Structure in Chemistry
SC7. The polarimetry experiment
In measuring optical rotation, plane-polarized light travels down a long tube containing the sample. If it is a liquid, the sample may be placed in the tube as a pure liquid (its is sometimes called a neat sample). Usually, the sample is dissolved in a solvent and the resulting solution is placed in the tube.
There are important factors affecting the outcome of the experiment.
Optical rotation depends on the number of molecules encountered by the light during the experiment.
Two factors can be controlled in the experiment and must be accounted for when comparing an experimental result to a reported value.
FigureSC7.1. The effect of concentration on optical rotation.
The more concentrated the sample (the more molecules per unit volume), the more molecules will be encountered.
Concentrated solutions and neat samples will have higher optical rotations than dilute solutions.
The value of the optical rotation must be corrected for concentration.
FigureSC7.2. The effect of path length on optical rotation.
[a] = a / (c x l)
Problem SC7.1. A pure sample of the naturally-occurring, chiral compound A (0.250 g) is dissolved in acetone (2.0 mL) and the solution is placed in a 0.5 dm cell. Three polarimetry readings are recorded with the sample: 0.775o, 0.806o, 0.682o.
a) What is [a]?
b) What would be the [a] value of the opposite enantiomer?
Problem SC7.2. A pure sample of the (+) enantiomer of compound B shows [a] = 32o. What would be the observed a if a solution of the sample was made by dissolving 0.150 g in 1.0 mL of dichloromethane and was then placed in a 0.5 dm cell?
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