Unlike diastereomers, enantiomers cannot be separated based on their physical properties (boiling points, solubilities, infrared spectra, and so on). The only way that enantiomers differ from each other is their behavior toward plane-polarized light. When a beam of plane-polarized light is passed through a solution of a single enantiomer, the plane of polarization rotates. Additionally, separate enantiomers will rotate the plane of light in equal but opposite directions. From the point of view of the observer, these directions are clockwise and counterclockwise. Enantiomers that rotate the light clockwise are designated dextrorotatory and labeled D or (+). Enantiomers that rotate the light counterclockwise are designated levorotatory and labeled L or (-). While these designations are related to the enantiomers' chirality, there is no correlation between these designations and the R,S naming system. An (R)-enantiomer can be the dextrorotatory enantiomer in one case and the levorotatory enantiomer in another.Enantiomers that rotate light are optically active compounds. An optically active compound is a substance that can rotate the plane of polarized light that is passed through it. An optically inactive compound is a substance that is not able to rotate the plane of polarized light that is passed through it. The rotation of plane-polarized light (light waves that vibrate only in one plane) by solutions of optically active compounds is measured in degrees by a polarimeter. A polarimeter is an instrument used to measure the effect of optically active compounds on plane-polarized light. This instrument is made up of a light source, a polarizer, a tube for holding the optically active substance, and a detector for measuring the angle (in degrees) that the plane-polarized light is rotated by the optically active substance.
Example of a Polarimeter
Since the observed rotation is positive and the specific rotation of the R isomer is negative, the mixture is mostly S.Use the Enantiomeric Excess formula to determine the %ee. Substitute the specific rotation observed in an experiment, (observed), with the 2-butanol observed specific rotation, +8.7º. Then substitute the accepted value of the specific rotation of a pure sample, (pure enantiomer), with (R)-2-butanol’s specific rotation, –13.5º.
Examine the results of the calculation. Determine the percentage of pure S.The calculation shows that the mixture is 64% pure S.
Determine the percentage of R and S.The rest of the mixture is a 50/50 mixture of R and S.