The stereoisomer of 2,3-dibromobutane shown has two stereocentres. Which statement about it is correct?

AIt is chiral and optically active
BIt is achiral because it has an internal plane of symmetry (a meso compound)
CIt is achiral because neither carbon is a stereocentre
DIt is a racemic mixture

Answer: B

Both C2 and C3 carry four different groups and are stereocentres. In the drawing the two halves of the molecule are mirror images of each other across the midpoint of the C2-C3 bond: one centre is

(R)

and the other is

(S)

. A molecule with an internal mirror plane is superimposable on its mirror image, so it is meso: achiral and optically inactive despite having stereocentres.

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