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MCAT Organic Chemistry Question — TLC

  • by John
  • Feb 18, 2017
  • MCAT Question of the Day

Thin-layer chromatography (TLC) is a laboratory technique used to separate non-volatile mixtures. Using a silica gel plate and a hexane solvent, which of the following compounds in a mixture containing CH3OH, HCOOH, CH2O, and C6H6 would have the highest Rf value?

 

  1. C6H6
  2. HCOOH
  3. CH2O
  4. CH3OH
Click for Explanation
This question is testing your familiarity with thin-layer chromatography (TLC), which is a technique used to separate mixtures. Silica gel is composed of a siloxane matrix (-O-Si-O-). The highly polar substance is capable of forming intermolecular bonds, including hydrogen bonding. As a result, polar compounds adhere to the silica gel plate and travel little from the initial starting position. Meanwhile, nonpolar compounds elute away from the starting position with the nonpolar hexane solvent via capillary action.

In the mixture above, HCOOH and CH3OH hydrogen bond with the siloxane matrix. Therefore, B and D will have the lowest Rf values. The oxygen atom in CH2O makes formaldehyde slightly polar allowing for intermolecular interactions between the gel plates. Benzene is the most nonpolar compound of the choices and therefore travels the farthest from the starting position along with the hexane solvent. Thus, A is the correct answer.

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