L-Threonine is (2S,3R)-2-amino-3-hydroxybutanoic acid. It is drawn as a Fischer projection with COOH (C1) at the top and CH3 (C4) at the bottom. Which placement of the NH2 group on C2 and of the OH group on C3 is correct? (Priorities: at C2, NH2 > COOH > C3 > H; at C3, OH > C2 > CH3 > H.)

NH2 on the left at C2 and OH on the right at C3

NH2 on the left at C2 and OH on the left at C3

NH2 on the right at C2 and OH on the right at C3

NH2 on the right at C2 and OH on the left at C3

JEE Advanced levelSingle correct optionDifficulty 3 of 3st105

L-Threonine is (2S,3R)-2-amino-3-hydroxybutanoic acid. It is drawn as a Fischer projection with $COOH$ (C1) at the top and $CH X_{3}$ (C4) at the bottom. Which placement of the $NH X_{2}$ group on C2 and of the OH group on C3 is correct? (Priorities: at C2, $NH X_{2}$ > $COOH$ > C3 > H; at C3, OH > C2 > $CH X_{3}$ > H.)

L-Amino acids have

NH X_{2}

on the left at C2, and the descriptor S at C2 confirms it. At C3 the top group is C2 (priority 2), the bottom is

CH X_{3}

(priority 3) and H is horizontal. With OH on the right, OH, then C2, then

CH X_{3}

runs anticlockwise, reversed to clockwise because H is horizontal, so R. OH on the right is therefore (3R).