The enantiopure bromide shown reacts with $\mathrm{NaCN}$ in DMSO by a pure SN2 mechanism. Give the CIP descriptors of the reactant and of the nitrile product, in that order.

The reactant is (S)-2-bromopropan-1-ol, $\mathrm{BrCH}(\mathrm{CH}{}_3)\mathrm{CH}{}_2\mathrm{OH}$, with priorities $\mathrm{Br}>\mathrm{CH}{}_2\mathrm{OH}>\mathrm{CH}{}_3>\mathrm{H}$, descriptor S. SN2 inverts the geometry, but in the product the priorities become $\mathrm{CH}{}_2\mathrm{OH}$ (O,H,H) $>$ $\mathrm{CN}$ (N,N,N) $>\mathrm{CH}{}_3>\mathrm{H}$ because O outranks N at the first point of difference. The new group CN occupies the old Br position in priority rank 2 rather than 1, so the label does not change: S to S (3-hydroxy-2-methylpropanenitrile, checked with RDKit), even though the geometry is inverted.