Purification of Organic Compounds

Purification of Organic Compounds

Most of the organic substances when synthetically prepared or isolated from natural source are impure due to the presence of some bye-products or reactants. Before the compound can be analysed qualitatively or quantitatively, it is necessary that it should be in a pure state. Purification is not always easy and various methods depending upon the nature of the compound and impurities are employed. The following methods are generally used for this purpose:

(1)    Crystallization

(2)    Fractional crystallization.

(3)    Sublimation.

(4)    Distillation at atmospheric pressure.

(5)    Distillation under reduced pressure.

(6)    Steam distillation.

(7)    Fractional distillation with or without the use of a fractionating column.

(8)    Chemical methods of purification.

1. Crystallization: Most of the solids are purified by crystallization. The technique employed is the same as employed for inorganic solids, except that in addition to water, organic solvents like alcohol, ether, chloroform, benzene etc. are frequently used because the organic compounds are more readily soluble in organic solvents. This is based on the principle “Similia Similibus Solventer” i.e., like dissolved by like. The process involves the following steps.

(i)                  Selection of the proper solvent: A proper solvent is that which (a) does not dissolve the impurities at all or dissolves them to a much larger extent so that either the impurity of the product remains in the mother liquor after separation of the solids, (b) the solvent selected should be such that it dissolves the maximum quantity of the solute when hot and throws out the maximum quantity when cooled.

The common solvents in use for this purpose are petroleum ether (323-333k); methyl alcohol (338K); acetone (329K); chloroform (334.2K); carbon tetrachloride (349—354K); ethyl acetate (350K); benzene (353.4k); toluene (383K) etc.

To select the proper solvent, small amounts (a few milligrams) of the substance are put into a number of small test tubes and treated with a small quantity of the common solvents. The suitable solvent will be that in which the substance dissolves on heating and from which it readily crystallizes out on cooling.

After selecting out the proper solvent, we must find out the amount of it to be used. Large excess of the solvent should be avoided otherwise a sufficient amount of the substance will remain dissolved in the mother liquor.

(ii)                Preparation of the solution: First of all the crude material is well powdered and a suitable amount is taken in a round bottom flask. A small amount of solvent is added and the flask heated on a water bath using a reflux condenser. If the substance does not dissolve completely.

preparation of the solution

then some more solvent is added. In case water is used as a solvent, the flask can be heated directly over the flame without using the reflux condenser. Sometimes only an air condenser is used.

(iii)               FiItration: The hot solution prepared as above is then filtered through a funnel fitted with an ordinary filter paper. Occasionally hot water funnel is used to prevent undue cooling during filtration. Otherwise crystallization

filtration

will start on the filter paper and choke it. It happens particularly when large volumes have to be filtered. The hot water funnel is an ordinary funnel surrounded by a circular double walled metal jaket with a side tube and filled with water, which is kept hot by a burner.

(iv)              Crystallization: The hot filtrate obtained above is allowed to cool. For getting pure and smaller crystals the

filtrate should be cooled rapidly. If larger crystals are required, the cooling is done slowly. But such crystals are not very pure as they may entrap minute amounts of impurities during their formation. The cooling should be continued until complete separation of the crystals has taken place. The soluble impurities remain in the mother liquor and do not come out with crystals.

(v)                Separation and drying of crystals: The crystals together with the mother liquor are transferred to a buchner funnel fitted on a suction flask which is connected to a water

suction pump. The funnel is previously fitted with a filter paper. The suction pump is started. The mother liquor passes down into the filtration flask quickly due to reduced pressure inside the flask. The crystals remain on the filter paper. They are washed two or three times with a small quantity of pure solvent to remove any adhering impurities. The suction is continued so as to dry the crystals as far as possible.

For further drying the crystals are first pressed between two pads of filter paper and then dried in air or in a temperature controlled over depending upon the nature of the substance. Sometimes the crystals are dried in a vacuum dessicator containing concentrated sulphuric acid or anhydrous calcium chloride. Sometimes it is convenient to

make use of a centrifugal machine. In industries the centrifugal machine is very commonly used. If crystals obtained are somewhat coloured due to the presence of trace of impurities, they are redissolved in the minimum quantity of the solvent, a little of animal charcoal is added, the suspension is boiled for a few minutes, filtered and crystallized as before. Crystallization is repeated until the solid has the same melting point on two successive operations.

2. Fractional Crystallization: The method of simple crystallization is suitable for the purification of a single substance, contaminated with small amount of impurities. But when two (or more) substances (both of which are soluble in the solvent used), are to be separated, the technique of fractional crystallization is employed. The hot solution of the two or more substances in a suitable solvent is cooled slowly.

The constituent which has a lower solubility crystallizes out first at a certain temperature. It should be filtered at this temperature. The other constituent will crystallize out on further cooling. The process is to be repeated several times for complete separation. The separation becomes more and more difficult when the solubilities of the constituents are nearly equal.

3. Sublimation: Certain substances when heated, first directly from the solid to the vapour state without melting.

The vapour when cooled, give back the solid substance. This process is known as Sublimation which is very helpful in separating a volatile solid from a non-volatile solid. It is, however, of limited importance as only a few substances such as naphthalene, camphor, benzoic acid etc. can be purified by this method.

The method consists in placing the impure material in a dish or big watch glass and covering this with an inverted funnel, the stem of which has been plugged with cotton. The substance is kept covered by a perforated filter paper which checks the sublimate from falling back into the dish. The dish or the water glass is heated very slowly on a sand bath with a free flame.

The upper edges of the funnel may be kept cooled by covering it with layers of wet filter papers. The solid vapouries and condenses on the walls of the funnel, leaving the non-volatile impurities in the dish.

The above procedure of sublimation is a crude one. Several more refined apparatus are available for this purpose. The one designed by Bruhl is very efficient. For substances which decompose under ordinary conditions of heating, vacuum sublimation is used. The substance in this case is heated under vacuum.

4. Distillation at Atmospheric Pressure: The process of distillation is employed for the purification of liquids from

non-volatile impurities. The impure liquid is boiled in a flask and the vapours thus obtained are collected and condensed in another vessel called receiver. The non-volatile impurities are left behind in the first flask. The apparatus used for simple distillation is shown above.

The impure liquid is taken in a round bottom distillation flask connected to a water condenser. (An air condenser can be used if the boiling point of the liquid is above 423K). The flask is heated on a water bath or sand bath depending upon the boiling point of the liquid. The vapours of the substance rise and come into the condenser tube where they liquify and are collected in a receiver placed at the lower end of the condenser.

It is better to protect the receiver from the flame especially in case of low boiling point liquids. The impurities remain behind in the distillation flask. It is desirable to add a few glass beeds, pumice stone or porcelain pieces to avoid bumping or super heating of the liquid in the distillation flask.

5. Distillation under Reduced Pressure: The process of distillation is suitable only for those liquids which boil without decomposition at atmospheric pressure. For those

organic liquids which decompose before their boiling point, distillation is carried out under reduced pressure when the liquid boils at a lower temperature. A liquid boils at a temperature when its vapour pressure becomes equal to the atmospheric pressure.

The apparatus used for this purpose is shown in the figure. To avoid increased chances of bumping and superheating a Claisen flask fitted with a capillary tube is used. The capillary is kept immersed in the liquid and by using a pressure tubing screw clip arrangement air is regulated through it, which eliminates bumping, or superheating.

The pressure inside the apparatus is reduced by using a filter pump, or for lower pressures, a vacuum pump. A mercury manometer is also used to indicate the pressure. Thus glycerine, which has a normal b.p. of 663K; can be distilled at 553K without decomposition at 12 mm pressure.

6. Steam Distillation: Many substances that are insoluble in water and are volatile in steam are purified by distillation in a current of steam and this process is known as Steam Distillation. In this method the non-volatile

impurities are left in the distillation flask. The apparatus is shown in figure given above. The substance to be purified is placed in a large round bottom flask, clamped at an angle, so as to prevent the solution being thrown into the condenser. The steam is generated in the steam generator and is bubbled through the impure substance by means of a tube.

The flask containing the substance is also heated gently on a sand bath in order to avoid too much condensation of steam in it.The liquid in the flask soon begins to boil and mixed vapours of the compound and steam pass over into the condenser and the condensed liquid is received in the receiver.

The distillation should be continued for about 15 minutes after oily drops or solid particles cease to appear in the condenser. In case of a soluble acid the distillation should be stopped when the distillate no longer gives an acid reaction to litmus.

The distillate in the receiver consists of water and the purified substance. In case the substance in an oily liquid of aniline, it is separated from water by means of a separating funnel. If it is an insoluble solid it is separated by filtration. If the substance is soluble in acid, the solution is exactly neutralized with and the solution evaporated to dryness.

From the pure sodium salt the acid is recovered by distilling with sulphuric acid. Essential oils, petroleum, turpentine oil, para separating funnel dichlorobenzene etc. are purified by steam distillation.

7. Fractional Distillation: A mixture of two or more volatile liquids can be separated by fractional distillation. If the boiling points differ by more than 40°C, the operation can be carried out with the help of ordinary distillation apparatus as discussed in distillation at atmospheric pressure. The more volatile over first and is collected in a receiver. Now temperature is again raised and the first receiver is disconnected and a new receiver is attached. Thus the distillate is collected in fractions and hence this process is known as Fractional Distillation.

But when the boiling points of the constituents are near, such as acetone (b.p. 329 K) and methyl alcohol (b.p. 338 K), they will not separate with the simple distillation apparatus. The apparatus is slightly modified. Instead of using a flask with a side tube, a simple round flask is used and the same is fitted with a suitable type of fractionating column.

The condenser is attached to the side tube of the fractionating column. Several designs of fractionating columns are in use. The real purpose of a fractionating column is to increase the cooling surface and to provide obstruction to the passage of ascending vapour or descending liquid.

In industrial processes, much longer and efficient columns are used. As the mixture containing the liquids A and B boiling say at 323 K and 333K respectively, is heated in this apparatus,

the vapours consisting more of A than B rise up. On coming into contact with the large cooling surface of the fractionating column, the vapours of B condense more than those A because B is less volatile. As the condensate consisting mostly of the less volatile component B flows down the fractionating column, it meets the fresh hot ascending vapours.

During this process, the ascending vapours are deprived of the less volatile component B which condenses and in exchange the down coming condensate loses its more volatile A which joins the upward going vapours is now much enriched in A.

This exchange of vapours is repeated throughout the length of the column, with the result that by the time the vapours reach the top of the column, they consist mainly of A while the down coming liquid mostly of B. In this way an almost complete separation is effected. If one fractionation is not sufficient, then it may be repeated for complete separation.

8. Chemical Methods of Purification: The physical methods of purifying and separating organic compounds, described in the above sections are of general application and of great value in separating substances that are chemically similar. On the other hand many chemical methods are also employed in case of mixture of substances that are chemically different. The exact details of the method employed have been discussed with the individual compounds. Only examples of a few cases have been given below:

(i)                  In purification of petroleum and coal tar products for their acidic, basic and neutral components, sulphuric acid is used to separate the basic component and caustic soda solution to separate the acid component.

(ii)                Separation of acetic acid from pyroligneous acid as its calcium salt and decomposing the latter with concentrated hydrochloric acid.

(iii)               Preparation of pure methyl alcohol free from acetone by conversion of alcohol into methyl oxatate, which is then decomposed by boiling with caustic soda solution.

(iv)              Separation of primary, secondary and tertiary amines by Hinsberg or Hoffmann methods using diethyl oxalate or benzene sulphonyl chloride for their separation.