Answer the question on basis of the passage given below:
In zwitter ionic form, amino acids show amphoteric behaviour as they react both with acids and bases.
Except glycine, all other naturally occurring \(\alpha \)- amino acids are optically active, since the \(\alpha \)-carbon atom is asymmetric. These exist both in 'D' and 'L' forms. Most naturally occurring amino acids have L-configuration. L-amino acids are represented by writing the \(NH_2\) group on the left hand side.
Optically inactive amino acid from the following is:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Glycine
All α-amino acids have the general formula \( \ce{H2N-CH(R)-COOH} \). The α-carbon carries four groups: –\( \ce{NH2} \), –COOH, –H and the side chain R. A carbon with four different groups is chiral, which makes the amino acid optically active.
The side chains of the given amino acids are:
Glycine: R = –H
Valine: R = –\( \ce{CH(CH3)2} \)
Alanine: R = –\( \ce{CH3} \)
Proline: the side chain forms a ring with the α-amino nitrogen (pyrrolidine ring)
In glycine, \( \ce{H2N-CH2-COOH} \), the α-carbon carries two hydrogen atoms, so it is not chiral. Glycine is the only common α-amino acid that is optically inactive.
Why the other options are incorrect
Option (1) Valine — Incorrect. The isopropyl side chain makes the α-carbon carry four different groups, so it is chiral and optically active.
Option (3) Alanine — Incorrect. Its α-carbon carries –\( \ce{NH2} \), –COOH, –H and –\( \ce{CH3} \), four different groups, so it is chiral.
Option (4) Proline — Incorrect. Its α-carbon is part of a ring and still carries four different groups (ring N, ring \( \ce{CH2} \), –COOH and –H), so it is chiral.
NCERT (2026-27 edition):
Classification of Amino Acids – …
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 06-06 S1 29-05 S1 | – | – | 13-06 S3 12-06 S2 | – |