Which one of the following statements is incorrect regarding amino acids?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Except glycine and alanine, all other naturally occurring α-amino acids are optically active.
An α-amino acid has the general formula \( \ce{R-CH(NH2)-COOH} \). The α-carbon carries four groups: H, \( \ce{NH2} \), COOH and the side chain R. A molecule is optically active when this carbon is chiral, that is, when all four groups are different.
Glycine (R = H): the α-carbon carries two H atoms, so it is not chiral. Glycine is optically inactive.
Alanine (R = \( \ce{CH3} \)): the four groups H, \( \ce{CH3} \), \( \ce{NH2} \) and COOH are all different, so it is chiral and optically active.
So glycine is the only naturally occurring α-amino acid that is optically inactive.
Option (1) Amino acids are classified as acidic, basic or neutral amino acids. — A correct statement about amino acids. The class depends on the relative number of amino and carboxyl groups.
Option (2) In zwitter ionic form, amino acids show amphoteric behaviour as they can react both with acids and bases. — A correct statement about amino acids. The –\( \ce{COO^{-}} \) group accepts a proton from an acid, and the –\( \ce{NH3^{+}} \) group gives a proton to a base.
Option (3) Except glycine and alanine, all other naturally occurring α-amino acids are optically active. — The incorrect statement. Alanine has a chiral α-carbon and is optically active; only glycine is not.
Option (4) Amino acids can form peptides or proteins via peptide bonds between them. — A correct statement about amino acids. The –COOH of one amino acid and the –\( \ce{NH2} \) of another join as an amide (–CO–NH–) link, the peptide bond.
NCERT (2026-27 edition):
Classification of Amino Acids – “Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule. …”
Structure of Proteins – “You have already read that proteins are the polymers of α-amino acids and they are connected to each other by peptide bond or peptide linkage. …”