Thin-layer chromatography was used to determine the best solvent system for purification of a mixture by column chromatography. One spot was observed near the origin using hexanes, one spot near the solvent front using ethyl acetate, and two spots separated using 1:1 hexanes/ethyl acetate. Which statement accurately describes the observations?

Question

Thin-layer chromatography was used to determine the best solvent system for purification of a mixture by column chromatography.  One spot was observed near the origin using hexanes, one spot near the solvent front using ethyl acetate, and two spots separated using 1:1 hexanes/ethyl acetate.  Which statement accurately describes the observations?

Quizplus · Accepted Answer

11ecba2d_12a5_72cc_a3bf_7de2476bdc76_MD0008_00 Thin-layer chromatography is a technique used to separate compounds based on polarity. In normal-phase thin-layer chromatography, the stationary phase is made up of a polar, adsorbent material, typically silica (SiO₂), and the mobile phase is an organic solvent that travels up the stationary phase via capillary action. The rate at which a compound travels up the plate is a function of the relative polarities of the compound and the solvent. If a mixture remains near the plate's origin, a more polar solvent is needed to increase the compound's affinity for the mobile phase relative to its affinity for the stationary phase. Conversely, if a mixture travels with the solvent front, a more nonpolar solvent is needed to decrease the compounds' affinity for the mobile phase relative to its affinity for the stationary phase. The 1:1 hexanes/ethyl acetate mixture is more polar than hexanes and less polar than ethyl acetate and achieved the compound separation essential for column chromatography. This solvent mixture decreased the compounds' affinity for the mobile phase relative to ethyl acetate. (Choice A) When using only hexanes, the compounds did not separate and remained near the origin. Therefore, hexanes are too nonpolar, and this results in the compounds' having a higher affinity for the stationary phase than the mobile phase. (Choice C) When ethyl acetate was used alone, both compounds moved with the solvent front. Therefore, ethyl acetate is too polar of a solvent, and this results in the compounds' having a higher affinity for the mobile phase than for the stationary phase. (Choice D) The 1:1 hexanes/ethyl acetate mixture increased the compounds' affinity for the mobile phase, rather than the stationary phase, relative to hexanes. Educational objective: Thin-layer chromatography is a technique used to separate compounds based on polarity. The rate at which a compound travels up the plate is a function of the compound and solvent polarities. Nonpolar solvents decrease a compound's affinity for the mobile phase, whereas polar solvents increase a compound's affinity for the mobile phase.

Thin-Layer Chromatography Was Used to Determine the Best Solvent System