METHODS OF PURIFICATION OF
ORGANIC COMPOUNDS
Once an organic compound is extracted from
a natural source or synthesised in the
laboratory, it is essential to purify it. Various
methods used for the purification of organic
compounds are based on the nature of the
compound and the impurity present in it.
The common techniques used for
purification are as follows :
(i) Sublimation
(ii) Crystallisation
(iii) Distillation
(iv) Differential extraction and
(v) Chromatography
Finally, the purity of a compound is
ascertained by determining its melting or
boiling point. Most of the pure compounds
have sharp melting points and boiling points.
New methods of checking the purity of an
organic compound are based on different
types of chromatographic and spectroscopic
techniques.
12.8.1 Sublimation
You have learnt earlier that on heating, some
solid substances change from solid to vapour
state without passing through liquid state.
The purification technique based on the above
principle is known as sublimation and is used
to separate sublimable compounds from nonsublimable impurities.
12.8.2 Crystallisation
This is one of the most commonly used
techniques for the purification of solid organic
compounds. It is based on the difference in
the solubilities of the compound and the
impurities in a suitable solvent. The impure
compound is dissolved in a solvent in which
it is sparingly soluble at room temperature
but appreciably soluble at higher
temperature. The solution is concentrated to
get a nearly saturated solution. On cooling
the solution, pure compound crystallises out
and is removed by filtration. The filtrate
(mother liquor) contains impurities and small
quantity of the compound. If the compound
is highly soluble in one solvent and very little
soluble in another solvent, crystallisation can
be satisfactorily carried out in a mixture of
these solvents. Impurities, which impart
colour to the solution are removed by
adsorbing over activated charcoal. Repeated
crystallisation becomes necessary for the
purification of compounds containing
impurities of comparable solubilities
DISTILLATION
This important method is used to separate (i)
volatile liquids from nonvolatile impurities and
(ii) the liquids having sufficient difference in
their boiling points. Liquids having different
boiling points vaporise at different
temperatures. The vapours are cooled and the
liquids so formed are collected separately.
Chloroform (b.p 334 K) and aniline (b.p. 457
K) are easily separated by the technique of
distillation (Fig 12.5). The liquid mixture is
taken in a round bottom flask and
heated carefully. On boiling, the
vapours of lower boiling component
are formed first. The vapours are
condensed by using a condenser and
the liquid is collected in a receiver. The
vapours of higher boiling component
form later and the liquid can be
collected separately.
Fractional Distillation: If the
difference in boiling points of two
liquids is not much, simple distillation
cannot be used to separate them. The
vapours of such liquids are formed
within the same temperature range and
are condensed simultaneously. The
technique of fractional distillation is
used in such cases. In this technique,
vapours of a liquid mixture are passed
through a fractionating column before
condensation. The fractionating
column is fitted over the mouth of the
round bottom flas
