Four structurally related compounds (W, X, Y, and Z) are injected individually onto a normal-phase silica gel column operated at constant flow rate with pure hexane as the initial mobile phase. The elution times (retention times) are listed below:
Compound
Elution time (min)
W
1.8
X
3.2
Y
7.5
Z
10.1
Molecular formulas and condensed structural information are provided:
• W: C6H14 (n-hexane)
• X: C6H12O (hexanal)
• Y: C6H12O2 (caproic acid)
• Z: C6H12O3 (caproic acid with an additional hydroxyl at C-4)
Assume column dimensions and detection parameters remain unchanged throughout the experiment.
(a) Rank the four compounds from least polar to most polar, citing specific functional groups and intermolecular forces in your justification.
(b) The chemist switches to a gradient elution, gradually increasing the percentage of ethyl acetate in hexane. Predict how the elution order will change, if at all, and explain the mechanistic basis for your prediction.
(c) Calculate the selectivity factor α for the pair of compounds that are separated most effectively under the isocratic hexane conditions. Show your work, defining all terms used. (Treat retention time as directly proportional to retention factor k for qualitative purposes.)
(d) Propose a modification to the stationary phase that would reverse the elution order observed in part (a). Explain the chemical principles that would cause the reversal.