GRE Subject Test: Chemistry : Stereochemistry

Study concepts, example questions & explanations for GRE Subject Test: Chemistry

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Example Questions

Example Question #1 : Stereochemistry

The molecules shown below are best described as __________.

Molecules

Possible Answers:

diastereomers

isomers

enantiomers

epimers

Correct answer:

isomers

Explanation:

The molecules in this problem are isomers because they each have unique configurations and do not share the same funcitonal groups at the same carbon positions. Enantiomers are reflections of each other. Diastereomers are stereoisomers that differ at one or more stereocenters, while epimers are stereoisomers that differ at only one stereocenter.

Example Question #1 : Isomers

Which of the following is not a geometric isomer of pentene?

Possible Answers:

All of these are geometric isomers of pentene.

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Screen shot 2015 12 08 at 8.52.05 pm

Screen shot 2015 12 08 at 8.52.08 pm

Screen shot 2015 12 08 at 8.52.13 pm

Correct answer:

Screen shot 2015 12 08 at 8.52.13 pm

Explanation:

Geometric isomers are compounds that have the same molecular formula but they differ in the way they are arranged spatially. Pentene carries the molecular formula, . The longest chain of carbon atoms should be five carbons. According to its name, pentene is an alkene due to the -ene form its name, therefore must also contain a  bond. The only compound without a chain of 5 carbons consecutively is Screen shot 2015 12 08 at 8.52.13 pmand this compound is not a geometric isomer of pentene.

Example Question #1 : Stereochemistry

A molecule has three chiral centers. How many stereoisomers of this compound will have different boiling points compared to the original molecule?

Possible Answers:

Seven

Two

Six

One

Correct answer:

Six

Explanation:

The first step is to determine how many stereoisomers there are for this molecule. Since the number of stereoisomers is dependent on the number of chiral carbons, we can solve according to the equation , where  is the number of chiral centers. Since there are three chiral centers, we determine that there are eight stereoisomers for this molecule. Keep in mind that this number includes the original molecule.

Next, we need to compare the different stereoisomers to the original molecule. The original molecule will have one enantiomer and six diastereomers. Remember that enantiomers have the same physical properties, so we will not include this isomer in the final answer. Diastereomers, on the other hand, have different physical properties compared to the original molecule. As a result, six stereoisomers will have different boiling points compared to the original molecule.

Example Question #1 : Help With Enantiomers

Penicillin

Shown above is the chemical structure for penicillin, a common prescription drug. How many chiral carbons does penicillin have?

Possible Answers:

Five

One

Zero

Two

Three

Correct answer:

Three

Explanation:

The correct answer is three. The key to finding chiral carbons is to look for carbons that are attached to four different substituents. We can immediately eliminate any carbons that are involved in double bonds, or that have two hydrogens attached. Given this, we find that there are three chiral carbons. Note that carbon chains of varying content will qualify as different substituents, allowing chiral carbons to bond to two other carbons.

Penicillin_chiral

 

Example Question #1 : Stereochemistry

 Compounds that are mirror images of each other are called __________.

Possible Answers:

diastereomers

stereoisomers

conformers

enantiomers

Correct answer:

enantiomers

Explanation:

Stereoisomers are isomers that differ in the orientation of atoms in space, but have the same bonding patterns and structures. Enantiomers are a specific class of stereoisomers that differ in orientation around a chiral center to create mirror image molecules.

Diastereomers are a type of stereoisomer that are not related through a reflection operation, and may differ at more than one chiral center. Conformers have the same structural formula, but different shapes due to bond rotation.

Example Question #1 : Help With Epimers

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The given molecules are __________.

Possible Answers:

identical

constitutional isomers

conformers

stereoisomers

None of these

Correct answer:

stereoisomers

Explanation:

Stereoisomers have different orientations around a single stereocenter. The two molecules are stereoisomers. Specifically, these molecules are epimers, meaning that they differ at only one stereocenter.

Constitutional isomers have the same molecular formula, but different structures. Conformers have different rotations around a single bond. The molecules are clearly not identical.

Example Question #1 : Help With Epimers

Which of the following carbons represents the stereogenic center between the given isomers?

Linear glucoseLinear galactose

Possible Answers:

Carbon 5

Carbon 2

Carbon 4

Carbon 3

Carbon 1

Correct answer:

Carbon 4

Explanation:

Epimers are isomers that have different configurations at only one carbon atom. This carbon atom is known as the stereogenic center. The given compounds are identical except for the orientation around carbon number 4; thus, carbon 4 is the stereogenic center.

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