SC20. More Practice with Stereochemistry
- Page ID
- 4127
Optical rotation
Problem SC20.61.
The (+) enantiomer of compound A has an optical rotation of 125o. If a pure sample of compound A has an optical rotation of 100o, what is the composition of the sample?
Problem SC20.62.
The (+) enantiomer of compound B has an optical rotation of 100o. If a pure sample of compound B has an optical rotation of 95o, what is the composition of the sample?
Problem SC20.63.
The (+) enantiomer of compound C has an optical rotation of 18o. If a pure sample of C contains 30% of the (+) enantiomer and 60% of the (-) enantiomer, what is the optical rotation value?
Problem SC20.64.
The (+) enantiomer of compound D has an optical rotation of 25o. If a pure sample of D contains 80% of the (+) enantiomer and 20% of the (-) enantiomer, what is the optical rotation value?
The polarimetry experiment
Problem SC20.61.
A pure sample of the naturally-occurring, chiral compound A (0.050 g) is dissolved in water (2.0 mL) and the solution is placed in a 0.5 dm cell. Three polarimetry readings are recorded with the sample: 0.625o, 0.706o, 0.682o. What is [a]?
Problem SC20.62.
A pure sample of the chiral compound B (0.540 g) is dissolved in ether (2.0 mL) and the solution is placed in a 1.0 dm cell. Three polarimetry readings are recorded with the sample: 1.225o, 1.106o, 1.182o. What is [a]?
Problem SC7.63.
A pure sample of the (+) enantiomer of compound C shows [a] = 42o. What would be the observed a if a solution of the sample was made by dissolving 0.250 g in 2.0 mL of acetontirile and was then placed in a 0.5 dm cell?
Problem SC20.64.
A student has prepared compound D in lab. She is sure the compound contains no impurities; a number of physical analyses have confirmed the structure and purity of the compound. A sample of compound D (0.10 g) is dissolved in methanol (2.0 mL) and the solution is placed in a 1.0 dm cell. Three polarimetry readings are recorded with the sample: 0.995o, 0.904o, 0.936o.
- What is [a]?
- The optical rotation of D has previously been reported as 25o. What is the optical purity of this sample?
- What is the enantiomeric excess of this sample?
- What is the composition of this sample?
- Why did the previous analyses show that there was only one compound present?
Problem SC20.65.
Suppose a student prepared a sample of a compound with known [a] = 40. The student did not realize that her sample contained 10% (by mass) of a non-chiral impurity. After correcting for c and l, what did she measure for [a]?
Diastereomers and Physical properties
Problem SC20.101.
From the following group of molecules, select:
- a pair that are the same compound.
- a pair that would have the same physical properties but opposite optical activities.
- a pair that have different physical properties but the same connectivity.
- a pair that have different connectivity.
- a compound that contains chiral centers but has no optical activity.
Problem SC20.102.
From the following group of molecules, select:
- a pair that are the same compound.
- a pair that would have the same physical properties but opposite optical activities.
- a pair that have different physical properties but the same connectivity.
- a pair that have different connectivity.