Match each Fischer projection in List I with its configuration in List II. In every projection the vertical chain is listed top to bottom and the single stereocentre carries the stated left and right groups. List I: (P) $CHO$ , C, $CH X_{2} OH$ ; left $OH$ , right $H$ . (Q) $COOH$ , C, $CH X_{2} SH$ ; left $NH X_{2}$ , right $H$ . (R) $CH X_{3}$ , C, $CH X_{2} CH X_{3}$ ; left $H$ , right $Cl$ . (S) $COOH$ , C, $CH X_{3}$ ; left $H$ , right $NH X_{2}$ . List II: (1) $R$ ; (2) $S$ .

Answer: A

In each projection

H

is on a horizontal bond, so the apparent sense is reversed. (P)

OH > CHO > CH X_{2} OH

OH

(left)

\to

top

\to

bottom is clockwise, so

S

. (Q)

NH X_{2} > CH X_{2} SH

(sulfur beats oxygen)

> COOH > H

NH X_{2}

(left)

\to

CH X_{2} SH

(bottom)

\to

COOH

(top) is anticlockwise, so

R

. (R)

Cl > CH X_{2} CH X_{3} > CH X_{3}

Cl

(right)

\to

bottom

\to

top is clockwise, so

S

. (S)

NH X_{2} > COOH > CH X_{3}

NH X_{2}

(right)

\to

top

\to

bottom is anticlockwise, so

R

. Match: P-2, Q-1, R-2, S-1.