experiment #2 - synthesis and recrystallization of
Introduction and Purpose
Recrystallization is, by far, the most common way in which solid organic compounds
Since experimental chemistry can be a challenge under the best of
circumstances, most practitioners try to operate under those circumstances.
among other things, working with pure compounds.
Consequently, solid products of
reactions are routinely recrystallized before subsequent use.
The main purpose of this
experiment is to give you some understanding of the process of purification of solids by
recrystallization and to give you some practical experience in carrying out this process.
The problem at hand
is that you have a solid substance that contains relatively small,
but significant, amounts of impurities, and you want to dispose of the impurities but keep
as much of the compound as you can.
[Recrystallization is less likely to be successful if there
are really large amounts of impurity.
See if you can figure out why, after you have read this.]
The solution to this problem is to find a solvent that has the following properties:
your compound is soluble in the hot solvent,
your compound is insoluble, or only slightly soluble, when the solvent is cold, and
the impurities are either completely soluble when the solvent is cold, or completely
insoluble when the solvent is hot.
This set of properties, combined with some work on your part, will enable you to clean up
impure solids by separating out the impurities and throwing them away.
The idea, then, is
to place the material to be recrystallized in an Erlenmeyer flask, add a small amount of
solvent that has the properties described above, and warm the mixture, bringing it to a boil.
If all of the compound does not dissolve after a few minutes of boiling, you slowly add more
solvent, keeping the mixture at a gentle boil, until all of the compound has dissolved.
insoluble impurities – rust, dandruff, floor sweepings,
. – are observed, use gravity
filtration (conical funnel and folded filter paper, not Buchner funnel with vacuum and flat
filter paper) to remove them and discard.
After you have done this, or if there were no
insoluble impurities, allow the filtrate to cool and collect the crystals of product that form,
on a Buchner funnel.
The soluble impurities are now in the filtrate, which is discarded.
solvent adhering to the crystals in the Buchner funnel is allowed to evaporate.
is now pure.
The Art of Recrystallization
O.K., in practice, things get a
more complicated than suggested above.
we're definitely not talking about brain surgery here.
Even a Hartwick student can perform
a successful recrystallization (well.
.., some of them).