A chemist must separate a three-component mixture containing benzaldehyde, benzoic acid, and ethyl benzoate by column chromatography. The stationary phase is silica gel. The column is initially eluted with pure hexane; once the first component has eluted, the eluent is switched to a 70:30 hexane–ethyl acetate mixture to finish the separation.
(a) Predict the order in which the three compounds will elute during the initial hexane portion of the run. Justify your answer by discussing relevant intermolecular interactions with both phases.
(b) Explain why the chemist switches to a more polar solvent mixture after the first component has eluted. In your explanation, relate solvent polarity to the differential adsorption of the remaining analytes.
(c) Suppose the chemist instead began elution with the 70:30 hexane–ethyl acetate mixture and never used pure hexane. Describe TWO practical consequences this change would have on (i) resolution between peaks and (ii) total separation time. Provide chemical reasoning for each consequence.