Question

Explain why all the amino acids except for glycine are chiral.

Short answer

All the standard amino acids except for glycine are chiral because they all have four different substituents attached to their carbon (C_α) atom.

Step-by-step solution

Definition of chiral molecules

The molecules that are non-superimposable on their mirror images are called as chiral molecules and the central atom of such molecules are called chiral centers or asymmetric centers.The α-carbon of the standard amino acids are the chiral centres and are called chiral carbon, because the C_αis attached to 4 different groups that is an amino group (NH₃), hydrogen atom (H), a carboxylic group (COO^-) and a R-group.

Explain the reasoning

Amino acid glycine contains a carboxylic group, an amino group and two hydrogen atoms attached to its central α-carbon atom as shown in the figure below:

Structure of Glycine

Because glycine does not contain 4 different substituents, it is not chiral and is the only exception in all the amino acids.