Question 1

The specific rotation of a pure substance is -5.90°. What is the percentage of this isomer in a mixture with an observed specific rotation of -2.95°?
A)25%
B)50%
C)75%
D)80%
E)0%

Accepted Answer

The answer of The specific rotation of a pure substance...

Question 2

Which of the following is not true of enantiomers?
A)They have the same melting point.
B)They have the same boiling point.
C)They have the same chemical reactivity with non-chiral reagents.
D)They have the same density.
E)They have the same specific rotation.

Accepted Answer

Enantiomers have opposite specific rotations. While they share physical and chemical properties such as melting point, boiling point, density, and reactivity with non-chiral reagents, their optical activities are mirror images of each other, meaning they rotate plane-polarized light in opposite directions.

Question 3

Provide a careful structure for (2R,3S,4S)-2-bromo-4-methyl-3-hexanol.

Accepted Answer

The answer of Provide a careful structure for (2R,3S,4S)-2-bromo-4-methyl-3-hexanol....

Question 4

Provide a careful structure for (2R,3S)-2,3-dibromohexane.

Accepted Answer

The answer of Provide a careful structure for (2R,3S)-2,3-dibromohexane....

Question 5

Which of the following statements correctly pertains to a pair of enantiomers?
A)They rotate the plane of polarized light by exactly the same amount and in opposite directions.
B)They rotate the plane of polarized light by differing amounts and in opposite directions.
C)They rotate the plane of polarized light by differing amounts and in the same direction.
D)They have different melting points.
E)They have the same melting point, but they have different boiling points.

Accepted Answer

Enantiomers are mirror images of each other that cannot be superimposed. They have the same physical and chemical properties except for their ability to rotate plane-polarized light in opposite directions by exactly the same amount.

Question 6

A sample of (+)and (-)-3-chlorohexane has a rotation of -20 degrees. Pure (+)-3-chlorohexane has a rotation of +40 degrees. Calculate the enantiomeric excess and the amount of (-)and (+)-3-chlorohexane in the original sample.

Accepted Answer

The answer of A sample of (+)and (-)-3-chlorohexane has a...

Question 7

(-)-Mandelic acid has a specific rotation of -158°. What would be the specific rotation of a solution which contains 40% (-)-mandelic acid and 60% (+)-mandelic acid?
A)+95°
B)+63°
C)+32°
D)-32°
E)-63°

Accepted Answer

The answer of (-)-Mandelic acid has a specific rotation of...

Question 8

A mixture of equal amounts of two enantiomers ________.
A)is called a racemic mixture
B)is optically inactive
C)implies that the enantiomers are meso forms
D)both A and B
E)none of the above

Accepted Answer

When a mixture contains equal amounts of two enantiomers, it is called a racemic mixture and is optically inactive. The enantiomers in a racemic mixture cannot be meso forms because meso forms are achiral molecules that contain chiral centers. Therefore, the correct answer is D, which states that a racemic mixture is optically inactive and called a racemic mixture.

Question 9

The specific rotation of a pure substance is 1.68°. What is the specific rotation of a mixture containing 75% of this isomer and 25% of the (-)isomer?
A)+1.68°
B)0°
C)+1.26°
D)+0.84°
E)+)042°

Accepted Answer

The answer of The specific rotation of a pure substance...

Question 10

The specific rotation of (R)-(+)-glyceraldehyde is +8.7°. If a mixture of glyceraldehydes enantiomers is 80% S and 20% R, what is the specific rotation of the mixture?

Accepted Answer

The answer of The specific rotation of (R)-(+)-glyceraldehyde is +8.7°....

Question 11

Label each asymmetric center as R or S.

Accepted Answer

The answer of Label each asymmetric center as R or...

Question 12

If (S)-glyceraldehyde has a specific rotation of -8.7°, what is the specific rotation of (R)-glyceraldehyde?
A)-8.7°
B)+8.7°
C)0)0°
D)cannot be determined from the information given

Accepted Answer

The specific rotation of (R)-glyceraldehyde would be the opposite of (S)-glyceraldehyde, so it would be a positive value of 8.7°.

Question 13

Provide a careful structure for (2S,3R,4R)-2-bromo-4-methyl-3-hexanol.

Accepted Answer

The answer of Provide a careful structure for (2S,3R,4R)-2-bromo-4-methyl-3-hexanol....

Question 14

Provide a careful structure for (2S,3S)-2,3-dibromohexane.

Accepted Answer

The answer of Provide a careful structure for (2S,3S)-2,3-dibromohexane....

Question 15

Provide a careful structure for (2S,3R)-2,3-dibromohexane.

Accepted Answer

The answer of Provide a careful structure for (2S,3R)-2,3-dibromohexane....

Question 16

A newly isolated natural product was shown to be optically active. If a solution of 2.0 g in 10 mL of ethanol in a 50 cm tube gives a rotation of +2.57°, what is the specific rotation of this natural product?

Accepted Answer

The answer of A newly isolated natural product was shown...

Question 17

Which of the following is/are optically inactive?
A)a 50-50 mixture of R and S enantiomers
B)a meso compound
C)every achiral compound
D)a racemic mixture
E)all the above

Accepted Answer

All the options listed are optically inactive. A 50-50 mixture of R and S enantiomers (racemic mixture) is optically inactive because the optical activities of the enantiomers cancel each other out. A meso compound is optically inactive due to its internal plane of symmetry, leading to internal compensation of optical activity. Every achiral compound is inherently optically inactive as it lacks chirality, which is necessary for optical activity.

Question 18

A solution containing 0.96 g of 2-bromooctane in 10 mL ether solution gave an observed rotation of -1.8° in a 10 cm cell at  Calculate the specific rotation of this solution.

Accepted Answer

The answer of A solution containing 0.96 g of 2-bromooctane...

Question 19

Twenty grams of a compound was dissolved in 100 mL of solvent and gave an observed rotation of -20 degrees. The sample tube is 4 dm long. Calculate the specific rotation.

Accepted Answer

The answer of Twenty grams of a compound was dissolved...

Question 20

Which of the following is a true statement?
A)All chiral molecules possess a plane of symmetry.
B)All achiral molecules are meso.
C)All molecules which possess a single chirality center of the S configuration are levorotatory.
D)A mixture of achiral compounds will be optically inactive.
E)All molecules which possess two or more chirality centers will be chiral.

Accepted Answer

A mixture of achiral compounds will not exhibit optical activity because the individual compounds do not have a chiral center. Optical activity is only exhibited by chiral molecules or mixtures of enantiomers.

Quiz 4: Isomers: the Arrangement of Atoms in Space

The specific rotation of a pure substance is -5.90°. What is the percentage of this isomer in a mixture with an observed specific rotation of -2.95°?

Which of the following is not true of enantiomers?

Provide a careful structure for (2R,3S,4S)-2-bromo-4-methyl-3-hexanol.

Provide a careful structure for (2R,3S)-2,3-dibromohexane.

Which of the following statements correctly pertains to a pair of enantiomers?

A sample of (+)and (-)-3-chlorohexane has a rotation of -20 degrees. Pure (+)-3-chlorohexane has a rotation of +40 degrees. Calculate the enantiomeric excess and the amount of (-)and (+)-3-chlorohexane in the original sample.

(-) -Mandelic acid has a specific rotation of -158°. What would be the specific rotation of a solution which contains 40% (-) -mandelic acid and 60% (+) -mandelic acid?

A mixture of equal amounts of two enantiomers ________.

The specific rotation of a pure substance is 1.68°. What is the specific rotation of a mixture containing 75% of this isomer and 25% of the (-) isomer?

The specific rotation of (R)-(+)-glyceraldehyde is +8.7°. If a mixture of glyceraldehydes enantiomers is 80% S and 20% R, what is the specific rotation of the mixture?

Label each asymmetric center as R or S.

If (S) -glyceraldehyde has a specific rotation of -8.7°, what is the specific rotation of (R) -glyceraldehyde?

Provide a careful structure for (2S,3R,4R)-2-bromo-4-methyl-3-hexanol.

Provide a careful structure for (2S,3S)-2,3-dibromohexane.

Provide a careful structure for (2S,3R)-2,3-dibromohexane.

A newly isolated natural product was shown to be optically active. If a solution of 2.0 g in 10 mL of ethanol in a 50 cm tube gives a rotation of +2.57°, what is the specific rotation of this natural product?

Which of the following is/are optically inactive?

A solution containing 0.96 g of 2-bromooctane in 10 mL ether solution gave an observed rotation of -1.8° in a 10 cm cell at Calculate the specific rotation of this solution.

Twenty grams of a compound was dissolved in 100 mL of solvent and gave an observed rotation of -20 degrees. The sample tube is 4 dm long. Calculate the specific rotation.

Which of the following is a true statement?