CH 223 - Organic Chemistry Laboratory I
Experiment #6 - Column and Thin-Layer Chromatography (TLC) of a Spinach Extract Pre-Lab Assignment
(1) List 5 ways that TLC can be used in an organic chemistry lab experiment. To identify an unknown, to monitor the course of a reaction and assess the purity of its product, to determine the best solvent for a column chromatography separation, to determine the somposition of each fraction from a column chromatography separation, and to determine whether a substance purified by recrystallization.
(2) The most common stationary phases or adsorbents using in TLC are silica gel, alumina, and cellulose. For each, (a) draw its chemical structure and (b) label it as polar or non-polar. Silica gel - polar
alumina - polar
cellulose - non-polar
(3) Why do “do-it-yourself” TLC plates give inconsistent results? Due to variations in the thickness of the absorbent layer.
(4) What concentration of compound(s) should be prepared to spot on a TLC plate? The sample should be dissolved in a suitable solvent to make around a 1% solution. The solute should be present at a concentration in the 0.2-2.0% range.
(5) Why should TLC plates be marked in pencil and not in pen? You should not mark a TLC plate with pen because the ink will travel up the plate with your solvent. Pencil will stay in place.
(6) Why do you want to make the spots as small as possible on a TLC plates? Large spots spread out too far to achieve accurate results.
(7) In choosing a TLC solvent, a student found that a 50:50 hexane : acetone and 65:35 hexane : acetone give similar results. Which one should they use for their TLC plate development? Why? They should use hexane for their TLC plate development because you want to use the least polar solvent possible to achieve good results, and acetone is more polar than hexane.
(8) Calculate the Rf values for compounds A, B and C below.
(9) If a TLC plate shows only one spot after development, does...
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