See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: A meso compound is an achiral molecule that contains two or more stereocenters but has an internal plane of symmetry, making it optically inactive despite having chiral centers. Step 1: Identify which compounds have stereocenters. - (a) 1,4-dimethylcyclohexane: C1 and C4 are stereocenters. However, due to the symmetry of the 1,4-substitution pattern, the cis isomer has a plane of symmetry and is a meso compound... but wait, we must check carefully. In 1,4-dimethylcyclohexane, C1 and C4 each bear H, CH3, and two ring portions. The two ring portions between C1 and C4 are identical (each is a -CH2CH2CH2- chain of 3 carbons), so C1 and C4 are not truly stereocenters in the classical sense because the two ring segments are identical. Actually they are pseudoasymmetric or the molecule has enough symmetry that no true chirality exists. - (b) 1,3-dimethylcyclohexane: C1 and C3 are stereocenters. The ring segments between C1 and C3 are: one segment has 2 carbons (C2) and the other has 4 carbons (C4, C5, C6), making C1 and C3 genuine stereocenters with different substituents. The cis-1,3-dimethylcyclohexane has both methyl groups on the same face; this isomer possesses an internal plane of symmetry (through C2 and C5, bisecting the ring), making it a meso compound. The trans-1,3-dimethylcyclohexane is chiral (exists as a pair of enantiomers). - (c) 1-ethyl-2-methylcyclohexane: C1 has ethyl and C2 has methyl - these are different substituents and the ring segments are also different, so stereocenters exist. However, for a meso compound we need two identical stereocenters with an internal plane of symmetry. Since C1 bears ethyl and C2 bears methyl (different groups), there is no internal plane of symmetry possible, so no meso form exists. - (d) 1,1-dimethylcyclohexane: C1 has two methyl groups, so it is not a stereocenter at all. No stereoisomers exist. Step 2: Identify the correct answer. Only 1,3-dimethylcyclohexane (option b) has a stereoisomer (the cis isomer) that is a meso compound, because the cis form has an internal plane of symmetry passing through C2 and C5 of the ring, even though it contains two stereocenters (C1 and C3). Step 3: Why other options fail. - (a) 1,4-dimethylcyclohexane: The carbons C1 and C4 are not true stereocenters because the two ring chains connecting them are identical (both are trimethylene chains), so no meso compound is possible in the strict sense. - (c) Different substituents at adjacent carbons means no internal plane of symmetry, no meso form. - (d) No stereocenter present, no stereoisomers at all. Therefore, the correct answer is B.