PURIFICATION OF ORGANIC COMPOUNDS
(a) Fractional crystallization: It is used when an organic compound is sparingly soluble in some solvent at
room temperature but considerably soluble at higher temperature and impurity is highly soluble. Its
solution can be heated dissolving the compound along with impurities and cooling it to form crystals of
highly pure organic compound.
Examples: p-Xylene , Benzoic acid, Fructose.
(b) Sublimation: It is the property of a substance to change its state from a solid state to a vapour state
without passing through a liquid state. Such compounds can be heated and separated from the impurity
by separating its vapour.
Example: Camphor, benzoic acid, anthracene.
(c) Chromatography: Chromatography is used for the separation of most of the compounds. There are two
phases used in this technique namely stationary phase and mobile phase. Stationary phase is immobile
throughout the process while mobile phase or eluent is a moving phase. Chromatography is based on its
solubility in mobile phase as well as adsorption of the compound in stationary phase.
Terms involved in chromatography:
(i) Rf factor: Retardation factor is used in identifying the product separated using paper or thin layer
chromatography.
Chromatogram In paper chromatography, if compounds are colourless, then ninhydrin spray is
used to visualize the compounds in the form of various colours.
The following are the different types of chromatography:
(i) Column chromatography: Column chromatography is a separation technique in which
the stationary bed is within a tube. The particles of the solid stationary phase or the
support coated with a liquid stationary phase may fill the whole inside volume of the
tube (packed column) or be concentrated on or along the inside tube wall leaving an
open, unrestricted path for the mobile phase in the middle part of the tube (open tubular
column). Differences in rates of movement through the medium are calculated to
different retention times of the sample. Silica gel is a commonly used stationary phase.
, (ii) Paper chromatography: Paper chromatography is a technique that involves placing a
small dot or line of sample solution onto a strip of chromatography paper. The paper is
placed in a jar containing a shallow layer of solvent and sealed. As the solvent rises
through the paper, it meets the sample mixture, which starts to travel up the paper with
the solvent.
(iii) Thin layer chromatography (TLC): Thin layer chromatography (TLC) is a widely
employed laboratory technique and is similar to paper chromatography. However,
instead of using a stationary phase of paper, it involves a stationary phase of a thin layer
of adsorbent like silica gel, alumina, or cellulose on a flat, inert substrate. Example:
Separation of vitamins.
(d) Distillation: There are two types of distillations based on the difference between the boiling points of
the organic compound and its impurity.
(i) Simple distillation: This method is used if there is sufficient difference in the boiling points of
volatile liquids. a) Chloroform and aniline b) ether and toluene
(ii) Fractional distillation: This method is used to separate two volatile liquids when the difference
in their boiling points is small. In such a case when one liquid starts boiling the other is also
converted to vapour considerably. So, we get the vapours of both compounds. Hence, to avoid
this fractionating column is used. Fractionating column opposes the vapour of one with higher
boiling point and its vapour again condenses and goes to solution.
Example: (a) Ethanol-water mixture (b) Petroleum products are separated by fractional
distillation, (c) Cyclohexane-toluene mixture, (d) Acetone and methyl alcohol.
(iii) Steam distillation: This method is used to separate the compounds which are steam volatile and
are insoluble in water. In this method, steam from a steam generator is passed through a heated
flask containing the liquid to be distilled. The mixture of steam and vapour is condensed and
collected in a receiver. Then the compound being insoluble in water can be separated easily.
Example: (a) Aniline-water mixture, (b) Isolation of citral, (c) Nitro benzene
(iv) Vacuum distillation: The lowering of pressure on the surface of a liquid lowers its boiling point.
As a result of this, a liquid can be boiled and distilled, without any decomposition, at temperature
much below its normal boiling point.
Example: Distillation of crude oil in oil refineries, Glycerol.
(e) Differential extraction: This method is based on the different solubilities of an organic compound in
different solvents. If a compound is soluble in water but more soluble in an organic solvent, it is
separated by shaking it with an organic solvent. As a result, the organic compound gets dissolved into a
organic solvent which can be extracted from water.
(a) Fractional crystallization: It is used when an organic compound is sparingly soluble in some solvent at
room temperature but considerably soluble at higher temperature and impurity is highly soluble. Its
solution can be heated dissolving the compound along with impurities and cooling it to form crystals of
highly pure organic compound.
Examples: p-Xylene , Benzoic acid, Fructose.
(b) Sublimation: It is the property of a substance to change its state from a solid state to a vapour state
without passing through a liquid state. Such compounds can be heated and separated from the impurity
by separating its vapour.
Example: Camphor, benzoic acid, anthracene.
(c) Chromatography: Chromatography is used for the separation of most of the compounds. There are two
phases used in this technique namely stationary phase and mobile phase. Stationary phase is immobile
throughout the process while mobile phase or eluent is a moving phase. Chromatography is based on its
solubility in mobile phase as well as adsorption of the compound in stationary phase.
Terms involved in chromatography:
(i) Rf factor: Retardation factor is used in identifying the product separated using paper or thin layer
chromatography.
Chromatogram In paper chromatography, if compounds are colourless, then ninhydrin spray is
used to visualize the compounds in the form of various colours.
The following are the different types of chromatography:
(i) Column chromatography: Column chromatography is a separation technique in which
the stationary bed is within a tube. The particles of the solid stationary phase or the
support coated with a liquid stationary phase may fill the whole inside volume of the
tube (packed column) or be concentrated on or along the inside tube wall leaving an
open, unrestricted path for the mobile phase in the middle part of the tube (open tubular
column). Differences in rates of movement through the medium are calculated to
different retention times of the sample. Silica gel is a commonly used stationary phase.
, (ii) Paper chromatography: Paper chromatography is a technique that involves placing a
small dot or line of sample solution onto a strip of chromatography paper. The paper is
placed in a jar containing a shallow layer of solvent and sealed. As the solvent rises
through the paper, it meets the sample mixture, which starts to travel up the paper with
the solvent.
(iii) Thin layer chromatography (TLC): Thin layer chromatography (TLC) is a widely
employed laboratory technique and is similar to paper chromatography. However,
instead of using a stationary phase of paper, it involves a stationary phase of a thin layer
of adsorbent like silica gel, alumina, or cellulose on a flat, inert substrate. Example:
Separation of vitamins.
(d) Distillation: There are two types of distillations based on the difference between the boiling points of
the organic compound and its impurity.
(i) Simple distillation: This method is used if there is sufficient difference in the boiling points of
volatile liquids. a) Chloroform and aniline b) ether and toluene
(ii) Fractional distillation: This method is used to separate two volatile liquids when the difference
in their boiling points is small. In such a case when one liquid starts boiling the other is also
converted to vapour considerably. So, we get the vapours of both compounds. Hence, to avoid
this fractionating column is used. Fractionating column opposes the vapour of one with higher
boiling point and its vapour again condenses and goes to solution.
Example: (a) Ethanol-water mixture (b) Petroleum products are separated by fractional
distillation, (c) Cyclohexane-toluene mixture, (d) Acetone and methyl alcohol.
(iii) Steam distillation: This method is used to separate the compounds which are steam volatile and
are insoluble in water. In this method, steam from a steam generator is passed through a heated
flask containing the liquid to be distilled. The mixture of steam and vapour is condensed and
collected in a receiver. Then the compound being insoluble in water can be separated easily.
Example: (a) Aniline-water mixture, (b) Isolation of citral, (c) Nitro benzene
(iv) Vacuum distillation: The lowering of pressure on the surface of a liquid lowers its boiling point.
As a result of this, a liquid can be boiled and distilled, without any decomposition, at temperature
much below its normal boiling point.
Example: Distillation of crude oil in oil refineries, Glycerol.
(e) Differential extraction: This method is based on the different solubilities of an organic compound in
different solvents. If a compound is soluble in water but more soluble in an organic solvent, it is
separated by shaking it with an organic solvent. As a result, the organic compound gets dissolved into a
organic solvent which can be extracted from water.
