Question 1

What is the percent ee of a mixture that has 70% of one enantiomer and 30% of the other?

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Question 2

A reaction mixture is three parts of one enantiomer and one part of the other. What is the percent ee of this mixture?

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Question 3

If a mixture has an ee of 90%, what is the percentage of each enantiomer?

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Question 4

A solution containing 0.08 g/mL of a pure R enantiomer in a 1 dm polarimeter rotates plane polarized light by +4°. What is the specific rotation of the S enantiomer?

Accepted Answer

The specific rotation is a property of a compound and is the observed rotation of plane-polarized light when it passes through a solution of 1 g/mL of a compound in a cell of length 1 dm. The formula to calculate specific rotation is given as:

[α] = α / (c*l)

Where [α] is the specific rotation in degrees, α is the observed rotation in degrees, c is the concentration in g/mL, and l is the path length in dm.

Given that the solution contains a pure R enantiomer and has a specific rotation of +4°, we can find the specific rotation of the S enantiomer using the following equation:

α(S) = -α(R)

[α](S) = α(S) / (c*l)

We know the concentration (c) is 0.08 g/mL and the path length (l) is 1 dm. Therefore,

[α](S) = (-4°) / (0.08 g/mL x 1 dm) = -50°

Thus, the specific rotation of the S enantiomer is -50°. Hence, the correct answer is option D.

Question 5

A natural product was isolated in the laboratory, and its observed rotation was +10° when measured in a 1 dm sample tube containing 1.0 g of compound in 10 mL of H₂O. What is the specific rotation of this compound?

Accepted Answer

The specific rotation $[α]$ is calculated using the formula $[α] = \frac{α}{l \times c}$, where $α$ is the observed rotation, $l$ is the path length in decimeters, and $c$ is the concentration in grams per milliliter. Plugging in the given values, $[α] = \frac{+10°}{1 \times 0.1} = +100°$.

Question 6

A solution containing 0.4 g/mL of a pure S enantiomer is a 1 dm polarimeter rotates plane polarized light by +5.6°. What is the rotation of a solution containing 0.8 g/mL of the S enantiomer in the same polarimeter.

Accepted Answer

The specific rotation of a compound, α, is defined as:

α = observed rotation (in degrees) / concentration (in g/mL) x path length (in dm)

Since the compound is a pure S enantiomer, the specific rotation is also the rotation of the enantiomer.

In the first solution, α = 5.6° / 0.4 g/mL x 1 dm = 14°.

In the second solution, we need to calculate the rotation using the same specific rotation:

rotation = α x concentration x path length = 14° x 0.8 g/mL x 1 dm = 11.2°

Therefore, the correct answer is B.

Question 7

How many isomers (constitutional and stereoisomers) exist for dimethylcyclobutane?

Accepted Answer

There are two constitutional isomers for dimethylcyclobutane: 1,1-dimethylcyclobutane and 1,2-dimethylcyclobutane. 
For 1,1-dimethylcyclobutane, there are no stereoisomers as both methyl groups are in the same position. 
For 1,2-dimethylcyclobutane, there are two stereoisomers because the methyl groups are on different carbons and can either be on the same side (cis) or opposite sides (trans) of the ring. 
Therefore, the total number of isomers is 2 constitutional isomers x 2 stereoisomers = 4 isomers.

Question 8

How much of the R enantiomer is present in 10 g of a mixture which has an enantiomeric excess of 30% of the S isomer?

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Quiz 5: Stereochemistry