Which amino acid is optically inactive?

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

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

• Serine: R = –\( \ce{CH2OH} \)

• Histidine: R = –CH₂–(imidazole 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) Leucine — Incorrect. Its α-carbon carries four different groups, so it is chiral and optically active.

• Option (3) Serine — Incorrect. The –\( \ce{CH2OH} \) side chain makes the α-carbon chiral.

• Option (4) Histidine — Incorrect. The imidazolylmethyl side chain makes the α-carbon 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: