Which of the following is the correct structure of L alanine?
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A
α-Amino acids have an amino group and a carboxyl group on the same carbon: \( \ce{H2N-CH(R)-COOH} \). Alanine has R = CH3. Except glycine, all α-amino acids are optically active, and their configuration is named D or L by comparison with glyceraldehyde.
In a Fischer projection, the –COOH group is written at the top and the side chain R at the bottom. Then:
L-amino acid: the –NH2 group is on the left.
D-amino acid: the –\( \ce{NH2} \) group is on the right.
Structure A has COOH at the top, CH3 at the bottom and NH2 on the left, so it is L-alanine. Naturally occurring amino acids in proteins mostly have the L configuration.
Why the other options are incorrect
Option (2) B — Incorrect. The side chain is \( \ce{CH2OH} \), so this is L-serine, not alanine.
Option (3) C — Incorrect. This carries a CHO group and no \( \ce{NH2} \) group; it is a sugar (glyceraldehyde), not an amino acid.
Option (4) D — Incorrect. This also has CHO, OH and \( \ce{CH2OH} \) groups and no amino group; it is glyceraldehyde, not alanine.
NCERT (2026-27 edition):
Classification of Amino Acids – …
