All GRE Subject Test: Chemistry Resources
Example Question #4 : Chromatography
A chemist carries out the synthetic scheme shown below. Unfortunately, the first two reactions are incomplete, and a mixture of compounds A, B, and C is obtained after the second step. The chemist purifies this mixture by normal phase chromatography, using silica gel as a stationary phase and a 10:1 hexanes-diethyl ether (v:v) solution as an eluent. In what order would compounds A, B, and C elute off the column?
For each choice, the first compound to elute is listed first.
B, C, A
C, A, B
B, A, C
C, B, A
A, C, B
C, A, B
In the normal phase chromatography system described, the most nonpolar compound would elute first and the most polar compound would elute last. The silica stationary phase will interact with more polar molecules, while the hexane mobile phase will carry nonpolar molecules. This would slow the progress of polar molecules as they bond to the silica, and enhance the progress of nonpolar molecules as they interact with the mobile phase.
Compound C is the most nonpolar compound because it contains only hydrogen and carbon. Compounds A and B are more polar because of the presence of oxygen, and hence the presence of polarized carbon-oxygen bonds. The alcohol group of compound B makes this compound the most polar of the three molecules by virtue of hydrogen bonding capabilities as well as the carbon-oxygen dipole. Compound B would thus elute last.