Racemic Mixtures

Edexcel A-level ChemistryChiralityLesson 4 of 6

A racemic mixture (or racemate) is a mixture containing equal amounts of the two enantiomers of a chiral compound.

A racemic mixture is optically inactive: it does not rotate plane-polarised light. Each enantiomer rotates the plane by the same angle in opposite directions, so the two effects cancel out. The individual molecules are still chiral; it is the mixture as a whole that shows no rotation.

Worked example: a solution of one pure enantiomer rotates plane-polarised light by 20° clockwise. A racemic mixture at the same total concentration gives a rotation of 0°. Now consider a mixture of 75% of the clockwise enantiomer and 25% of the other. The 25% of the anticlockwise form cancels 25% of the clockwise form, leaving a 50% excess of the clockwise enantiomer. The rotation is 50% of 20° = 10° clockwise.

More in Chirality

• Chirality
• Optical Isomers
• Plane-Polarised Light
• Racemates from Nucleophilic Addition
• Identifying Chiral Centres

All 6 lessons in Chirality · All Edexcel A-level Chemistry topics