Premed HQ

MCAT Solvent Choice

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MCAT Prep Notes

For the first part of the recrystallization process the compound needs to be more soluble in the solvent, but for the second part of the process the compound needs to be less soluble in the solvent.
That said, a solvent must be chosen that has a good balance. By looking at the compound’s solubility in different compounds, a solvent that has an intermediate solubility should be chosen.

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MCAT Prep Notes

In general, solutes are more soluble in a solvent at higher temperatures and less soluble at lower temperatures.
This relationship between solubility and temperature can be used to purify a substance that can easily form a crystalline (solid) structure.
In recrystallization, an impure solid (a solid mixture) is placed in a solvent and heated to form a liquid mixture. As the solid is heated, it dissolves in the solvent, resulting in a mixture that contains the solvent, the compound, and the other compounds (impurities) that were originally present in the solid.
This solution is then filtered to remove the large impurities that were in the solid.
This is done using hot gravity filtration, which involves keeping the solution hot while it is being filtered to ensure that the compound is as soluble as possible.
After filtration, the solution is covered and is allowed to cool (normally it is placed in an ice bath).
The idea is to decrease the temperature of the solution, causing the solubility of the compound to decrease, causing the compound to form a solid (crystal).
The newly formed solid will be pure and can be removed from the solution by gravity filtration (this time you want the solution to remain cold to make sure that the compound is insoluble, so that it can be filtered out).
Slower cooling and more time will ensure larger crystals and a more pure crystal, which will be easier to filter.

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MCAT Prep Notes

Stationary Phase: thin layer of a polar medium (silica gel or alumina) on a small plate
Mobile Phase: a polar or non-polar organic solvent
Using a capillary tube, a small amount of the (liquid) mixture is placed at the bottom of the TLC plate.
After the mixture dries on the plate, the TLC plate is placed upright in the mobile phase.
The mobile phase will elute up through the TLC plate, and when the solvent approaches the top of the plate, the TLC plate is removed from the solution and allowed to dry.
The components of the mixture that had a higher affinity for the organic solvent (mobile phase) will be “carried” up the TLC plate further, separating the mixture.
Each component of the mixture will have a measureable retention factor (Rf) (a ratio of “distance the spot moved from the origin” to “distance the mobile phase moved from the origin”)
By comparing retention factors for particular components, TLC can be used qualitatively to identify the presence of a given compound in a mixture.

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MCAT Prep Notes

Stationary Phase: absorbent paper (most be porous for the mobile phase to elute)
Mobile Phase: a polar or non-polar organic solvent
A drop of the (liquid) mixture is placed on the paper, placed in the solvent, and the solvent will elute up through the paper.
Components of the mixture that “stick better” (have a higher affinity) to the paper will take longer to elute through the paper, resulting in separation of the mixture.

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MCAT Prep Notes

Stationary Phase: thick, viscous, waxy material
Mobile Phase: inert gas (want a gas that will not react with the mixture)
Using a gas chromatogram, the mixture is vaporized and placed in the stationary phase (oil), followed by the addition of the mobile phase (gas).
The gas will move the components of the mixture through the oil in a column at different rates, depending on the interaction of the individual components with the oil.
The rate at which the components elute through the column is detected, recorded and graphed by the gas chromatogram.
Components that have a higher affinity (attraction) to the stationary phase will take longer to elute through the column, resulting in separation of the mixture.