Q.Which of the following amino acids are achiral?

Chirality at the $\alpha$-carbon requires four different substituents; glycine's $\alpha$-carbon carries $-NH_2$, $-COOH$, and two identical $-H$ atoms, so it fails this test and glycine is achiral, unlike proline, alanine, and leucine.

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A chiral carbon must carry four different groups. In glycine, the α-carbon bears two identical H atoms, so it is not chiral.

All standard amino acids have the general structure $H_2N-CH(R)-COOH$, where the α-carbon is bonded to an $-NH_2$ group, a $-COOH$ group, an H atom, and the side chain R. For the α-carbon to be a chiral (asymmetric) centre, all four substituents attached to it must be different.

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The 20 or so standard amino acids that make up proteins are all $\alpha$-amino acids, i.e. the amino group is on the carbon adjacent to the carboxyl group.

In an $\alpha$-amino acid, the $-NH_2$ group is attached to the $\alpha$-carbon (the carbon directly bonded to $-COOH$); these units condense via peptide ($-CONH-$) bonds to form the polypeptide chains of proteins. $\beta$-amino acids and $\alpha$/$\beta$-hydroxy acids are real classes o …