MCAT Organic Chemistry Question 49: Answer and Explanation

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Question: 49

4. Rank the following in order of decreasing leaving group ability: H2O, HO–, Br–, H–

  • A. H2O > Br– > HO– > H–
  • B. H2O > HO– > Br– > H–
  • C. HO– > Br– > H2O > H–
  • D. HO– > H– > H2O > Br–

Correct Answer: A

Explanation:

Good leaving groups are weak bases, which are the conjugates of strong acids. Leaving groups must also be stable once they leave the molecule. H2O is, by far, the most stable leaving group and will be extremely unreactive once it leaves the molecule through heterolysis. Br– is the conjugate base of HBr; HO– is the conjugate base of water. HBr is a much stronger acid than water, so Br– is a better leaving group than HO–. Finally, hydride (H–) is a very poor leaving group because it is extremely unstable in solution.

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