Problem 45 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following amino acid is optically inactive? (a) phenyl amine (b) asparagine (c) glutamic acid (d) glycine

Step-by-Step Solution

Answer

Glycine is optically inactive.

Step 1: Understand Optical Activity

Optical activity is a property of chiral molecules. A chiral molecule has a non-superimposable mirror image, typically due to the presence of an asymmetric carbon atom (a carbon atom with four different groups attached to it). Molecules are optically active if they can rotate the plane of polarized light.

Step 2: Identify Chirality in Amino Acids

Examine each of the given amino acids to identify whether they have a chiral center. 1. **Phenyl amine** is a basic aromatic amino compound and may not be chiral by structure. 2. **Asparagine** has a central carbon atom bonded to four different groups, making it chiral. 3. **Glutamic acid** also has a central asymmetric carbon atom, thus is chiral. 4. **Glycine** has two hydrogen atoms attached to the central carbon, making it achiral, meaning it's optically inactive.

Step 3: Determine Optically Inactive Amino Acid

Among the amino acids listed, only glycine lacks a chiral center because its central carbon atom has two hydrogen atoms. Therefore, glycine does not rotate plane-polarized light and is the only optically inactive amino acid in the list.

Key Concepts

Chiral Molecules

Chiral molecules are like left and right hands - they are mirror images of each other but cannot be superimposed. This property is also called "handedness." In chemistry, chirality usually arises when a carbon atom, often found in the center of a molecule, is attached to four different groups. When this happens, the spatial arrangement of these groups becomes unique, creating a molecule that can exist in two non-superimposable forms known as enantiomers. Each form can rotate plane-polarized light differently, a property known as optical activity.
In amino acids, chirality is crucial because it influences how these building blocks of proteins behave in biological systems. For example:

• Chiral amino acids can rotate polarized light to the right or left.
• The specific direction depends on the arrangement of their atoms.
• This property affects how they interact with other chiral molecules in the body.

Achiral Centers

An achiral center, on the other hand, is a part of a molecule that does not result in non-superimposable mirror images. These centers have symmetry, which means they cannot rotate plane-polarized light. The presence of identical groups on a central atom often leads to achirality.
In the context of amino acids, glycine is a perfect example of a molecule that has an achiral center. Glycine is the simplest amino acid and has a hydrogen atom as its side chain, resulting in two hydrogen atoms being attached to its central carbon. This gives the central carbon a symmetry that most other amino acids don't have, making glycine optically inactive. It does not rotate polarized light, which differentiates it from other, chiral amino acids.

Asymmetric Carbon Atom

An asymmetric carbon atom is a carbon atom that is bonded to four different groups or atoms, creating a chiral center in the molecule. This asymmetry is what allows chiral molecules to exhibit optical activity, as their structures cannot be superimposed onto their mirror images.
Here's a simpler way to understand:

• Each of the four attached groups must be different.
• This arrangement prevents the molecule from having a plane or center of symmetry.
• Therefore, it results in two different forms (enantiomers) that rotate light in distinct directions.

In amino acids, except for glycine, you often see an asymmetric carbon at the core of the molecule. For example, asparagine and glutamic acid both have central asymmetric carbon atoms, making them chiral and enabling them to interact uniquely with polarized light. This property is fundamental in the interaction of amino acids with biological molecules.