racemic
Summary
A racemic mixture contains equal amounts of two enantiomers, written (±), so the optical rotations cancel and the mixture is optically inactive. It typically forms when a stereocenter is created from a planar, achiral intermediate, as in SN1 at a carbocation.
Detail
Meaning: A racemic mixture (racemate) contains equal amounts of two enantiomers. Enantiomers rotate plane-polarized light by equal amounts in opposite directions, so the rotations cancel and the observed rotation is 0°. Each molecule is still chiral; the mixture is inactive only because of the cancellation. Racemic products form when a stereocenter is created with no chiral influence, such as a nucleophile attacking a planar carbocation (SN1) or a planar carbonyl carbon. Both faces are equally likely to be attacked.
Equation: ee = |R − S| / (R + S) × 100%, where R and S are the amounts (mol) of each enantiomer and ee is the enantiomeric excess, a dimensionless percentage. A racemic mixture has ee = 0%.
On the MCAT: A passage shows a reaction that makes a new stereocenter from achiral reactants, or an SN1 at a chiral carbon, and asks whether the product is optically active (it is not). Resolution of a racemate uses a chiral reagent to make diastereomers, which differ in physical properties, or chiral chromatography.
Common confusion: Racemic is not meso. A meso compound is one achiral molecule with an internal mirror plane. A racemate is a mixture of two chiral molecules. Both show no net rotation. SN2 at a stereocenter gives inversion, not racemization.
Sources
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
- Organic Chemistry (Clayden)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.