16.9 Stereoisomerism

Stereoisomerism is a form of isomerism where molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. This phenomenon is also known as configurational isomerism.

There are two main types of stereoisomerism:

1. Optical Isomerism
2. Geometric Isomerism

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1. Optical Isomerism

Optical isomerism is exhibited by molecules that contain a chiral carbon, which is a carbon atom bonded to four different atoms or groups of atoms. A molecule with a chiral carbon is described as being chiral. A chiral carbon is also called an asymmetric carbon because it lacks a plane of symmetry.

Optical isomers share the same molecular formula but have different spatial arrangements of atoms. Their key distinguishing feature is their effect on plane-polarized light.

Polarized Light and Optical Activity

• Unpolarized Light: Ordinary light consists of electromagnetic waves vibrating in all possible planes perpendicular to the direction of propagation.
• Polarized Light: When unpolarized light passes through a polarizer (like in a polarimeter), the emerging light waves vibrate in only a single plane.

A pair of optical isomers, known as enantiomers, rotate the plane of polarized light by equal amounts but in opposite directions (one clockwise, the other anticlockwise). Such compounds are said to be optically active.

Enantiomers and Chirality

Enantiomers are stereoisomers that are non-superimposable mirror images of each other, much like a person's left and right hands. Any object that cannot be superimposed on its mirror image is said to possess chirality.

A classic example is lactic acid, which has one chiral carbon atom.

Molecules with Multiple Chiral Centers

If a molecule contains n chiral centers, it can have a maximum of $2^n$ stereoisomers. For a molecule with two chiral carbons, there can be up to four optical isomers.

• Optically Active Isomers: Isomers I and II are enantiomers (mirror images) and are optically active.
• Optically Inactive Isomers:
  • Meso Compound (Isomer III): This isomer has a plane of symmetry, causing the rotational effects of its two chiral centers to cancel each other out. This is called internal compensation.
  • Racemic Mixture (Isomer IV): This is an equimolar ($50/50$) mixture of dextrorotatory (clockwise) and levorotatory (anticlockwise) enantiomers. The net rotation of polarized light is zero because the effects of the two enantiomers cancel each other out.

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Diastereomers

Diastereomers are stereoisomers that are not mirror images of one another. They arise in molecules containing two or more chiral centers.

For example, in 2,3-dichloropentane, structures I and III are diastereomers, as are I and IV.

Enantiomers, on the other hand, are mirror images of each other, while diastereomers are not.
The difference is illustrated below:

Optical Activity in Cyclic Compounds

Substituted cyclic compounds can also be optically active if they contain asymmetric carbon atoms. For example, in 2-aminocyclohexanol, carbon-1 is chiral because it is attached to four different groups: $-H$, $-OH$, $-CH(N{H}_2)(C{H}_2{)}_4-$, and $-(C{H}_2{)}_{4}CH(N{H}_2)-$.

The optical activity of a substance is measured using an instrument called a polarimeter. A solution of known concentration is placed in the instrument, and the degree and direction of rotation of polarized light are determined.

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2. Geometric Isomerism

Geometric isomerism, also known as cis-trans isomerism, occurs in molecules where there is restricted rotation around a bond. This is common in:

• Unsaturated compounds with a carbon-carbon double bond ($C=C$).
• Substituted cyclic compounds with carbon-carbon single bonds within the ring.

For geometric isomerism to exist, each carbon atom involved in the restricted rotation must be bonded to two different atoms or groups.

• cis isomer: The isomer where identical or high-priority groups are on the same side of the double bond or the ring. (cis is Latin for "on the same side").
• trans isomer: The isomer where identical or high-priority groups are on opposite sides of the double bond or the ring. (trans is Latin for "opposite side").

Examples: