Properties of Enantiomers

Optical activity is an experimentally useful property and usually is measured as the angle of rotation (α) of the plane of polarization of polarized light passing through solutions of the substances under investigation (Figure 5-4). Where measurable optical activity is present, it is found that one enantiomer rotates the plane of polarization in one direction, whereas the other causes the plane to rotate equally but in the opposite direction. With reference to the plane of incident light, the enantiomer that rotates the plane to the right is called dextrorotatory and is symbolized by either d or (+); the enantiomer that rotates the plane to the left is levorotatory, symbolized by l or (−). A racemic mixture then can be designated as dl or (±), and will have no net optical rotation. It is very important to know that d, l, (+), or (−) do not designate configurations. Thus, although (+)-2-butanol actually has configuration 5 and (−)-2-butanol has configuration 6, there is no simple way to predict that a particular sign of rotation will be associated with a particular configuration. Methods used in assigning the true configurations to enantiomers will be discussed later.

Figure 5-4: Schematic representation of the rotation of the plane of polarization of polarized light by an optically active compound. Plane-polarized light is different from ordinary light in that its electrical component vibrates in a plane rather than in all directions. The angle αα is the angle between the plane of polarization of light entering the sample and the plane of polarization of the emerging light.

A very important point to keep in mind about any pair of enantiomers is that they will have identical chemical and physical properties, except for the signs of their optical rotations, with one important proviso: All of the properties to be compared must be determined using achiral reagents in a solvent made up of achiral molecules or, in short, in an achiral environment. Thus the melting and boiling points (but not the optical rotations) of 5 and 6 will be identical in an achiral environment.

In the older literature, chiral centers often are called asymmetric centers and you may be confused by the difference between asymmetric and dissymmetric. Both asymmetric and dissymmetric molecules (or objects) are chiral. An asymmetric object has no symmetry at all and looks different from all angles of view. Formulas 3 and 44 represent asymmetric molecules. A dissymmetric molecule is chiral, but looks the same from more than one angle of view. A helical spring is dissymmetric - it looks the same from each end. We will encounter dissymmetric molecules later.

The tetrahedral carbon was first proposed by E. Paterno in 1869 , but he apparently did not recognize its implications for chirality. These implications were recognized first by van't Hoff and Le Bel, with van't Hoff proceeding on the basis of bonds to carbon being directed to the corners of a regular tetrahedron. Le Bel was opposed to such a rigid formulation of the bonds to carbon.

33An interesting account and references to van't Hoff's early work can be found in "The Reception of J. H. van't Hoff's Theory of the Asymmetric Carbon" by H. A. M. Snelders, J. Chem. Educ. 51, 2 (1974). A century has passed since van't Hoff first published his theory, which he did before he obtained his doctoral degree from the University of Utrecht. van't Hoff was the first recipient of the Nobel Prize in chemistry (1901) for his later work in thermodynamics and chemical kinetics.