See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 – Identify the molecular formula and degree of unsaturation. C8H14 has degree of unsaturation = (2×8 + 2 - 14)/2 = (18-14)/2 = 2. So the molecule has 2 degrees of unsaturation (e.g., one ring + one double bond). Step 2 – Understand the key conditions. (i) Compound A is optically active (chiral, no internal plane of symmetry). (ii) Catalytic hydrogenation of A gives a meso compound (the product has stereocenters but an internal plane of symmetry making it achiral). Step 3 – Analyze option (b): 1,4-dimethylcyclohex-2-ene with the two methyl groups in a trans (one wedge, one dash) configuration. - The molecule has a ring (1 degree) and a double bond (1 degree), total = 2. Consistent with C8H14. - With the double bond at C2-C3 and methyl groups at C1 (wedge) and C4 (dash), C1 and C4 are stereocenters. The trans arrangement means one methyl is above and one is below the ring plane, giving a chiral molecule (no internal mirror plane) — optically active. This satisfies condition (i). - Upon catalytic hydrogenation (H2/catalyst), the double bond is reduced. The hydrogens add to the same face (syn addition). The ring becomes 1,4-dimethylcyclohexane. Because the starting material has the methyls trans to each other (one up, one down), the hydrogenation saturates the ring giving cis-1,4-dimethylcyclohexane (the two methyls end up on the same side relative to the ring) — wait, let us re-examine: trans-1,4-dimethylcyclohex-2-ene on hydrogenation. With syn addition of H2 across the 2,3-double bond, the existing trans relationship at C1 and C4 is preserved in going to 1,4-dimethylcyclohexane. Trans-1,4-dimethylcyclohexane is actually a meso compound because C1 and C4 are mirror-image stereocenters with an internal plane of symmetry bisecting the ring. This satisfies condition (ii). Step 4 – Why other options fail. (a) 1,4-dimethylcyclohex-2-ene with both methyls on wedge (cis): this is a meso compound itself (has an internal plane of symmetry), so it is NOT optically active. Fails condition (i). (c) The structure shown has a gem-dimethyl group (two methyls on the same carbon) with a ring double bond — this would not produce a meso compound upon hydrogenation (no new stereocenters created in a useful way for meso). (d) Ethylbenzene (C8H10, not C8H14) does not match the molecular formula, and hydrogenation of a benzene ring does not straightforwardly give a meso product under simple catalytic conditions described. Step 5 – Conclusion: Option (b), trans-1,4-dimethylcyclohex-2-ene (with methyls in opposite orientations), is optically active and gives the meso trans-1,4-dimethylcyclohexane upon hydrogenation. Therefore, the correct answer is B.