New Methods of Optical Rotation

Two valuable methods for characterizing organic compounds by optical rotation have been developed (1) the monochromatic-rotation predictions of WhilTen and Brewster, and (2) the optical rotatory dispersion methods of Djerassi. As Eliel has pointed out, these methods are complementary, as optical rotatory dispersion is mainly useful for compounds chromophoric in the near ultraviolet, and rotation predictions are principally of use for saturated compounds. 

The rotatory contribution of an individual molecule depends, presumably, on two fixed properties, structure and configuration, and on two time-variable properties, conformation and orientation with respect to the incident beam of light. The observed rotation of a compound is the statistical summation of molecular contributions. Probably, even inactive molecules (with rare exceptions) make non-zero contributions in most conformations and orientations. 

In the Whiflen and Brewster approaches, the conformation is explicitly considered. The partially successful, earlier rotation rules for carbohydrates ignored conformation that the rules hold may be attributed to molecular rigidity or to accidental preponderance of suitable conformations of the compounds considered. 

For many cyclitols, a convenient, short-cut version of the Whiffen-Brewster calculations may be used, namely (1) consider the most stable chair conformation of the molecule, for which purpose, Dreiding stereomodels are helpful (2) then assume that each ee or ea —CHOH—CHOH— grouping contributes -t-45 if it is in the front-left conformation (149), and —45° if it is in the front-right conformation (143). Each diaxial —CHOH—CHOH— grouping, and each isolated asymmetric center, makes a contribution of zero. The sum of the numbers is then equal to the molecular rotation predicted (for water and the d line of sodium). 

As shown in Table XII, agreement between predicted and found values is usually good for polyhydroxycyclohexanes. The precision is often poor when substituents other than the hydroxyl group are present, but, even then, the trend for any two stereoisomers is usually in the correct direction. 

---