GRE Subject Test: Chemistry : Isomers

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

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Example Questions

Example Question #442 : Organic Chemistry

The molecules shown below are best described as __________.

Molecules

Possible Answers:

isomers

epimers

enantiomers

diastereomers

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:

Screen shot 2015 12 08 at 8.52.13 pm

Screen shot 2015 12 08 at 8.51.59 pm

All of these are geometric isomers of pentene.

Screen shot 2015 12 08 at 8.52.05 pm

Screen shot 2015 12 08 at 8.52.08 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.

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