Q: A mixture contains 70% of the R enantiomer and 30% of the S enantiomer. What is the enantiomeric excess ($$ee) of $$the mixture? Use the formula $$ee = \frac{ |R - S| }{ R + S } \times 100\%.$$

$$ee = \frac{70 - 30}{70 + 30} \times 100\% = 40\%$$

Question 1

Which of the following statements best explains why enzymes in our bodies are sensitive to the stereochemistry of molecules?

Accepted Answer

The 3D shape and stereochemistry of a molecule determine how it fits into the enzyme's active site, affecting its reactivity.

Question 2

A mixture contains 70% of the R enantiomer and 30% of the S enantiomer. What is the enantiomeric excess ($$ee) of $$the mixture? Use the formula $$ee = \frac{ |R - S| }{ R + S } 	imes 100\%.$$

Accepted Answer

$$ee = \frac{70 - 30}{70 + 30} 	imes 100\% = 40\%$$

Question 3

Which of the following is true about a racemic mixture?

Accepted Answer

It contains equal amounts of both enantiomers and has an enantiomeric excess of 0%.

Question 4

When assigning priorities to substituents on a chiral center, which rule should be followed first?

Accepted Answer

Assign the highest priority to the substituent with the highest atomic number directly attached to the chiral center.

Question 5

Consider the molecule 2-bromobutane. Which of the following statements is correct regarding its stereochemistry?

Accepted Answer

It has one chiral center and can exist as a pair of enantiomers.

Question 6

Which of the following best describes a meso compound?

Accepted Answer

A compound with two or more chiral centers that is achiral due to an internal plane of symmetry.

Question 7

If a Fischer projection of a chiral molecule is rotated by $$180^{\circ} in $$the plane, what happens to its configuration?

Accepted Answer

The configuration remains the same.

Question 8

Which of the following is true about diastereomers?

Accepted Answer

They have different physical properties and are not mirror images.

Question 9

A student is asked to assign the R/S configuration to a chiral center. The lowest priority group is pointing towards them (on a wedge). After assigning priorities, they find the sequence 1 → 2 → 3 is clockwise. What is the correct configuration?

Accepted Answer

S

Question 10

Which of the following real-world examples best illustrates the importance of stereochemistry in pharmaceuticals?

Accepted Answer

The difference between R- and S-thalidomide, where one enantiomer is a teratogen and the other provides morning sickness relief.

Question 11

Which of the following statements about enantiomers is correct?

Accepted Answer

Enantiomers have identical physical properties except for their interaction with plane-polarized light and chiral environments.

Question 12

Which technique is commonly used to separate enantiomers in a laboratory setting?

Accepted Answer

Chromatography using a chiral column

Question 13

Which of the following molecules is achiral due to rapid interconversion between conformations?

Accepted Answer

A molecule with conformationally mobile atoms that move too fast to isolate a single chiral form

Question 14

Which of the following is NOT a correct step in assigning R/S configuration to a chiral center?

Accepted Answer

Assign the highest priority to the group with the lowest atomic number.

Question 15

Which of the following best describes the difference between enantiomers and diastereomers?

Accepted Answer

Enantiomers are mirror images; diastereomers are not mirror images and have different physical properties.

Question 16

A compound has two chiral centers. How many stereoisomers are possible, assuming no meso forms?

Accepted Answer

4

Question 17

Which of the following statements is true regarding Fischer projections?

Accepted Answer

Horizontal lines represent bonds coming out of the plane towards the viewer.

Question 18

Which of the following is a correct statement about the optical activity of a meso compound?

Accepted Answer

A meso compound is optically inactive due to internal symmetry.

Question 19

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

Accepted Answer

$$ee = \frac{ |R - S| }{ R + S } 	imes 100\%$$

Stereochemistry in Organic Chemistry I: Chirality, R/S Assignment, and Stereoisomers

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