Purification of Organic Compounds

September 29, 2020

The purification of organic compounds is tedious unlike inorganic compounds because large number of them decomposes on heating, some are sensitive to other reagents and resist the solvent action of water.
Depending on the nature of the compound and nature of impurities present, various methods can be employed for the purification of a particular compound.
The various processes used for this purpose are

Crystallization
Sublimation
Distillation
Fractional distillation
Steam distillation
Distillation under reduced pressure
Differential extraction
Chromatography

1) Crystallization

This is the most common method used for the purification of solid organic compounds.
This method is based upon the differences in their solubility in a given solvent or a mixture of solvents.
Since most of the compounds are not fairly soluble in water, other solvents such as alcohol, acetone, chloroform, ether, etc. are also commonly used for this purpose.
The following steps are involved in this process.

a) Selection of solvent

The substance that we need to purify, in small quantity is firstly heated with2-3 ml of solvent under examination.
When it will dissolve partially or completely, we need to cool it.
If a considerable proportion of the dissolved substance separates out in the form of crystals, the solvent is considered suitable.
Thus, a suitable solvent is one in which the substance dissolves on heating and readily crystallizes out on cooling.
The solvent must not react chemically with the substance.

b) Preparation of the solution

The impure substance is powdered and is heated with the selected solvent in the flask.
The amount of solvent should be just sufficient to dissolve the substance on heating.
In the case of solvents with low boiling points, the flask is fitted with a water condenser or an air condenser to avoid the loss of solvent on heating.
If the solution obtained is colored, then it is decolorized by heating with animal charcoal.

c) Filtration

The hot saturated solution prepared above is then filtered preferably through a fluted filter paper in a glass funnel.
The use of fluted filter paper makes the filtration rapid.
If the organic compound crystallizes during filtration, the filtration cannot be done with ordinary filter paper as the crystals will be formed on the filter paper itself and proper filtration will not be possible.
In such cases, the filtration is done with the help of hot water funnel.
A hot water funnel is an ordinary funnel surrounded by a double walled copper jacket between which hot water is circulated.
This keeps the funnel hot, and thus prevents cooling and consequent crystallization of the solid over the filter paper during the filtration.

d) Crystallization

The hot filtrate is then allowed to cool in a shallow container when the pure solid starts depositing in the form of crystals leaving behind the impurities in the mother liquor.
Scratching the sides of the container often facilitates crystallization.

e) Separation and drying of crystals

The crystals thus obtained are separated from the mother liquor by filtration.
This may be done rapidly by using a Buchner funnel by applying a little vacuum using a filter pump.
When the whole of the mother liquor has been removed, the crystals are washed two or three times with small quantities of the pure solvent.
Crystals thus obtained may be further purified by recrystallization.
These are dried by pressing between folds of filter paper or by using a porous plate.
If the substance is stable and high melting then the crystals can also be dried by placing them in a stream of air-oven.
The crystals are finally dried over sulphuric acid or calcium chloride in a vacuum desiccator.

Fractional crystallization

If the impure substance is mixture of two compounds which do not have large difference in their solubility’s, the process of fractional crystallization is used for their separation.
A saturated solution of the mixture is prepared and allowed to cool as usual.
On cooling the less soluble substance crystallizes out first.
The mother liquor is now very rich in the more soluble substance which separated out in the crystalline form on further concentration and cooling of the mother liquor.
The less soluble substance separated in this way is contaminated with a little of the more soluble substance and vice-versa.
A complete separation of the substances is achieved by repeating the above process several times.

2) Sublimation

Normally, a solid when heated first melts into a liquid and on strong heating, is changed into vapours and the reverse changes take place on cooling.
However, there are certain solids which on heating directly change to vapours and the vapours on cooling form the solid. This phenomenon is called sublimation.
This property of sublimation is found very useful in the purification and separation of those compounds which sublime on heating from non-volatile solids.
In this method, the impure substance is heated in a dish covered with a perforated asbestos sheet over which an inverted funnel is placed.
The stem of the funnel is plugged with a little cotton or glass wool and the sides are kept cooled from outside with the help of wet-cloth or wet- filter paper.
Vapours of the solids, which sublimes, pass through the holes in the asbestos sheet and condense on the cooler walls of the funnel.
The non-volatile impurities are left behind in the dish.
Naphthalene, camphor, anthracene, anthraquinone, benzoic acid, etc. are purified by this method.

3) Distillation

It is the process of conversion of a liquid into vapours and condensing the vapours back to the liquid.
This method is used for the purification of liquids which boils without decomposition and are associated with non-volatile impurities.
The impure liquid is boiled and the vapours formed are condensed in a separate vessel to get the pure liquid.
It involves a set of apparatus consisting of a distillation flask having a side tube connected with a condenser.
An air condenser, which is a long glass tube is, used for liquids with boiling points above 100⁰C.and a water condenser in the case of other liquids.
The mouth of the flask is closed with the cork through which passes a thermometer and the bulb of the thermometer is kept just below the opening of the side tube.
A receiver is attached to the lower end of the condenser.
The substance is placed in the distillation flask and few pieces of unglazed porcelain or glass beads are added to prevent bumping of the liquid during distillation.
The flask is heated on a water bath or sand bath in the case of highly volatile and inflammable liquids, while it is heated directly.
The liquid changes into vapours which pass through the condenser and are condensed back into the liquid form.
The liquid distillate is collected in the receiver while the impurities are left behind in the distillation flask.

4) Fractional distillation

This process is used to separate mixture of two or more miscible liquids having different boiling points.
When the boiling points of the liquids do not differ by more than 40⁰C, the separation can be done with the help of ordinary distillation apparatus.
On heating the distillation flask, the temperature rises until the boiling point of the low boiling component is reached.
At this stage, the temperature becomes constant and the low boiling liquid distils over and is collected in the receiver.

$What is the difference between distillation and fractional distillation? - Quora$

When the temperature again begins to rise, the first receiver is disconnected.
A new receiver for the collection of the second component is attached as soon as the temperature becomes constant, once again, at the boiling point of the second component.
The two fractions, thus obtained are redistilled separately to ensure complete separation of the liquids.
If the boiling points of the liquids in the mixture are very close to each other, a fractionating column is fitted with distillation flask which in turn is connected to the condenser.
A fractionating column is a long tube provided with obstruction to the passage of the vapours upwards and that to liquid downwards.
Any one among different types of fractionating columns can be used which is fitted in-to mouth of a round bottom flask containing the mixture of liquids.
The column itself is fitted with a thermometer and is attached to a condenser.
This method has been found very useful in industry, especially in the separation of petroleum, coal tar and crude alcohol.

5) Steam distillation

This method is used for the purification of those substances which

Have non-volatile impurities
Are insoluble in water, and
Possess high vapour pressure at about 100⁰C.

For example: aniline, o- nitrophenol, nitrobenzene etc. are purified by this method.

Steam distillation is based on the fact that:

i) A liquid boils at a temperature at which its vapour pressure becomes equal to the atmospheric pressure.

ii) The vapour pressure of the mixture of two immiscible liquids is equal to the sum of vapour pressure of the individual liquids.

In actual process, steam is continuously passed through the impure organic liquid.
Steam heats the liquid but itself get practically condensed to water.
After sometime, the mixture of the liquid and water begins to boil because the vapour pressure of the mixture becomes equal to atmospheric pressure.
Obviously this happens at a temperature which is lower than the boiling point of the substance or that of water.
In steam distillation, the liquid get distilled at a lower temperature than its boiling point when it might decompose.
Therefore, the main purpose of steam distillation is to avoid decomposition.
The proportion and liquid in the mixture that distills over is given by

W₁/W₂ = (p₁× 18)/(p₂ × M)

Where W₁ and W₂ stand for the masses of water and the organic liquid that distills over, p₁ and p₂ represents vapour pressures of water and the liquid at the distillation temperature and M is the molecular weight of liquid.

6) Distillation under reduced pressure

Many organic liquids decompose at or below their boiling points, thus they cannot be purified by distillation under atmospheric pressure.
Such compounds are, therefore, purified by distillation under reduced pressure.
Under reduced pressure, the liquid will boil at a low temperature and the temperature of decomposition will not be reached.
Glycerol is one of the examples of it as it boils with decomposition at 290^0.
If the pressure is reduced to 12 mm of hg, it boils at 180⁰C without decomposition and hence can be safely distilled.
The liquid to be distilled is taken in a two necked flask called “Claisen’s flask”.
It is fitted with a long capillary tube in the main neck and the thermometer in the side neck.
The side tube of the second neck is connected to a condenser carrying a receiver at the other end.
The receiver is further connected to a vacuum pump and a manometer which indicates the pressure.
The whole apparatus is tested to be air tight. The flask is usually heated on an oil bath or sand bath.
During the distillation a stream of air bubbles is allowed to pass through the capillary tube to prevent bumping by ensuring steady pressure.
The desired pressure is maintained by working the vacuum pump.
Distillation under reduced pressure find a wide applicability in various industries, e.g., in the concentration of sugarcane juice in sugar industry and separation of glycerol from spent lye in soap industry.

7) Differential extraction

The process of removing a substance from its aqueous solution by shaking with a suitable organic solvent is known as differential extraction.
This method is used to extract organic compounds (solid or liquid) when present in aqueous solution.
The aqueous solution of the organic substance is shaken with a suitable solvent which is immiscible with water but in which the organic compounds is highly soluble.
Some of the commonly used solvents are benzene, chloroform, ether, and carbon tetrachloride, etc.
The aqueous solution of the organic compound is mixed with a small quantity of the organic solvent in a separating funnel.
The funnel is stoppered and its contents shaken for some time when the organic solvent dissolves out the solute.
The mixture is now allowed to settle and in this way two separate layers are formed.
The lower aqueous layer is run out by opening the tap and the solvent layer is collected separately.
The remaining aqueous layer is again subjected to above treatment with a fresh quantity of organic solvent.
The solvent layer is then kept over anhydrous calcium chloride, which absorbs any water present.
Finally, it is filtered and the solvent is distilled off when the organic compound is obtained as residue.

8) Chromatography

It is a modern technique used for separation of mixture, isolation of compounds from natural sources, purification and identification of the constituents of a mixture and also in the concentration of product that occurs in nature in very dilute state.
The technique of chromatography is based upon the fact that the different constituents of the mixture are absorbed to different extents by a given adsorbent.
Tswett in 1903 described this method for the first time for the separation of coloured substances into individual components.
Since then the method has undergone tremendous modifications so that now-a-days various types of chromatographic methods are in use to separate mixture, whether coloured or colourless into its constituents and to test purity of these constituents.
Some of these methods are: column chromatography, paper chromatography, thin layer chromatography, and vapour phase chromatography, etc.
The simplest chromatographic method is column chromatography.

It is carried out in a long glass column having a stop-cock near the bottom.
A plug of cotton or glass wool is placed at the bottom of the column to support the adsorbent.
The tube is packed uniformly with a suitable adsorbent which is usually taken in the form of slurry in petroleum ether. This constitutes the stationary phase.
The commonly used adsorbents are: activated aluminium oxide (alumina), magnesium oxide, silica gel, starch and Fuller’s earth.
Nearly one-fourth of the tube is left empty.
A loose plug of cotton or glass wool is then placed at the top of the adsorbent column.
The substance to be purified is added, as such if t is a liquid or in the form of its solution in some suitable solvent is it is a solid, at the top of the column and allowed to pass slowly through it.
As it passes through the column, the different components of the mixture get adsorbed.
As soon as the last portion of the substance enters the column, a selected solvent, known as eluent, is added to the column. This acts as a moving phase.
More than one eluent may be used in certain cases.
The eluents dissolve out the different components selectively.
The weakly adsorbed component is eluted more rapidly than a strongly adsorbed component.
Such a progressive separation of a mixture occurs in this chromatography.
Different components of the mixture are collected in the form of different fractions in separate conical flasks.
The eluent from each fraction is then distilled off to get the various components in pure form.
This technique is now widely used in research laboratories for the purification of different substances and for the separation of mixtures.
For example: a mixture of an alcohol and a liquid hydrocarbon may be separated by using adsorbent alumina and petroleum ether as eluent.