Which of the following cyclic molecules is a meso compound?

• A. $\text{cis-1,2-dibromocyclohexane}$
• B. $\text{trans-1,2-dibromocyclohexane}$
• C. $\text{cis-1,2-dimethylcyclobutane}$
• D. $\text{trans-1,3-dimethylcyclopentane}$

Answer: (A)

Verified step by step guidance

1. Recall that a meso compound is an achiral molecule that contains multiple stereocenters and an internal plane of symmetry, making it optically inactive despite having stereocenters.
2. Identify the stereocenters in each cyclic molecule and consider their relative configurations (cis or trans) to determine if the molecule can have a plane of symmetry.
3. For cis-1,2-dibromocyclohexane, analyze the chair conformation to see if the two bromine substituents can be positioned such that the molecule has a mirror plane dividing it into two symmetrical halves.
4. For trans-1,2-dibromocyclohexane and the other options, check if any internal symmetry exists; typically, trans-1,2-disubstituted cyclohexanes lack such symmetry and are chiral.
5. Conclude that the molecule with an internal plane of symmetry and multiple stereocenters, which is cis-1,2-dibromocyclohexane, is the meso compound.