A Fischer projection of 2-hydroxypropanoic acid (lactic acid) is drawn with the vertical chain, top to bottom: H, C2, CH3 (here the top vertical group is the hydrogen atom). On C2 the left group is OH and the right group is COOH. What is the configuration of C2?

R only after rotating the projection by 90 deg

the projection is invalid, as the carbon chain must be vertical

JEE Advanced levelSingle correct optionDifficulty 3 of 3st052

A Fischer projection of 2-hydroxypropanoic acid (lactic acid) is drawn with the vertical chain, top to bottom: $H$ , C2, $CH X_{3}$ (here the top vertical group is the hydrogen atom). On C2 the left group is $OH$ and the right group is $COOH$ . What is the configuration of C2?

Priorities:

OH > COOH > CH X_{3} > H

. Here

H

is on a vertical bond, so it points away from the viewer and the apparent sense is the true sense (no reversal).

OH

(left)

\to

COOH

(right)

\to

CH X_{3}

(bottom) is clockwise:

R

. (Equivalently, swapping

H

with a horizontal group inverts the descriptor, and a second swap restores it.)