Question 1

Label each asymmetric center as R or S.

Accepted Answer

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Question 2

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

Accepted Answer

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Question 3

(-)-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?

Accepted Answer

The specific rotation of the solution is calculated by taking the weighted average of the specific rotations of the two enantiomers. The specific rotation of the solution is: (0.4 * -158°) + (0.6 * 158°) = -63.2° + 94.8° = +31.6°, which rounds to +32°.

Question 4

A mixture of equal amounts of two enantiomers

Accepted Answer

A mixture of equal amounts of two enantiomers is called a racemic mixture and is optically inactive. It does not necessarily imply that the enantiomers are meso forms. Choice D encompasses both correct statements.

Question 5

Which of the following is/are optically inactive?

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 6

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

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Question 7

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

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Question 8

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

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Question 9

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

Accepted Answer

Enantiomers have equal and opposite specific rotations. The sign of optical rotation depends on the direction of rotation of plane-polarized light, which is determined by the absolute configuration of the molecule. Therefore, if (S)-glyceraldehyde has a specific rotation of -8.7°, (R)-glyceraldehyde, its enantiomer, will have a specific rotation of +8.7°.

Question 10

Which of the following is a true statement?

Accepted Answer

A mixture of achiral compounds will not rotate plane-polarized light and thus will be optically inactive.

Question 11

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

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Question 12

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 20°C.Calculate the specific rotation of this solution.

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Question 13

Which of the following is not true of enantiomers?

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 14

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.

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Question 15

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

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 16

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

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Question 17

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°?

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Question 18

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

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Question 19

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?

Accepted Answer

The specific rotation of the mixture is calculated by taking 75% of the specific rotation of the pure substance and subtracting 25% of the specific rotation of the (-) isomer, which is the negative of the pure substance's specific rotation. Thus, (0.75 * 1.68) + (0.25 * -1.68) = +0.84°.

Question 20

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

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Quiz 4: Isomers: the Arrangement of Atoms in Space