This document provides a detailed overview of the methods and techniques used to purify organic compounds in laboratory settings. Purification is an essential step in organic chemistry to isolate a compound from impurities, ensuring that it is suitable for research, analysis, or practical applicati...
The methods to be employed depend on the physical state of the compound.
CRYSTALLISATION: This method is based on the differences in the solubility of the organic
compound and its impurities in a solvent.
(a) Preparation of the solution: Organic substances is powdered and is dissolved in a suitable solvent
by heating. The amount of solvent should be just sufficient to dissolve the whole of the solid on
heating.
Choice of Solvent. The choice of solvent is very important in the crystallisation process. The main
conditions of the solvent are:
I. The organic substance should dissolve in the solvent upon heating and it should get
separated on cooling.
II. The solvent should not dissolve the impurities.
III. The solvent should not react chemically with the substance.
For example, suppose we want to purify sugar containing an impurity of common salt. This can
be done by treating the mixture with ethanol around 350K. The sugar will dissolve whereas common
salt remains insoluble.
(b) Filtration of the solution: The hot saturated solution is filtered preferably through a fluted filter
paper placed in a glass funnel. The use of the fluted filter paper makes the filtration rapid. The
jacketed vessels of the hot water funnel is heated from outside and this keeps the solution hot in the
glass funnel. This will prevent the formation of crystals during filtration.
(c) Separation of the crystals: The crystals formed are separated from the mother liquor by
filtration. The filtration is normally done by use of Buckner funnel and a suction pump. This enables
the filtration under reduced pressure and is therefore, quite fast.
(d) Crystallisation: The hot filtration is allowed to cool slowly and undisturbed in a beaker or in
crystallising dish. After some time, the crystals of the pure compound are formed.
(e) Drying of crystals: The crystals are dried by pressing between the folds of filter paper and then
placed in a steam of air oven for some time. The crystals are finally dried over sulphuric acid or
calcium chloride in a desiccator.
SUBLIMATION: Certain organic solids directly change from solid to vapour state on heating. This
process is called sublimation. This process is called sublimation. The vapours on cooling change back
to the solid form. The sublimation process is used for the separation of those solids which sublime on
heating from non-volatile solids. The process is generally used for the purification of camphor,
DISTILLATION
This method is used for the purification of liquids which boil without decomposition and contain
non- volatile impurities. The simple distillation involves its boiling point so that it is converted into
vapours. On cooling the vapours, pure liquid is obtained.
The distillate contains pure liquid while the impurities are left behind in the distillation flask
e.g. Ether from ethyl alcohol.
Fractional Distillation:
This process is used to separate a mixture of two or more miscible liquids which have boiling
points close to each other. The fractionating column is a long tube provided with obstructions to the
passage of the vapours moving upwards and liquid moving downwards. This method may be used to
separate a mixture of acetone (b.p. 330K) and methyl alcohol (b.p. 338.K).
Distillation under Reduced pressure (Vacuum Distillation):
Certain liquids tend to decompose at a temperature below their boiling points. Such liquids cannot
be purified by ordinary distillation. Therefore, vacuum distillation is used for liquids which
decompose at a temperature below their normal boiling points.
Ex. glycerol boils with decomposition at 563K.
Steam distillation:
The process of steam distillation is used for the separation and purification of liquid which is
appreciably volatile in steam, from non-volatile components of a mixture. Thus, the process of steam
distillations is used to purify the substances which
i. are volatile in steam but are not miscible with water
ii. possess sufficiently high vapour pressure at the boiling point temperature of water (100°C)
iii. contain non-volatile impurities.
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