In: Chemistry
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.
Ans
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.