4.5 : Proprietà degli enantiomeri e attività ottica

A chiral object and its mirror image, such as our feet, interact differently with other chiral objects, such as a pair of shoes.

For example, our left and right feet can only fit in the left and right shoes, respectively, and not vice versa. In contrast, an achiral object such as a sock can be worn equally well on either foot.

Similarly, enantiomers of a molecule exhibit different properties only when they interact with other chiral media.

For instance, enantiomers interact differently with plane-polarized light, a phenomenon known as optical activity.

Polarized light has electric field vectors oscillating in a single plane, which is rotated by a certain amount when the polarized light passes through a solution of an enantiomer.

Here, the polarized light can be construed as a superposition of chiral left- and right-handed circular polarizations of light. When the polarized light passes through the solution, the enantiomer molecules interact more with one circular polarization. This results in the rotation of the polarized light in a specific direction.

For example, (R)-2-butanol rotates the plane in the counterclockwise direction and is called laevorotatory. The other enantiomer, (S)-2-butanol, rotates the plane in the clockwise direction and is called dextrorotatory.

At a given temperature, the degree of observed rotation of a solution of an enantiomer depends on the specific rotation of the enantiomer, the concentration of the enantiomer, and the pathlength of the cell.

Enantiomers, such as (R)-2-butanol and (S)-2-butanol, have the same magnitude of specific rotation but with opposite signs. As such, an equimolar mixture of enantiomers exhibits no net rotation of polarized light. Such a mixture is referred to as a racemic mixture.

The observed rotation from a sample can be used to calculate the relative abundance of one enantiomer over the other, defined as enantiomeric excess or ee. While a pure solution of one enantiomer has an ee of 100%, racemic mixtures have an ee of 0%.