Optically pure (R)-2-iodobutane is left for a long time in dry acetone containing excess $NaI$ . The optical activity of the solution will

Abecome that of pure (S)-2-iodobutane
Bremain unchanged because the substituent is identical
Cchange to that of a meso compound
Dgradually fall to zero because iodide displaces iodide with inversion repeatedly, giving the racemic mixture

Answer: D

Each

S_{N} 2

attack of

I X^{-}

on 2-iodobutane inverts the stereocentre, converting (R) into (S) and (S) into (R). Repeated exchange drives the mixture to equal amounts of the two enantiomers, a racemic mixture with zero rotation.

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Answer: D

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