Enantiomers and Optical Activity
Enantiomers have identical physical properties, except for their behavior towards polarized light. One enantiomer rotates the plane of polarized light clockwise, it is dextrorotatory (+). The other enantiomer causes rotation in the counter-clockwise direction, it is levorotatory (-). This phenomenon associated with chiral substances is known as optical activity.
Measurement of Light Rotation
Optical rotation is measured with a polarimeter that consists of a light source, a polarizer that emits light oscillating in a single plane, the cuvette that contains the enantiomer, and an analyzer that allows measuring the light rotation.
Observed and Specific Optical Rotation
The rotation measured in the polarimeter is called observed optical rotation and is represented by α. Its value depends on numerous variables such as temperature, wavelength, concentration, solvent, and type of substance. To avoid these dependencies, specific optical rotation is defined.
Enantiomeric Excess or Optical Purity
When mixing two enantiomers in equal proportion, the optical rotation is null [α]=0, the rotation of the dextrorotatory is compensated by that of the levorotatory (racemic mixture). If enantiomers are mixed in different proportions, the optical rotation can be calculated through the enantiomeric excess or optical purity, which represents the percentage of enantiomer that causes the rotation of light.
