MCAT Organic Chemistry Question 9: Answer and Explanation

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Question: 9

9. What is the IUPAC name for the following structure?

  • A. 2,5-dimethylheptane
  • B. 2-ethyl-5-methylhexane
  • C. 3,6-dimethylheptane
  • D. 5-ethyl-2-methylhexane

Correct Answer: A

Explanation:

The first task in naming a compound is identifying the longest carbon chain. In this case, the longest chain has seven carbons, so the parent alkane ends in –heptane. Choices (B) and (D) can therefore be eliminated. Then, we must make sure that the carbons are numbered so that the substituents’ position numbers are as small as possible. This compound has two methyl groups; minimizing their position numbers requires us to number the chain from right to left. These methyl groups are attached to carbons 2 and 5, so the correct IUPAC name is 2,5-dimethylheptane. Choice (C) is incorrect because the position numbers of the substituents are not minimized.

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