Optical isomerism 3.
Optical rotation
Chiral chemical compounds are able to rotate the plane of polarization of a beam of plane-polarized light. This is known as optical activity. Optical rotation is measured with an instrument called polarimeter. A polarimeter comprises a source of monochromatic light, a polarizing filter, a tube containing a solution of the compound to be analysed, and the analysator (a second rotatable polarizing filter). On placing the sample into the instrument the plane of polarized light rotates and becomes dark from the point of view of the observer. The second polarizer is rotated a given angle α until the light is transmitted again.
The observed rotation α is a function of concentration, length of the sample tube, wavelength of light, temperature. To eliminate these, the following eqution is used to calculate the [α] specific rotation:
[α] = 100 * α / l * c
c: concentration of the solution in grams/100 ml, l: length of the tube in decimeters.
If the molecule rotates the polarized light clockwise, the rotation is defined as positive and called dextrorotatory, the counterclockwise direction is defined as negative, levorotatory. Earlier these were marked as d and l, nowadays (+) and (-) are the official prefixes.
Enantiomers rotate plane-polarized light to equal and opposite extents. An optically pure compound contains only one enantiomer. If both isomers are present, this lessens the rotation. The optical rotation of 1:1 mixtures (racemates) is 0°.
The optical purity or enantiomeric excess (e.e.) is defined as:
optical purity = % enantiomeric excess = % enantiomer(1) - % enantiomer(2) = 100 * [α]mixture / [α]pure compound
e.e. = 100 * ([R]-[S]) / ([R]+[S]), where [R] and [S] are the concentrations of the two isomers.
Excercise:
The specific rotation of (+)-asparagic acid is +25,4°. The observed rotation of the mixture of isomers is +7,62°. Calculate the optical purity of the mixture.
Calculate the % ratio of enantiomers.