MCAT Organic Chemistry Question 24: Answer and Explanation

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Question: 24

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

I. Cistrans isomers

II. Diastereomers

III. Enantiomers

  • A. I only
  • B. II only
  • C. I and II only
  • D. I and III only

Correct Answer: C

Explanation:

(E)-2-butene can also be called trans-2-butene; (Z)-2-butene can also be called cis-2-butene. As such, they are cistrans isomers. Remember that cistrans isomers are a subtype of diastereomers in which the position of substituents differs about an immovable bond. Diastereomers are molecules that are non-mirror-image stereoisomers (molecules with the same atomic connectivity). These are not enantiomers because they are not mirror images of each other.

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