MCAT Organic Chemistry Review

Isomers

Practice Questions

1.    Which of the following does NOT show optical activity?

1.    (R)-2-butanol

2.    (S)-2-butanol

3.    A solution containing 1 M (R)-2-butanol and 2 (S)-2-butanol

4.    A solution containing 2 M (R)-2-butanol and 2 M (S)-2-butanol

2.    How many stereoisomers exist for the following aldehyde?

1.    2

2.    8

3.    9

4.    16

3.    Which of the following compounds is optically inactive?

1.   

2.   

3.   

4.   

4.    Cholesterol, shown below, contains how many chiral centers?

1.    5

2.    7

3.    8

4.    9

5.    Which isomer of the following compound is the most stable?

1.   

2.   

3.   

4.    They are all equally stable.

6.    The following reaction results in:

1.    retention of relative configuration and a change in the absolute configuration.

2.    a change in the relative and absolute configurations.

3.    retention of the relative and absolute configurations.

4.    retention of the absolute configuration and a change in the relative configuration.

7.    The following molecules are considered to be:

1.    enantiomers.

2.    diastereomers.

3.    meso compounds.

4.    structural isomers.

8.    (+)-glyceraldehyde and (–)-glyceraldehyde refer to the (R) and (S) forms of 2,3-dihydroxypropanal, respectively. These molecules are considered:

1.    enantiomers.

2.    diastereomers.

3.    meso compounds.

4.    structural isomers.

9.    Consider (E)-2-butene and (Z)-2-butene. This is a pair of what type(s) of isomers?

1.    Cistrans isomers

2.    Diastereomers

3.    Enantiomers

1.    I only

2.    II only

3.    I and II only

4.    I and III only

10.3-methylpentane and hexane are related in that they are:

1.    enantiomers.

2.    diastereomers.

3.    constitutional isomers.

4.    conformational isomers.

11.(R)-2-chloro-(S)-3-bromobutane and (S)-2-chloro-(S)-3-bromobutane are:

1.    enantiomers.

2.    diastereomers.

3.    meso compounds.

4.    the same molecule.

12.A scientist takes a 0.5 M solution of an unknown pure dextrorotatory organic molecule and places it in a test tube with a diameter of 1 cm. He observes that a plane of polarized light is rotated 12° under these conditions. What is the specific rotation of this molecule?

1.    –240°

2.    –24°

3.    +24°

4.    +240°

13.Omeprazole is a proton pump inhibitor commonly used in gastroesophageal reflux disease. When omeprazole, a racemic mixture, went off-patent, pharmaceutical companies began to manufacture esomeprazole, the (S)-enantiomer of omeprazole, by itself. Given 1 M solutions of omeprazole and esomeprazole, which solution(s) would likely exhibit optical activity?

1.    Omeprazole only

2.    Esomeprazole only

3.    Both omeprazole and esomeprazole

4.    Neither omeprazole nor esomeprazole

14.(2R,3S)-2,3-dihydroxybutanedioic acid and (2S,3R)-2,3-dihydroxybutanedioic acid are:

1.    meso compounds.

2.    the same molecule.

3.    enantiomers.

1.    I only

2.    III only

3.    I and II only

4.    I and III only

15.If the methyl groups of butane are 120° apart, as seen in a Newman projection, this molecule is in its:

1.    highest-energy gauche form.

2.    lowest-energy staggered form.

3.    middle-energy eclipsed form.

4.    highest-energy eclipsed form.

PRACTICE QUESTIONS