Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Stereochemistry
5.5 Properties of Stereogenic Compounds

Since enantiomers are different molecules, they must have different properties. For a pair of enantiomers, there is only one physical property difference observed and that is their interaction with polarized light. Ordinary light can be thought of as electromagnetic radiation, which consists of electric and magnetic field vectors, which oscillate in all directions. Polarized light, on the other hand, has vectors in only one plane; vectors in the other planes have been filtered out, this light is also known as monochromatic polarized light. When ordinary light is passed through a prism, such as a calcite crystal (CaCO₃) prism, the light that results is polarized light. If polarized light is passed through a solution of an enantiomer of a stereogenic compound, the plane of the polarized light is rotated to the right or to the left depending on the absolute configuration of the enantiomer. Based on this property, enantiomers are also referred to as optically active compounds. This property, specific rotation [α], is measured using a polarimeter and is defined by Eq. (5-1).

(5-1)

One enantiomer of an optically active compounds will rotate the plane of polarized light in one direction and its enantiomer will rotate the plane of polarized light by an equal amount, but in the opposite direction. Optically active compounds that rotate the plane of polarized light to the right (or clockwise) are called dextrorotatory (d) compounds, whereas compounds that rotate the plane of polarized light to the left (or counter clockwise) are called levorotatory (l) compounds. Another method that is often used to differentiate between the enantiomers of optically active compounds is the use of a positive sign (+) for dextrorotatory compounds and a negative sign (−) for levorotatory compounds. This system was an arbitrary assignment by Emil Fischer, a German organic chemist. He arbitrarily assigned the glyceraldehyde enantiomer that rotates the plane of polarized light to the right with a (+) sign and called it the d isomer. Optically active compounds are assigned + or − (or d and l) based on experimental results, and as a result, these experimental assignments do not necessarily correlate with the R and S absolute assignments discussed earlier. Thus, there is no direct relationship between R and S and l and d or R and S and the direction that enantiomers rotate the plane of polarized light. The experimentally determined specific rotation values of glyceraldehyde are shown below.

The specific rotation for an equal mixture of enantiomers will be zero since for every molecule that rotates the plane of polarized light in one direction, its enantiomer will rotate it in the opposite direction. A mixture of equal amounts of enantiomers is called a racemic mixture. Racemic mixtures are optically inactive.