Glyceraldehyde, 2,3-dihydroxypropanal, is shown drawn with wedges. Which option gives the correct configuration and the correct reason?

A $(R)$ ; $CHO$ ranks above $CH X_{2} OH$ because its carbon is attached to (O,O,H)
B $(S)$ ; $CH X_{2} OH$ ranks above $CHO$ because it carries more hydrogens
C $(R)$ ; $CH X_{2} OH$ ranks above $CHO$ because it has a higher molar mass
D $(S)$ ; $CHO$ ranks above $CH X_{2} OH$ because its carbon is attached to (O,O,H)

Answer: A

Priorities at C2:

OH

first. Between the two carbon groups, the

CHO

carbon has the duplicated set (O,O,H) while the

CH X_{2} OH

carbon has (O,H,H); the first point of difference favours

CHO

H

is last. With

H

away from the viewer the sequence

OH \to CHO \to CH X_{2} OH

is clockwise in the drawing, so the compound shown is

(R)

-glyceraldehyde (D-glyceraldehyde).