Optical Activity

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Chapter: Organic Chemistry : Stereochemical and Conformational Isomerism

We have seen how stereochemical relationships can be designated and distinguished. Now let us see how the stereochemistry influences the chemical and/or physical properties of molecules.


OPTICAL ACTIVITY

We have seen how stereochemical relationships can be designated and distinguished. Now let us see how the stereochemistry influences the chemical and/or physical properties of molecules.

Individual pure enantiomers are identical to each other in most respects in that they have the same physical properties, melting point (m.p.), boiling point (b.p.), refractive index, polarity, and solubility. The only difference between indi-vidual enantiomers is that they behave differently in chiral environments. For example, each enantiomer of an enantiomeric pair produces a rotation of the plane of plane-polarized light to an equal but opposite extent. This is because plane-polarized light is itself chiral and each enantiomer interacts differently with the light (Figure 6.1).


If a compound rotates plane-polarized light, it is termed optically active. To be optically active, a compound must be chiral and one enantiomer of the compound must be present in excess over its mirror image. The enantiomer which produces clockwise rotation of plane-polarized light is designated the positive enantiomer, and the enantiomer which produces counterclockwise rotation of plane-polarized light is designated the negative enantiomer. At a given wavelength under standard conditions of concentration (1 g/mL) and pathlength (10 cm), a pure enantiomer will give the maximum rotation in degrees. Its pure mirror image will give an equal but opposite rotation.

The rotation of plane-polarized light by a pure enantiomer is an inherent prop-erty of that enantiomer. However, the amount of rotation actually measured is dependent on the concentration of molecules in the light beam, the pathlength, and the wavelength of the light used. To account for these variables, the observed rotation is converted to the specific rotation [α], which is defined as the rota-tion observed for a solution of 1 g/mL concentration in a 10-cm-pathlength cell. Furthermore a wavelength of 589 nm, the D line of sodium, is normally the standard wavelength for measuring the specific rotation and 25 C is the standard temperature of the measurement. The specific rotation [α]25D, in which the super-script indicates the temperature of the measurement and the subscript D (or number) indicates the wavelength of light used to measure the optical rotation, is calculated from


where αobs is the observed rotation, l is the pathlength of the cell used for the measurement, and C is the concentration of the sample in grams per milliliter.

When both enantiomers are present in solution, the observed rotation will reflect the enantiomeric composition of the mixture. If equal amounts of enan-tiomers are present, the solution will not exhibit optical activity, because for each molecule that rotates light in one direction there will be another molecule that rotates light in the opposite direction and the net rotation is zero. Such a mixture is called a racemic mixture and is indicated by (±). Thus (±)-2-butanol is an equal mixture of the R and S enantiomers of 2-butanol. In the liquid state, racemic mixtures have the same physical properties as the individual enantiomers.

If one enantiomer is present in excess over the other, then the solution will have a net rotation corresponding in sign (+ or ) to that of the more abundant enantiomer. The composition of the mixture is denoted by the optical purity or the percent enantiomeric excess (ee%). The enantiomeric excess is defined as ee = % major enantiomer % minor enantiomer and is a measure of the optical purity of the sample. Values range from 100% (pure enantiomer, ee = 100% 0%) to 0% (racemic mixture or ee = 50% 50%). A sample which has an optical purity of 92% is thus a mixture of 96% of one enantiomer and 4% of the other enantiomer.

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