/H2), more substituted alkenes adsorb preferentially and react faster. However, the terminal (less hindered) double bond can also be more accessible. In practice with limonene and one equivalent H2 over PtO2, the less hindered terminal vinyl (isopropenyl CH2=) group is hydrogenated selectively to give an isopropyl group, while the more hindered trisubstituted endocyclic double bond is left intact. Step 4: Product of hydrogenating the exocyclic CH2= double bond: the isopropenyl group (-C(CH3)=CH2) becomes an isopropyl group (-CH(CH3)2), while the ring double bond (trisubstituted, more hindered) remains. This gives 1-methyl-4-isopropylcyclohex-1-ene, which matches option (b): cyclohexene ring with methyl at top and isopropyl at C4. Why other options fail: - (a) shows the ring double bond reduced and side chain still with C=CH2 - wrong selectivity direction. - (c) shows a gem-dimethyl substituted ring which is not consistent with the starting material structure. - (d) shows the ring double bond reduced and the side chain as isopropylidene (trisubstituted) - incorrect product. Therefore, the correct answer is B.