(S)-Butan-2-ol is treated with $\mathrm{SOCl}{}_2$ in the presence of pyridine. The main product is

With pyridine present, the chlorosulphite intermediate is attacked by free $\mathrm{Cl}{}^{-}$ from the back side (an ${\mathrm{S}}_{\mathrm{N}}2$ step), so the configuration is inverted. In both the alcohol and the chloride the priority order is substituent (OH or Cl) > ethyl > methyl > H, so spatial inversion changes the descriptor from (S) to (R).