Identify the indicated faces in the following molecules as Re or Si:

(a)

(b)

In stereochemistry, prochirality is converting achiral compounds into chiral ones. There are two faces in prochirality, one is Re face, and the other is Si face.Re face is when priority groups are arranged clockwise, and Si face is when priority groups are arranged anticlockwise.

(a) According to C.I.P. sequence order, Re face is when priority substitutents are arranged in clockwise order. When we turn over the molecule, the priority substituents become anticlockwise, this is Si face. The ‘Re’ face is given to upper planar face. Here, the carbonyl group is given 1^st priority, followed by anion which is given 2^nd priority, and the 3^rd priority is given to group. The ‘Si’ face is given to lower planar face which is anticlockwise.

Re and Si faces in pyruvate