Which of the following molecules can be classified as meso?

Question

BlackTom AI · Accepted Answer

Meso compounds are achiral despite having stereocenters because they contain an internal plane (or center) of symmetry that makes the molecule superimposable on its mirror image. Key requirements: at least two stereocenters, identical substituent patterns on the stereocenters, and opposite absolute configurations (R and S) so the stereochemical effects cancel.

Step-by-step analysis of the three structures:

I. This structure shows a single stereocenter (a carbon bearing Cl, H, and two different carbon substituents). A single stereocenter produces a chiral center (non-superimposable mirror image) and therefore cannot be meso. Meso status requires two or more stereocenters.

II. This molecule has two stereocenters but both are drawn with the same relative stereochemistry (both wedged substituents on the same face). That corresponds to the (R,R) or (S,S) stereoisomer. There is no internal mirror plane that maps one stereocenter onto the other while reversing configuration; the molecule and its mirror image are non-superimposable enantiomers, so it is chiral — not meso.

III. This is the classic meso example: 2,3-dibromobutane drawn with one bromine wedge (out of the page) and the other bromine dashed (into the page). The two stereocenters have opposite configurations (one R, one S), and the substituent sets on C-2 and C-3 are identical. A plane of symmetry bisects the molecule between the two central carbons, reflecting one half onto the other. That internal mirror operation makes the molecule superimposable on its mirror image and therefore achiral (meso), even though there are two stereocenters.

Why the other options are incorrect: I is excluded because it has only one stereocenter (chiral), and II is excluded because its two stereocenters have the same configuration giving a chiral enantiomeric pair.

Therefore the correct meso compound is III, which matches the provided answer.

Which of the following molecules can be classified as meso? Single choice

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