Q.2,3-Dibromobutane has two stereocentres. State which combination of configurations, $(2R,3R)$, $(2S,3S)$ or $(2R,3S)$, gives a meso (achiral) compound, and explain why the other two are a pair of enantiomers.

2,3-Dibromobutane's two stereocentres, C2 and C3, carry an identical set of substituents as each other (each is attached to $\text{Br}$, $\text{H}$, a methyl group, and the other stereocentre), which is what makes a meso possibility available here. In the $(2R,3S)$ (equivalently written $(2S,3R)$, since the molecule is symmetric) combination, one half of the molecule is the mirror image of the other half, related by an internal mirror plane running between C2 and C3 -- the molecule is therefore superimposable on its own mirror image and is achiral, despite containing two stereocentres; this is the meso compound. In the $(2R,3R)$ and $(2S,3S)$ combinations, both stereocentres share the same handedness, there is no such internal mirror plane …

Check specifically for an internal mirror plane relating the two halves of a molecule with two identically-substituted stereo …

Assuming any (R,S)-labelled compound is automatically 'the racemic form' rather than checking whether it is actually a single achiral meso structure, which is a distinct po …