Question 1

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

Accepted Answer

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Question 2

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] = α_obs / (l * c), where α_obs is the observed rotation, l is the path length in decimeters, and c is the concentration in g/mL. Here, α_obs = +10°, l = 1 dm, and c = 1.0 g/10 mL = 0.1 g/mL. So, [α] = +10° / (1 * 0.1) = +100°.

Question 3

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 rotation of a solution is directly proportional to its concentration. As the concentration of the S enantiomer has doubled from 0.4 g/mL to 0.8 g/mL, it is expected that the rotation will also double. Therefore, the rotation of the solution containing 0.8 g/mL of the S enantiomer in the same polarimeter is +11.2° (2 x 5.6°), which is option B.

Question 4

Which of the following compounds is a meso compound?

Accepted Answer

A meso compound contains chiral centers but is superimposable on its mirror image due to an internal plane of symmetry. (2R,3S)-dichlorobutane is a meso compound because it has two chiral centers but is achiral overall due to its symmetry.

Question 5

Which of the following cyclic molecules is a meso compound?

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Question 6

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

Accepted Answer

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Question 7

Which of the following statements about meso compounds is not true?

Accepted Answer

A meso compound is not chiral because it has an internal plane of symmetry. This means that although it may have stereogenic centers, the molecule as a whole is not chiral and cannot rotate polarized light.

Question 8

What is the relationship between the following compounds?

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Question 9

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

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Question 10

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 11

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 of a substance is defined as the angle of rotation caused by 1 g/mL of the substance in a polarimeter tube of 1 dm length. Since the given solution contains only the R enantiomer and its specific rotation is +4°, we can use the following formula to calculate the specific rotation of the S enantiomer:

[solution] × [length of tube] = ([concentration in g/mL] × [specific rotation]) × [length of tube]

0.08 × 1 = (x × -4) × 1
x = -0.02

Therefore, the specific rotation of the S enantiomer is -20°. However, some sources define specific rotation in terms of the observed rotation of the enantiomeric mixture (i.e. the racemic mixture), which would give a specific rotation of -40° for the S enantiomer. Both answers are acceptable, but the one closest to -20° is D.

Question 12

Which of the following compounds can have a meso stereoisomer?

Accepted Answer

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Question 13

Which of the following compounds are meso?

Accepted Answer

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Question 14

Which of the following compounds does not have a plane of symmetry?

Accepted Answer

The compound II does not have a plane of symmetry, as in its mirror image, the two methyl groups will be on opposite sides of the molecule. The other compounds have a plane of symmetry present.

Question 15

What is the relationship between the following compounds?

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Question 16

How many stereoisomers of 1,3-dibromocyclopentane exist?

Accepted Answer

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Question 17

What is the relationship between the following compounds?

Accepted Answer

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Question 18

What is the relationship between the following compounds?

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Question 19

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

Accepted Answer

An enantiomeric excess of 30% S means 70% of the mixture is racemic (50% R and 50% S). Therefore, 35% of the mixture is R (50% of 70%). 35% of 10 g is 3.5 g.

Quiz 5: Stereochemistry

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

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?

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?

Which of the following compounds is a meso compound?

Which of the following cyclic molecules is a meso compound?

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

Which of the following statements about meso compounds is not true?

What is the relationship between the following compounds?

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

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

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?

Which of the following compounds can have a meso stereoisomer?

Which of the following compounds are meso?

Which of the following compounds does not have a plane of symmetry?

What is the relationship between the following compounds?

How many stereoisomers of 1,3-dibromocyclopentane exist?

What is the relationship between the following compounds?

What is the relationship between the following compounds?

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

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?