Why is that when an optically active (R)-2-Bromobutane is added to a solution of NaBr...?

Question

Why is that when an optically active (R)-2-Bromobutane is added to a solution of NaBr in acetone, the solution gradually becomes optically inactive?

Answer 1

The mechanism is the Walden inversion. Br- attacks the bromobutane from the underside, like a breeze blowing along the handle of an umbrella. The breeze turns the umbrella inside out. In this case, the umbrella is the CH3, H, and CH2CH3 that get turned inside out, and the Br is expelled as Br-. So that results in (S)-2-bromobutane. But after a long while, with Br- constantly attacking (R)- and (S)-molecules, the result is a 50/50 mix of the two.