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MCAT Organic Chemistry Question — Chromatography

  • by Kerry Goldstein
  • Jan 23, 2015
  • MCAT Blog, MCAT Organic, MCAT Question of the Day

A thin layer chromatography is performed on both the reactants and products of a reaction. It is found that the products have an Rf value that is significantly higher than the reactants. Which of the following could adequately describe this reaction:

A) SN2 reaction converting an alkyl bromide to an alkyl chloride
B) Addition reaction converting an alkene to an alcohol
C) Nucleophilic acyl substitution reaction converting an ester to an anhydride
D) Elimination reaction converting an alcohol to an alkene

Explanation

A) SN2 reaction converting an alkyl bromide to an alkyl chloride

B) Addition reaction converting an alkene to an alcohol

C) Nucleophilic acyl substitution reaction converting an ester to an anhydride

D) Elimination reaction converting an alcohol to an alkene

In a typical TLC, a high Rf value indicates a nonpolar molecule that moves with the solvent and a low Rf value indicates a polar molecule that sticks to the silicate of the glass slide. The problem describes products with a higher Rf value so we’re looking for a reaction that goes polar → less polar.

Choice (D) is the correct answer as it shows a highly polar molecule, an alcohol, being converted to a less polar molecule, an alkene. The remaining choices:

A: polar → polar

B: less polar → more polar

C: polar → slightly more polar

 

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