MCAT Organic Chemistry Question 105: Answer and Explanation

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Question: 105

15. The catalytic production of dihydroxyacetone and glyceraldehyde 3-phosphate (2-hydroxy-3-oxopropyl dihydrogen phosphate) from fructose-1,6-bisphosphate ({[(2S,3S,4S,5R)-3,4-dihydroxy-5-[(phosphonooxy)methyl]oxolan-2-yl]methoxy}phosphonic acid) is what type of reaction?

  • A. Aldol condensation
  • B. Retro-aldol reaction
  • C. Dehydration
  • D. Nucleophilic attack

Correct Answer: B

Explanation:

The nomenclature in this question is well above what one needs to be able to draw on the MCAT; however, we can discern that we are forming a ketone and an aldehyde from a single molecule. The hallmark of a reverse aldol reaction is the breakage of a carbon–carbon bond, forming two aldehydes, two ketones, or one of each. In an aldol condensation, choice (A), we would expect to form a single product by combining two aldehydes, two ketones, or one of each. A dehydration reaction, choice (C), should release a water molecule, rather than breaking apart a large organic molecule into two smaller molecules. A nucleophilic attack, choice (D), should feature the formation of a bond between a nucleophile and an electrophile; again, we would not expect to break apart a large organic molecule into two smaller molecules. Note that simply noting how many reactants and products are present in the reaction is sufficient to determine the answer.

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