Practice Problems 605566

1. A mixture contains 3g of (+)-2-bromobutane and 2g of (–)-2-bromobutane. What is the rotation of the mixture, given that (+)-2-bromobutane has a specific rotation of +23.1°. Ans: +4.6° 2. What is the ee of a solution with a specific rotation of –90o where the pure solution rotates at –135o? Ans: ee 66.7%

3. A sample of 50% optical purity and 0.4 g/mL concentration shows a rotation of 10o when placed in a 5 cm polarimeter tube. Calculate the specific rotation of the minor enantiomer. Ans: -100°

4. The specific rotation of neat (S)-carvone is +61o. The specific rotation of a neat sample of carvone of unknown optical purity was measured -23o. Calculate the enantiomeric ratio and ee. Ans: ee 37.7%; enantiomeric ratio (er) 69:31 5. Sample A is a mixture of two enantiomers. A solution of A is made by dissolving 5.00 g of the sample in enough ethanol to bring the volume of solution to 25.0 mL. Some of the solution is placed in a 10 cm polarimeter cell and its optical rotation is measured at 25°C using light of the sodium D line wavelength (589.6 nm). The observed rotation is - 8.3°. a) What is the specific rotation of this sample? b) An enantiomerically pure sample of the S enantiomer of A has a specific rotation of + 72.3°. What is the % enantiomeric excess of sample A? Ans: a) –41.5°; b) ee 57.4%