A racemic mixture is an equimolar mixture of two enantiomers, meaning it contains equal amounts of each non-superimposable mirror-image form. It is optically inactive because the enantiomers rotate plane-polarized light equally in opposite directions, so their rotations cancel.
Why Racemic Mixtures Form
Enantiomers exist when a molecule is chiral, usually because it contains a carbon atom bonded to four different groups. The two enantiomers have the same connectivity but different three-dimensional arrangements.
A racemic mixture can form when a reaction produces a new chiral centre through a trigonal planar intermediate. For example, electrophilic addition of hydrogen bromide to but-1-ene may form a planar secondary carbocation. The bromide ion can attack either face of this carbocation with equal probability under achiral conditions, producing equal amounts of the two enantiomers of 2-bromobutane.
Each enantiomer is optically active on its own. If one rotates plane-polarized light by a particular angle clockwise, the other rotates it by the same angle anticlockwise.
| Sample | Composition and optical effect |
|---|---|
| Pure enantiomer | Contains one enantiomer and rotates plane-polarized light |
| Racemic mixture | Contains a 1:1 molar ratio of enantiomers; net rotation is zero |
| Achiral compound | Does not have non-superimposable mirror images; net rotation is zero |
The common misconception is that molecules in a racemic mixture are individually optically inactive. This is incorrect: each enantiomer is optically active, but their opposite rotations cancel at the macroscopic level.
IB Exam Technique
This is HL content linked to Structure 3.2 and stereoisomerism. For an “explain” question, state the 1:1 ratio, identify the molecules as enantiomers, and explain that equal and opposite optical rotations produce zero net rotation. If asked how a racemic mixture forms, refer to equal attack on both sides of a planar intermediate rather than simply stating that “two products form.”