An optical rotation study gives a result of alpha=+140 degrees. Suggest a dilution experiment to test whether the result is indeed +140 degrees, and not -220 degrees.
-could polarimetry experiment be used to find the r and s isomer?
Yes, I think it can. This problem has been posted before and I suggested diluting the sample by a factor of 10 and measuring the rotation. If the original was indeed 140 degrees, then the new measurement would show 14.0 degrees. If the original measurement was -220 degrees, the new measurement would be -22.0 degrees. There would be no confusion between +14 and -22. Check my thinking.
To test whether the reported optical rotation result of +140 degrees is accurate, you can perform a dilution experiment using the following steps:
1. Prepare a series of dilutions of the sample solution with a known concentration. Ideally, prepare at least three dilutions, each with different concentrations.
2. Measure the optical rotation of each diluted solution using a polarimeter. Make sure to note the angle of rotation for each dilution.
3. Plot a graph with the angle of rotation on the y-axis and the concentration on the x-axis.
4. If the reported optical rotation result of +140 degrees is accurate, the graph should show a linear relationship between the concentration and the angle of rotation. The slope of the line should correspond to the specific rotation (alpha) value.
5. Compare the plotted line with the expected linear relationship for a positive optical rotation. If the slope of the line matches the expected positive rotation, it confirms that the reported result of +140 degrees is correct.
Regarding the question about using polarimetry to distinguish between the R and S isomers, it is not possible to directly determine the isomeric identity using solely optical rotation measurements. Optical rotation is a general property of chiral molecules, and it cannot specifically differentiate between different stereoisomers (such as R and S). Other techniques like NMR spectroscopy or X-ray crystallography are usually used to determine the absolute configuration or isomeric identity of a compound.
To determine if the optical rotation result is indeed +140 degrees and not -220 degrees, you can perform a dilution experiment. Here's how you can do it:
1. Prepare a sample of the substance with the known optical rotation value (+140 degrees).
2. Dilute the sample with a suitable solvent, such as water or ethanol, to create a series of dilutions. Keep track of the dilution ratios.
3. Measure the optical rotation of each dilution using a polarimeter.
4. Plot a graph of the optical rotation against the concentration (or dilution ratio).
5. If the result is consistent with the known value (+140 degrees), then it supports the original measurement. However, if the measured values deviate significantly from +140 degrees, it may indicate a possible error in the initial measurement.
Regarding the second part of your question, a polarimetry experiment alone cannot be used to determine the configuration of individual molecules (such as "r" or "s" isomers). Polarimetry measures the overall optical activity of a compound but does not provide information about the specific arrangement of the molecules. Isomer determination usually requires additional techniques such as NMR spectroscopy, X-ray crystallography, or chemical analysis.