Purification of Organic compounds : Types, methods and principles
Introduction
We know that, Organic compounds are obtained from the natural sources. They are also synthesized in the laboratory. It is essential to obtain an organic compound in pure form for determination of its properties and molecular formula. i.e. removal of impurities associated with organic compounds. There are many methods for purification of organic compounds based on nature of the organic compounds to be purified and the impurities present in it.
Following methods are used for purification of organic compounds:
1) Crystallization
2) Fractional crystallization
3) Sublimation
4) Distillation
5) Fractional distillation
6) Steam distillation
7) Solvent extraction
8) Chromatography, etc.
Criteria of purity:
Once an organic compound is purified, it becomes necessary to test it's purity. A pure organic compound has characteristics physical properties such as crystalline form, refractive index, specific gravity, melting point, boiling point etc. If a sample of a given compound shows the properties that the pure compound is known to process, the sample may be considered as the pure compound. The properties of these compounds change with the nature and the amount of impurities present.
There are two criteria for purity of solids and liquids, respectively-
1) Melting point:
Melting point is criteria for purity of solids. Organic solids melts at a definite temperature while an impure substance does not melt at definite temperature.
Melting point of a solid is the temperature at which the solid and liquid phases of the substance exist in equilibrium with each other under one atmospheric pressure.
How melting point is determined?
There is a apparatus used for the determination of boiling point.
A capillary tube 5-6 cm long and 1 mm in diameter is sealed at one end and filled with finely powdered crystals of the organic compound to 3-4 mm in length from the bottom. The capillary tube is tied with the help of thread to a thermometer. The thermometer is partly inserted in Thiele tube containing paraffin oil. The Thiele tube is clamped to a stand and gradually heated. The heating can be controlled by adjusting the flame.
When the compound in the capillary tube just shows sign of melting, the heating is discontinued. The temperature at which the compound melts completely is recorded. It is the melting point of the organic compounds.
If the melting point determined is the same as that reported for the pure compound, the compound is said to be pure.
2) Boiling point:
The boiling point is criteria for purity of liquids. It is normally reported at standard atmospheric pressure. A pure organic liquids boils at a fixed temperature and the atmospheric pressure and is the characteristic of the compound.
Boiling point of a liquid is the temperature at which the vapour pressure of the liquid is equal to the atmospheric pressure.
How Boiling point is determined?
A little quantity of liquid is taken in an ignition or fusion tube and is tied to thermometer with a thread. A capillary tube is sealed at one end. The open end is immersed into the liquid in the ignition tube. The thermometer is then inserted into the Thiele tube containing paraffin oil. The Thiele tube is heated slowly. The stage at which brisk and continuous bubbles come out from the open end of the capillary, tube heating is stopped and temperature at which the bubbles just stop and the liquid rises into the capillary tube is noted.
If the boiling point of given liquid is equal to that of the pure liquid, given sample of liquid is pure.
Methods of purification-
1) Purification of Solids crystallization:
Crystallization means the process of obtaining a pure solid in a crystalline form from its solution.
Principle:
At higher temperature generally the substance is more soluble than at lower temperature in a given solvent.
Process:
The impure solid substance to be purified is dissolved at high temperature in a solvent in which it is insoluble or sparingly soluble at room temperature but appreciably soluble at highest temperature. The solution is heated to get the saturated solution. This hot solution is filtered through a hot water funnel to remove insoluble impurities.
The filtrate is allowed to cool slowly at room temperature when fine crystals of pure substance are formed. The crystals are removed from the mother liquor by filtration and washed with the same solvent to remove adhered impurities. The crystals are then dried. Repeated crystallization may be necessary for the purification of a substances containing impurities of comparable solubility.
If the compound is colourless but the filtration is coloured, then some coloured impurities are present in the solution. These are removed by adding small quantity of activated charcoal to the solution and warmed. It is then filtered while not. The coloured impurities are removed by their adsorbtion on the charcoal.
Choice of solvent:
The solvent to be used for crystallization should have the following properties:
1) It should have high solubility for the compound at higher temperature and less solubility at room temperature.
2) The impurities should be either insoluble or sparingly soluble in it.
3) It should not react chemically with the substance to be purified.
4) As far as possible, it should be low boiling liquid so that it's removal becomes easy.
5) It should not be viscous.
Example: Water, Acetone, methanol, ethanol, ethyl acetate, petroleum ether, toluene etc.
Crystallization of benzoic acid :
Benzoic acid can be purified by crystallization from hot water. Impure benzoic acid is mixed in minimum quantity of water. The solution is heated to higher temperature to get a saturated solution. The solution filtered while hot to remove any insoluble impurities. The filtrate is allowed to cool slowly to the room temperature when the needle shaped crystals of pure benzoic acid separate out. The crystals are filtered, washed with little cold water and dried in air.
Fractional crystallization-
When the given substance contains impurities in large proportion or it is a mixture of two or more substances, instead of simple crystallization, fractional crystallization is used.
Principle:
Fractional crystallization is based on the differences in solubilities of the different compounds in the same solvent at the same temperature. The substance which is less soluble crysatllizes out first than the more soluble substance.
Example 1) Separation of urea and acetanilide from their mixture.
Urea (NH2CONH2) is more soluble than acetanilide (C6H5NHCOCH3) in water at ordinary temperature. A saturated solution of mixture is prepared at higher temperature and filtered to remove insoluble impurities while hot. The crystals of acetanilide separate out while cooling. Crystals are removed by filtration. The mother liquor which contains urea is further concentrated and allowed to cool; when, we get the crystals of urea, which are removed by filtration. These crystals of both acetanilide and urea are purified by repeated crystallizations. Purity is tested by determining melting points.
Example 2 : Separation and purification of glucose and fructose from their mixture:
Glucose and fructose, both being highly soluble in water, can not be separated by fractional crystallization using water as a solvent. Alcohol is the suitable solvent to effect their separation from their mixture. Glucose being almost insoluble in alcohol crystallizes on cooling while fructose being more soluble than glucose, remains in the solution.
A mixture of glucose and fructose is taken in 90% ethanol and heated below 323K. The solution is allowed to cool when crystals of glucose start appearing. In case, if there is no crystal formation at this stage, then few crystals of pure glucose are added to induce crystallization. This is called as 'seeding'.
Glucose being less soluble in ethanol crystallizes out first, while fructose remains in the solution. It is filtered and further purified by recrystallization from hot ethanol. Fructose can be obtained in the crystalline form by concentrating the mother liquor once again and evaporating the solvent ethanol.
2) Purification of Liquids:
Organic liquids containing soluble impurities can be purified by distillation method. Various methods employed for the purification of liquids depend upon the nature of the liquid to be purified and the impurities present. The miscible liquids and soluble solids as impurities are removed by distillation.
Simple Distillation:
Impure liquids containing soluble, non-volatile impurities and boil under atmospheric pressure without undergoing decomposition are purified by simple distillation.
Principle:
Distillation is a process in which liquid is converted into its vapours at its boiling point and the vapours are condensed to liquid on cooling.
Procedure:
The apparatus used for distillation consists of distillation flask fitted with a thermometer in its neck and Liebig's water condenser at the side tube. The impure liquid to be purified is taken in the distillation flask. Few pieces of porcelin are added to avoid bumping during heating. The thermometer is adjusted in such a way that it's bulb is near the opening of the side - tube. This ensures the exact measurement of the temperature of the vapour passing over the condenser. A suitable vessel called receiver is attached to the other end of the condenser to receive the condensed liquid.
On heating the distillation flask, the thermometer first records a rise in temperature. Any liquid coming out is rejected. When temperature becomes constant, most of the liquid passes over and is collected in the receiver as a distillate. When temperature starts rising once again, the distillation is stopped and the receiver is disconnected. The receiver contains pure liquid whereas impurities remain in the distillation flask.
For the distillation of highly volatile and inflammable liquid, the distillation flask is heated in a water -bath and when very high boiling liquid is to be distilled, the flask is heated on sand bath.
Example 1) Purification of low boiling liquid, carbon tetrachloride.
Impure CCl4 is taken in a distillation flask. Flask is fitted with a water condenser and a thermometer. A receiver is attached to the other end of the condenser. Since CCl4 is a volatile liquid, the distillation flask is heated on a water- bath. CCl4 distills at 350 K. The vapours after condensation are converted into liquid and pure CCl4 is collected in the receiver. When the temperature starts rising above 350 K, the distillation is stopped and the receiver is removed.
Example 2) Purification of high boiling liquid, Aniline
Impure aniline is reddish in colour. It is taken in a distillation flask, which is filtered with a water condenser and a thermometer. A receiver is attached to the other end of the condenser. Aniline distills at 457K. The vapours after condensation are converted into liquid. Pure aniline is collected in the receiver as a colourless liquid. When temperature starts rising above 457K , the distillation is stopped and the receiver is removed.
Fractional distillation:
It is a process used for the separation of two or more miscible liquids having different boiling points from their mixtures.
Principle:
The more volatile liquid distills first leaving behind the less volatile liquid in the distillation flask. As the distillate is collected in fractions, the process is called as fractional distillation.
Example: Separation of mixture of acetone and water
Procedure:
A mixture of acetone and water is taken in a distillation fitted with thermometer and water condenser. The flask is heated on the water bath when thermometer shows a constant temperature of 329K, acetone being more volatile distills out first and is collected as a first fraction in a receiver. When acetone is completely distilled out, the temperature starts rising and their receiver at a constant temperature of 373K. The two fractions may further purified separately, by repeated distillation.
