Q: A sample of (S)-(+)-2-butanol has a specific rotation of $$+13.52^ ext{o}. If a $$mixture of the two enantiomers shows a specific rotation of $$+6.76^ ext{o}$$, what is the enantiomeric excess (ee) of the mixture?

$$ee = \frac{+6.76}{+13.52} \times 100 = 50\%$$

Q: What is the specific rotation $$[\alpha]$$ for a 1.50 g sample of coniine dissolved in 10.0 mL ethanol, placed in a 5.00 cm pathlength cell, with an observed rotation of $$1.21^ ext{o}$$?

$$[\alpha] = \frac{1.21}{0.150 \times 0.5} = 16.13^ ext{o}$$

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

Which of the following compounds is most likely to be chiral?

Accepted Answer

A compound with a carbon atom bonded to four different substituents.

Question 3

A molecule of 2-butanol has the following groups attached to its second carbon: $$-OH$$, $$-H$$, $$-CH_3$, and -CH_2CH_3$. What is the correct method to assign its absolute configuration?

Accepted Answer

Rank the groups by atomic number, orient the lowest priority group away, and determine if the sequence from highest to lowest priority is clockwise (R) or counterclockwise (S).

Question 4

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 5

A sample of (S)-(+)-2-butanol has a specific rotation of $$+13.52^	ext{o}. If a $$mixture of the two enantiomers shows a specific rotation of $$+6.76^	ext{o}$$, what is the enantiomeric excess (ee) of the mixture?

Accepted Answer

$$ee = \frac{+6.76}{+13.52} 	imes 100 = 50\%$$

Question 6

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 7

What is the specific rotation $$[\alpha]$$ for a 1.50 g sample of coniine dissolved in 10.0 mL ethanol, placed in a 5.00 cm pathlength cell, with an observed rotation of $$1.21^	ext{o}$$?

Accepted Answer

$$[\alpha] = \frac{1.21}{0.150 	imes 0.5} = 16.13^	ext{o}$$

Question 8

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

Accepted Answer

Optical rotation direction

Question 9

A racemic mixture is best described as:

Accepted Answer

A 50:50 mixture of two enantiomers that does not rotate plane-polarized light.

Question 10

Which of the following statements about chirality centers is correct?

Accepted Answer

A chirality center is a point in a molecule where four different groups are attached to a single atom.

Question 11

Which of the following molecules will NOT be optically active?

Accepted Answer

A meso compound with two chiral centers and a plane of symmetry.

Question 12

What is the effect of a chiral compound on plane-polarized light?

Accepted Answer

It rotates the plane of polarization, making the compound optically active.

Question 13

Which of the following is true about diastereomers?

Accepted Answer

They have different physical properties and are not mirror images.

Question 14

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

Accepted Answer

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

Question 15

Which of the following is a correct statement about the resolution of racemic mixtures?

Accepted Answer

Resolution is achieved by separating enantiomers using a chiral resolving agent to form diastereomers, which can be separated by their differing solubility.

Question 16

Which of the following atoms can potentially be a chirality center in organic molecules, but is usually not due to rapid inversion?

Accepted Answer

Nitrogen

Question 17

A compound with two chiral centers can have which of the following relationships between its stereoisomers?

Accepted Answer

Enantiomers and diastereomers

Question 18

Which of the following is the correct sequence for assigning R/S configuration using the Cahn-Ingold-Prelog rules?

Accepted Answer

Rank groups by atomic number, orient the lowest priority group away, and trace the sequence from highest to lowest priority.

Question 19

Which of the following best explains why trivalent nitrogen is usually not a stable chirality center?

Accepted Answer

It rapidly inverts, preventing isolation of individual enantiomers.

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

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