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Chapter 22: Problem 102

Why is glycine not optically active?

Short Answer

Expert verified
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

01

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.
02

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.
03

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.
04

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.

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