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Unformatted text preview: rotation for the sample is then read from the digital display.
The value of the rotation is dependent upon the number of molecules of the optically active
compound in the path of the plane polarized light. Each molecule will rotate the plane of
polarized light by the same number of degrees. The greater the number of the molecules, the
higher the reading will be. The specific rotation is corrected for concentration effects. To
calculate the specific rotation for a particular compound, the observed rotation is divided by the
concentration in g/ml and the length of sample tube in decimeters:
[α]Dt = α / cl where α is in degrees of rotation (+/-), c is in g/ml, and l is in dm. The temperature (t) of the measurement is also noted as a superscript outside the brackets above
the wavelength (denoted as D if the sodium D line is used). Example: A sample of one of the enantiomers of 2-butanol gives an observed
rotation of +6.5° when measured in a 10 cm long sample cell. If the
concentration of the solution was 0.5 g/ml, what is the specific rotation
of this enantiomer of 2-butanol? What is the specific rotation of the
[α]Dt = α / cl = (+6.5°) / (0.5)(1) = +13° (sample measured)
Since enantiomers have equal, but opposite specific rotations, the
specific rotation of the enantiomer must be -13°. I f the specific rotations of two enantiomers are measured separatel...
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This note was uploaded on 03/06/2012 for the course CHEM 301 taught by Professor Sahli during the Spring '07 term at VCU.
- Spring '07