(R)-2-Bromobutane, CH3CH2-CH(Br)-CH3, is treated with sodium iodide in dry acetone. The major product is

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(R)-2-Bromobutane, $CH X_{3} CH X_{2} - CH (Br) - CH X_{3}$ , is treated with sodium iodide in dry acetone. The major product is

A secondary bromide with a good nucleophile in a polar aprotic solvent reacts by SN2 with inversion. The priorities at the stereocentre are halogen

> C X_{2} H X_{5} > CH X_{3} > H

for both substrate and product, so inversion of geometry changes the descriptor from R to S: (S)-2-iodobutane.