Choose the optically inactive amino acid from the following.

The correct answer is Option (1) → 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 (asymmetric), which makes the amino acid optically active.

The side chains of the given amino acids are:

• Glycine: R = –H

• Alanine: R = –\( \ce{CH3} \)

• Leucine: R = –\( \ce{CH2CH(CH3)2} \)

• Isoleucine: R = –\( \ce{CH(CH3)CH2CH3} \)

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 (2) Alanine — Incorrect. Its α-carbon carries –H, –\( \ce{CH3} \), –\( \ce{NH2} \) and –COOH, four different groups, so it is chiral and optically active.

• Option (3) Leucine — Incorrect. The isobutyl side chain makes the α-carbon chiral, so leucine is optically active.

• Option (4) Isoleucine — Incorrect. Its α-carbon is chiral, and the side chain has a second chiral carbon (the one carrying –\( \ce{CH3} \)), so it is optically active.

NCERT (2026-27 edition):
Classification of Amino Acids – “Except glycine, all other naturally occurring α-amino acids are optically active, since the α-carbon atom is asymmetric. These exist both in ‘D’ and ‘L’ forms. Most naturally occurring amino acids have L-configuration. L-Aminoacids are represented by writing the –NH₂ group on left hand side.”

Note: The same/similar question has been asked in different years by NTA as per the details given below: