Thin-layer chromatography (TLC) plates coated with alumina are often used to monitor the progress of esterification reactions. Consider the reaction of salicylic acid with methanol to produce methyl salicylate (oil of wintergreen). A student spots the TLC plate with pure salicylic acid in lane 1, pure methyl salicylate in lane 2, and a reaction mixture in lane 3. The plate is developed in a 15:85 ethyl acetate–hexane mobile phase. After development, lane 1 shows a single spot at Rf=0.18 and lane 2 shows a single spot at Rf=0.70. Lane 3 contains two spots at Rf=0.19 and Rf=0.69.
(a) Identify which spot in lane 3 corresponds to the reactant and which corresponds to the product, and explain your choices on the basis of polarity differences.
(b) The student wishes to obtain better separation between the two compounds. Propose a single modification to the mobile phase composition that would increase the distance between the two spots. Predict qualitatively how each Rf value would change and justify your prediction.
(c) Describe how hydrogen bonding between the analytes and the stationary phase influences the observed Rf values in this TLC experiment.