Amongst the following amino acids, the (R)-enantiomer is represented by · ABC26OR0742
Subject: Organic Chemistry · Chapter: Stereochemistry · Topic: Stereochemistry – General · Exam: GATE 2000-2022 · Marks: 2 · Difficulty: Easy
Amongst the following amino acids, the (R)-enantiomer is represented by
(a)
(b)
(c)
(d)
Answer
Answer: B ✓ checked by 4AB · confidence high
The source book printed A; on checking, B is correct — see the explanation.
Fischer analysis: (a) S, (b) R, (c) S, (d) S. Only (b), D-alanine, is R. The book's printed key for this chapter also reads 1. (b). The stored key (a) is S-alanine.
Explanation
Priorities: NH₂ > COOH > CH₃ > H.
(a) H is on the vertical bond. NH₂ (left) → COOH (right) → CH₃ (top) runs anticlockwise, so (a) is S.
(b) H is on a horizontal bond. NH₂ (right) → COOH (top) → CH₃ (bottom) runs anticlockwise; reversing because H is horizontal gives R.
(c) H is horizontal. NH₂ (top) → COOH (bottom) → CH₃ (left) runs clockwise; reversing gives S.
(d) H is vertical. NH₂ (right) → COOH (top) → CH₃ (left) runs anticlockwise, so (d) is S.
Only (b), which is D-alanine, is the (R)-enantiomer.
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