Q: A solution contains 65% (S)-enantiomer and 35% (R)-enantiomer. If the specific rotation of pure (S) is $$[\alpha] = -118^\circ$$, what is the observed rotation?

$$[\alpha]_{obs} = -118^\circ \times 0.30 = -35.4^\circ ($$where 0.30 is the enantiomeric excess)

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

$$ee = |\%\text{major enantiomer} - \%\text{minor enantiomer}|$$

Question 1

Which of the following statements best describes a chiral molecule?

Accepted Answer

A molecule that is not superimposable on its mirror image.

Question 2

What is the relationship between the two molecules shown in the first image (amino alcohols with opposite configurations)?

Accepted Answer

They are enantiomers.

Question 3

Which test is most useful for quickly determining if a molecule is achiral?

Accepted Answer

Plane of symmetry test

Question 4

How many stereoisomers are possible for a molecule with three chiral centers, assuming no meso forms?

Accepted Answer

$$2^3 = 8$

Question 5

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

Accepted Answer

It is a meso compound and achiral.

Question 6

Which of the following is true about the optical activity of a racemic mixture?

Accepted Answer

It does not rotate plane-polarized light.

Question 7

Assign the R/S configuration to the following chiral center: a carbon attached to Br, Cl, F, and H. (Atomic numbers: Br \u003e Cl \u003e F \u003e H)

Accepted Answer

R, if the lowest priority group (H) is in the back and the sequence Br → Cl → F is clockwise.

Question 8

A solution contains 65% (S)-enantiomer and 35% (R)-enantiomer. If the specific rotation of pure (S) is $$[\alpha] = -118^\circ$$, what is the observed rotation?

Accepted Answer

$$[\alpha]_{obs} = -118^\circ 	imes 0.30 = -35.4^\circ ($$where 0.30 is the enantiomeric excess)

Question 9

Which of the following statements about meso compounds is FALSE?

Accepted Answer

Meso compounds are always optically active.

Question 10

Which of the following atoms can serve as a stereogenic center in organic molecules, according to the study materials?

Accepted Answer

Carbon, nitrogen, phosphorus, and sulfur

Question 11

A compound has the following Fischer projection:

$$\begin{array}{c} 	ext{COOH} \ | \ 	ext{H} - 	ext{Br} \ | \ 	ext{H} - 	ext{Br} \ | \ 	ext{CH}_2	ext{NH}_2 \end{array}$$

What is the relationship between the two chiral centers?

Accepted Answer

They are equivalent and the molecule is meso.

Question 12

Which of the following best describes the effect of a chiral molecule on plane-polarized light?

Accepted Answer

It rotates plane-polarized light, but the direction (dextrorotary or levorotary) is not directly related to R/S configuration.

Question 13

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

Accepted Answer

$$ee = |\%	ext{major enantiomer} - \%	ext{minor enantiomer}|$$

Question 14

Which of the following cyclohexane derivatives is always achiral according to TEST 3?

Accepted Answer

Cis-1,2-dimethylcyclohexane

Question 15

A molecule has two chiral centers and is atomically symmetrical, with both chiral centers being mirror images of each other. What is the total number of stereoisomers, including meso forms?

Accepted Answer

$$2^n - 	ext{meso forms}$$

Question 16

Which of the following is NOT a criterion for a compound to be classified as meso?

Accepted Answer

It is optically active.

Question 17

Which of the following best describes the relationship between two molecules that have the same molecular formula, the same connectivity, but differ in the configuration at only one of two chiral centers?

Accepted Answer

They are diastereomers.

Question 18

Which of the following is the correct formula for calculating the observed rotation ($$\alpha_{obs}) of a $$mixture given the specific rotation $$[\alpha]$$, concentration $$c$$, and path length $$l$$?

Accepted Answer

$$\alpha_{obs} = [\alpha] 	imes c 	imes l$$

Question 19

Which of the following statements about Fischer projections is TRUE?

Accepted Answer

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

Isomers and Chirality: The Arrangement of Atoms in Space

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