Q.Among the following the achiral amino acid is (AIIMS)

Step 1. A carbon is a stereocentre (chiral) only if it is bonded to four DIFFERENT groups; an amino acid is achiral only if its α-carbon fails this test, i.e. carries two identical substituents.

Step 2. Check each option's α-carbon. (a) 2-ethylalanine: alanine's α-carbon (bonded to CH₃, NH₂, COOH, H) gains an ethyl group in place of the H, giving four different groups (CH₃, C₂H₅, NH₂, COOH) -- chiral. (c) 2-hydroxymethylserine: serine's α-carbon already carries NH₂, COOH, H and a -CH₂OH side chain; adding a further hydroxymethyl group still leaves four different substituents -- chiral. (d) Tryptophan: a standard proteinogenic amino acid with an indolylmethyl side chain, NH₂, COOH and H on its α-carbon -- four different groups, chiral. …

For each option, draw the α-carbon's four substituents and check whether any two are identical; two identical substituents make …

• Taking '2-methylglycine' at face value as literally one methyl added to glycine -- that description actually yields alanine (chiral). The option is a known naming inconsistency carried over from the adapted exam question, whose original intended compound (2-amino-2-methylpropanoic acid / α-amino …