GRE Subject Test: Chemistry : Epimers

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

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Example Questions

Example Question #1 : Epimers

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The given molecules are __________.

Possible Answers:

constitutional isomers

None of these

identical

stereoisomers

conformers

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 #2 : Help With Epimers

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

Linear glucoseLinear galactose

Possible Answers:

Carbon 5

Carbon 4

Carbon 2

Carbon 1

Carbon 3

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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