Which amino acid does not have a chiral α-carbon?

The correct answer is Option (3) → Glycine

A carbon is chiral (asymmetric) when it is joined to four different groups. The α-carbon of an α-amino acid, \( \ce{R-CH(NH2)-COOH} \), carries H, –NH₂, –COOH and the side chain R, so it is chiral unless R is also a hydrogen atom.

Option (1) Alanine — Has a chiral α-carbon. R = \( \ce{-CH3} \), so the α-carbon has H, –NH₂, –COOH and \( \ce{CH3} \): four different groups.

Option (2) Valine — Has a chiral α-carbon. R = \( \ce{(CH3)2CH-} \) is a fourth different group.

Option (3) Glycine — Does not have a chiral α-carbon. In \( \ce{H2N-CH2-COOH} \) the side chain R is a hydrogen atom, so the α-carbon carries two H atoms and glycine is optically inactive.

Option (4) Leucine — Has a chiral α-carbon. R = \( \ce{(CH3)2CH-CH2-} \) is a fourth different group.

NCERT (2026-27 edition):
10.2.2 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.”