The specific rotation for a molecule is also dependent on the wavelength of the plane polarized light. A common light source for simple polarimeters is a lamp with enhanced output at the sodium D-line"; in this instance, the specific rotation would be shown as [a] Sample problem A pure enantiomer has an observed optical rotation of -0 82 when measured in a one dm tube at a concentration of 0.3 g/10 mL. Calculate the specific rotation for this molecule Solution The concentration of 0.3 g/10 mL is equivalent to 0.03 g/mL; c=0.03 g/mL The length of the analysis tube is one dm: 1=1.0 dm The specific rotation is therefore [a]=082)103gmLx10dm) [a]=-27 3"- dr If the specific rotation of a pure enantiomer is known, the observed rotation can also be used to calculate optical purity, or the level of contamination of one compound with its enantiomer, using the simple convention optical purity =( of one enantiomer )-(% of the other enantiomer) Sample problem The specific rotation for a pure enantiomer is known to be -39 g mL- dmr. A sample containing both enantiomers is found to have an observed rotation of 0. 62. in a one dm tube at a concentration of 3. 5 g/100 mL. What is the optical purity of the sample Solutio For this sample, the apparent specific rotation is [a]=-17 7 g"' mL- drThe specific rotation for a molecule is also dependent on the wavelength of the plane polarized light. A common light source for simple polarimeters is a lamp with enhanced output at the "sodium D-line"; in this instance, the specific rotation would be shown as []D. Sample problem: A pure enantiomer has an observed optical rotation of -0.82o when measured in a one dm tube at a concentration of 0.3 g/10 mL. Calculate the specific rotation for this molecule. Solution: The concentration of 0.3 g/10 mL is equivalent to 0.03 g/mL; c = 0.03 g/mL The length of the analysis tube is one dm; l = 1.0 dm The specific rotation is therefore: [] = -0.82o) /(0.03 g/mL x 1.0 dm) [] = -27.3o g -1 mL-1 dm-1 If the specific rotation of a pure enantiomer is known, the observed rotation can also be used to calculate optical purity, or the level of contamination of one compound with its enantiomer, using the simple convention: optical purity = (% of one enantiomer) - (% of the other enantiomer) Sample problem: The specific rotation for a pure enantiomer is known to be -39o g -1 mL-1 dm-1. A sample containing both enantiomers is found to have an observed rotation of -0.62o in a one dm tube at a concentration of 3.5 g/100 mL. What is the optical purity of the sample? Solution: For this sample, the apparent specific rotation is: [] = -0.62o) /(0.035 g/mL x 1.0 dm) [] = -17.7o g -1 mL-1 dm-1