# 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** --- ## 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. --- ### 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: $-H$, $-OH$, $-CH(NH_2)(CH_2)_4-$, and $-(CH_2)_4CH(NH_2)-$. 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. --- ## 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:** | cis-Isomers | trans-Isomers | | :---: | :---: | | **cis-but-2-ene** | **trans-but-2-ene** | |

| | **cis-1,3-dimethylcyclopentane** | **trans-1,3-dimethylcyclopentane** | |