chapter eleven
11.1 Introduction
When a crude product has been isolated from a reaction work-up, the next
step is to purify it. The degree of purity required will depend to some
extent on the use for which the sample is intended. A synthetic intermedi-
ate might only require rough purication, whereas a product for elemen-
tal analysis would require rigorous purication. This chapter describes
the most important purication, crystallization, distillation, sublimation,
and chromatography techniques. It is assumed that the reader is famil-
iar with the basic principles of these methods, so the emphasis here is
on more demanding applications such as the purication of air-sensitive
materials and purications on a microscale.
11.2 Crystallization
11.2.1 Simple crystallization
All crystallizations are based on the same principle: a solid compound
is fully dissolved in a solvent (or solvents) at a particular temperature,
the conditions of the solution are changed such that the solution becomes
supersaturated in the compound to be crystallized, and the compound
then crystallizes from the solution. Many compounds can crystallize in
more than one polymorphic form. For example, snow and hail are differ-
ent polymorphs of ice. The polymorph you get depends on the conditions
under which the crystallization takes place. Polymorphs have different
melting points and stabilities, so if you have difculty in obtaining good
crystals from one crystallization system it is worth trying other systems.
There are two distinct phases of crystallization:
1. Nucleation: This is when a solid rst starts to precipitate from a solu-
tion. The formation of a crystalline solid rather than oil or wax can be
highly solvent dependent, so the correct choice of solvent is crucial.
2. Crystal growth: This is the second phase of crystallization. In gen-
eral, the slower the rate of crystallization the larger the size of the
crystals formed.
There are several different ways to make a solution supersaturated
in order to induce crystallization. The simplest way is to generate a hot

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