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7 Cards in this Set
- Front
- Back
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Compound X has an Rf of 0.22 with hexane as the eluent. using ethyl acetate as the eluent, would the Rf of Compound X be expected to be greater than, less than, or equal to 0.22? Why?
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Greater than, because ethyl acetate is a more polar solvent so it will travel farther, and Rf is calculated as distance of the spot/distance of the solvent
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Once the column has been prepared, why is it important to allow the level of the solvent to drop to the level of the silica before adding your mixture?
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If drops below, column will crack; air holes will form in long lines in the silica that look like cracks, which reduces the efficiency of separation
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Which method, TLC or flash column chromatography, would be a better way to check the purity of a compound? To separate 5 g of a product mixture?
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TCL: purity; chromatography: separate
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What do you think is the advantage of using a gradient elution vs. just one solvent system?
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With gradient, composition of eluent changes over time
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Why is it important for the spots on a TLC plate to be above te level of the solvent in the TLC chamber?
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Otherwise, they will be dissolved in the solvent pool
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Predict the order of elution of a mixture of biphenyl and benzoic acid on a TLC plate in a 1:1 ethyl acetate:hexane solution; and what the order would be if the solvent system were changed.
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Since benzoic acid has a
really polar carboxylic acid group, it will elute more slowly than biphenyl and on the TLC plate, bezoic acid will have a lower Rf value. Even if you change the solvent system, benzoic acid will still interact with the silica more than biphenyl and will elute more slowly. |
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Why are solvents in a gradient system increasingly polar? What would happen if you tried to do the reverse?
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you start with a low polarity solvent and then all the really non-polar compounds elute first and as you gradually increase the polarity, the mobile phase (solvent) is better able to interact with the polar compounds and makes them elute. The other alternative is to use the same polarity for the entire column, but if you have compounds with varying polarities, the polar ones would probably take an extrememly long time to come off the column if you choose a low polarity solvent. If you wanted to use a single solvent that was high polarity, then the polar and non-polar compounds would probably just both fly off the column together and you'd get really poor separation. Again, the basic idea is that silica is polar and the polarity of your solvent will determine how well your compounds move with the solvent as it flows through the column (or up the TLC plate).
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