Q: A polarimeter is used to measure the optical rotation of a solution. If the observed rotation is $$+9.06\degree$$, the sample tube length is $$1.25\ \mathrm{dm}$$, and the concentration is $$0.473\ \mathrm{g/mL}$$, what is the specific rotation $$[\alpha]$$?

$$[\alpha] = \dfrac{+9.06}{1.25 \times 0.473} = +15.3$$

Q: Which of the following is the correct formula for calculating specific rotation $$[\alpha]$$?

$$[\alpha] = \dfrac{\text{observed rotation}}{\text{pathlength} \times \text{concentration}}$$

Question 1

Which of the following statements best describes a chiral molecule?

Accepted Answer

A molecule that is not superimposable on its mirror image and lacks a plane of symmetry.

Question 2

Consider the following molecule: $$\mathrm{CH_3CH(OH)COOH} ($$lactic acid). Which feature makes it chiral?

Accepted Answer

It has a tetrahedral carbon attached to four different groups.

Question 3

Which of the following is true about enantiomers?

Accepted Answer

They have identical physical properties except for their interaction with plane-polarized light and other chiral substances.

Question 4

A carbon atom is attached to the following groups: $$\mathrm{Cl}$$, $$\mathrm{Et}$$, $$\mathrm{Me}$$, and $$\mathrm{H}. $$What is the correct term for this carbon?

Accepted Answer

Chirality center

Question 5

Which of the following molecules is achiral?

Accepted Answer

A molecule with a plane of symmetry.

Question 6

Assign the configuration (R or S) to a chiral center with the following priority order (highest to lowest): $$\mathrm{Br}$$, $$\mathrm{OH}$$, $$\mathrm{CH_3$}$$, $$\mathrm{H}$$, with $$\mathrm{H}$$ pointing away from you. If the sequence from highest to lowest priority is clockwise, what is the configuration?

Accepted Answer

R

Question 7

Which of the following best describes a meso compound?

Accepted Answer

A compound with a plane of symmetry and is achiral despite having chiral centers.

Question 8

A sample of (S)-(+)-2-butanol has a specific rotation of $$+13.52\degree. If a $$mixture of the enantiomers shows a specific rotation of $$+6.76\degree$$, what is the enantiomeric excess (ee) of the mixture?

Accepted Answer

$$50\%$$

Question 9

Which of the following statements about racemic mixtures is correct?

Accepted Answer

They do not rotate plane-polarized light.

Question 10

A polarimeter is used to measure the optical rotation of a solution. If the observed rotation is $$+9.06\degree$$, the sample tube length is $$1.25\ \mathrm{dm}$$, and the concentration is $$0.473\ \mathrm{g/mL}$$, what is the specific rotation $$[\alpha]$$?

Accepted Answer

$$[\alpha] = \dfrac{+9.06}{1.25 	imes 0.473} = +15.3$$

Question 11

Which of the following is NOT a property shared by enantiomers?

Accepted Answer

Optical rotation direction

Question 12

Which of the following is the correct formula for calculating specific rotation $$[\alpha]$$?

Accepted Answer

$$[\alpha] = \dfrac{	ext{observed rotation}}{	ext{pathlength} 	imes 	ext{concentration}}$$

Question 13

Which of the following is a correct method for resolving a racemic mixture?

Accepted Answer

Reaction with a chiral resolving agent to form diastereomers, which are then separated by their differing solubility.

Question 14

Which of the following statements about chirality at nitrogen is correct?

Accepted Answer

Trivalent nitrogen rapidly flips, preventing isolation of individual enantiomers.

Question 15

A compound has two chiral centers and a plane of symmetry. What is the best description of this compound?

Accepted Answer

It is a meso compound and achiral.

Question 16

Which of the following is the correct formula for calculating enantiomeric excess (ee)?

Accepted Answer

$$ee = \dfrac{	ext{observed specific rotation}}{	ext{specific rotation of pure enantiomer}} 	imes 100$$

Question 17

Which of the following is true about diastereomers?

Accepted Answer

They are stereoisomers that are not mirror images.

Question 18

Which of the following would NOT result in optical activity?

Accepted Answer

A solution of a racemic mixture.

Question 19

Which of the following is a real-world application of chirality in organic chemistry?

Accepted Answer

Designing drugs that interact specifically with biological targets due to their handedness.

4. Stereochemistry at Tetrahedral Centers: Chirality, Enantiomers, and Symmetry

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