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In: Chemistry

12.You have a mixture of solids in the ratio of 50:3 for A : B. You...

12.You have a mixture of solids in the ratio of 50:3 for A : B. You are planning to use the technique of recrystallization to purify A and realize that you will first need to determine which solvent or solvent pair would be appropriate. Describe the solubility properties of the solvent that you would want to use, briefly explaining your rationale.

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Recrystallization is the primary method for purifying solid organic compounds. Compounds obtained from
natural sources or from reaction mixtures almost always contain impurities. The impurities may include
some combination of insoluble, soluble, and colored impurities. To obtain a pure compound these impurities
must be removed. Each is removed in a separate step in the recrystallization procedure.


To understand the recrystallization process, solubility behavior must first be considered. It is often stated that
"like dissolves like". More correctly, it should be stated that, "compounds having similar structural features
will be soluble in one another". Some obvious structural features that may affect solubility include polarity
and the ability to hydrogen bond. For example, a compound having just a few carbons and an alcohol
functional group (FG) would be expected to be soluble in solvents that have a few carbons and an alcohol FG
or in some other polar solvent, and to be less soluble in nonpolar solvents. Conversely an alkane would be
expected to show the opposite solubility behavior. In most cases though it is not as simple as this. If for
example a compound has lots of carbons and hydrogens (> 6 C's) and just one alcohol group, the solubility
will be dominated more by the alkyl part of the molecule than by the alcohol part, and the compound will
show a solubility behavior more like that of an alkane. For known compounds it is useful to consider the
structure of the compound when choosing a recrystallization solvent. An educated guess can save some time.
Usually however, the structure of a compound may not be known so the solvent must be chosen by carrying
out solubility tests.


The first part of this experiment involves carrying out solubility tests on known compounds. Later on such
solubility tests will be used to find a suitable recrystallization solvent for an unknown compound. A
compound usually exhibits one of three general solubility behaviors: (1), the compound has a high solubility
in both hot and cold solvent, (2), the compound has a low solubility in both hot and cold solvent, and (3), the compound has a high solubility in hot solvent and a low solubility in cold solvent. Solvents which exhibit the first two behaviors are not useful for recrystallizing a compound. A solvent showing the third behavior, that is, high solubility at high temperatures and low solubility at low temperatures, is one that is suitable for use as a recrystallization solvent.


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