Question

Why is glycine not optically active?

Short answer

Glycine is not optically active because it is an achiral molecule. This is due to the presence of a plane of symmetry in its molecular structure, with the α carbon bonded to two groups that are the same, namely a hydrogen atom (H) and a methylene group (CH_2). As the α carbon is not bonded to four unique groups, there is no chiral center, thus glycine cannot rotate the plane of polarized light.

Step-by-step solution

Understand optical activity in compounds

Optical activity is the ability of some molecules to rotate the plane of polarized light. These molecules, called chiral molecules, typically have a carbon atom bonded to four different groups. Compounds with such carbons are called optically active and the carbons are referred to as chiral centers, stereocenters, or asymmetric centers. A molecule is achiral (not optically active) if it has a plane of symmetry or a center of symmetry, meaning that it can be superimposed on its mirror image.

Examine the molecular structure of glycine

Glycine has the general structure of an α-amino acid, where an -NH2 (amine) group and a -COOH (carboxylic acid) group are both attached to a central carbon (α carbon). The structure of glycine can be represented as HOOC-CH_2-NH_2. In this structure, the α carbon is attached to the two functional groups mentioned above, as well as a hydrogen atom (H) and a methylene group (CH_2), which has two hydrogens attached to it.

Identify any planes or centers of symmetry

The α carbon serves as a symmetry center in glycine, as it is bonded to groups -NH_2, -COOH, -H, and -CH_2. If we observe the molecule closely, we'll notice that the central carbon is essentially symmetric. The molecule is non-chiral because the methylene group (-CH_2) can be superimposed on its mirror image, and the molecule has a plane of symmetry passing through the functional groups and the central atom. This symmetry makes glycine an achiral compound.

Conclude why glycine is not optically active

Since glycine has a plane of symmetry and the α carbon is not bonded to four different groups, it is an achiral molecule. As a result, glycine is not optically active, meaning it cannot rotate the plane of polarized light.