Organic Chemistry MCAT Practice Exam

What defines a meso compound?

1. Having more than one chiral center

2. Symmetrical chiral centers with opposite R/S configurations

3. Nonsuperimposable mirror images

4. Different properties and optical activity

The correct answer is: Symmetrical chiral centers with opposite R/S configurations

A meso compound is characterized by having more than one chiral center, but what distinctly defines it is the presence of a plane of symmetry within the molecule. This means that despite having multiple chiral centers, the overall molecule is achiral because the symmetry leads to superimposability with its mirror image. For a compound to be classified as a meso compound, it must have chiral centers that are configured in such a way that they produce enantiomers that are not optically active due to the internal plane of symmetry. This is precisely what option B addresses: symmetrical chiral centers configured oppositely (R/S configurations), leading to a structure that is identical to its mirror image. The other options do not accurately capture the essence of what defines a meso compound. While it is true that a meso compound can have multiple chiral centers, not all compounds with more than one chiral center are meso; they must also exhibit symmetry. Additionally, nonsuperimposable mirror images and differences in optical activity pertain more directly to compounds that are true enantiomers rather than meso compounds.