MCAT Organic Chemistry Question 20: Answer and Explanation

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Test Information

Question: 20

5. Which isomer of the following compound is the most stable?

  • A.
  • B.
  • C.
  • D. They are all equally stable.

Correct Answer: B

Explanation:

This molecule is a chair conformation in which the two equatorial methyl groups are trans to each other. Because the axial methyl hydrogens do not compete for the same space as the hydrogens attached to the ring, this conformation ensures the least amount of steric strain. Choice (A) would be less stable because the diaxial methyl group hydrogens are closer to the hydrogens on the ring, causing greater steric strain. Choice (C) is incorrect because it is in the more unstable boat conformation.

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