An enantiomer is one of a pair of stereoisomers that are non-superimposable mirror images. Optical activity is the ability of a substance to rotate the plane of plane-polarized light; the two enantiomers rotate it equally in opposite directions.
Why Enantiomers Form
Enantiomers arise from chirality, meaning that a molecule and its mirror image cannot be superimposed. In IB Chemistry, this commonly occurs when a tetrahedral carbon atom is bonded to four different atoms or groups; this atom is called a chiral carbon or stereogenic centre.
For example, a carbon bonded to H, OH, CH₃ and CH₂CH₃ has four different substituents. Its two possible three-dimensional arrangements are mirror images, but rotating either structure cannot make it coincide with the other.
Enantiomers generally have identical physical properties in an achiral environment, including melting point and boiling point. However, they differ in their effects on plane-polarized light and may interact differently with other chiral substances, such as enzymes.
| Sample | Effect on plane-polarized light |
|---|---|
| One pure enantiomer | Rotates the plane clockwise, designated (+), or anticlockwise, designated (-) |
| Its opposite enantiomer | Produces an equal rotation in the opposite direction |
| 1:1 mixture of both enantiomers | Shows no net rotation because the effects cancel |
An equal mixture of two enantiomers is a racemic mixture and is optically inactive. A polarimeter measures the direction and angle of optical rotation.
A common misconception is that the three-dimensional configuration automatically predicts the direction of rotation. It does not: the (+) or (-) sign must be determined experimentally and should not be inferred from a structural label.
Exam Technique
This is AHL content within Structure 3.2. In an IB response, identify the stereogenic centre, state that four different groups are attached, and explain that the mirror images are non-superimposable. If optical activity is asked, include both the rotation of plane-polarized light and the cancellation in a racemic mixture.
