PURIFICATION AND SEPARATION OF ORGANIC COMPOUNDS
Organic compounds, whether solids , liquids or gaseous, when are separated from organic reactions are seldom pure. They may be contaminated with other compounds which are formed as a byproduct, owing to a side reaction.
These organic compounds have to be purified before using them in other chemical reaction. Several methods of purification have to be :-
1- Crystallization.
2- Sublimation.
3- Extraction.
4- Distillation.
5- Chromatographic adsorption.
The method employed depends upon the nature of the material to be separated and purified.
1- Purification of organic compound by crystallization:-
Crystallization is one phenomena which is involved in the production of pure crystals form solution , after dissolving the impure substance in a suitable solvent or mixture of solvents.
The choice of the solvent is very important in this process, water or any other organic solvent ( ethyl alcohol, methyl alcohol, acetone, glacial acetic acid, ethyl acetate, benzene, petroleum ether, ether, chloroform………..etc.) can be used.
Ether could be avoided whenever possible for its high inflammability.
A distinct relation has to be existed between the constitution of the organic compound and the solvent, so they may undergo into solution. As the old rule said “like dissolves like “, therefore, polar organic compound dissolve in non polar solvents and so on.
It is also assumed that:-
1) The solvent should not react chemically with the substance to be purified.
2) It should dissolve the impurities readily.
3) It should well-formed crystals of the purified compound.
4) It must be capable of easy removal from the crystals of the purified compound.
In some cases mixture of two of the solvents might be used, especially when a substance is highly soluble in one solvent and sparingly soluble in other solvent, the mixture of the two solvents can be used for crystallization.
Purification by crystallization may be improved by means of some adsorbing material such as animal charcoal or boneblack.
These materials have the property of adsorbing, on their surface areas, colored impurities present in many organic substances.
The following discussion illustrates the techniques used in purification by crystallization. The impure substance is placed in a flask, add the chosen solvent gradually with constant stirring, heat the mixture on a water bath, nearly to boiling ,away from direct flame ( since most of the solvents mentioned above are inflamable ) until a clear solution is formed. Filter the hot solution through a fluted- filter paper, and collect the hot filtrate in a dry clean flask.
( Fluted filter paper – is a folded filter paper several times , back and forth in pleats like . )
The funnel has to be kept hot , to avoid crystallization on its stem . set your filtration apparatus as is shown on the board . leave the filtrate to cool down gradually . after the crystallization is complete, the crystals are filtered form the remaining “mother liquor” by means of suction filtration with a Buchner funnel .
The crystals are well pressed on the filter with the aid of the flat top of a glass stopper, and washed a few times with small portions of the cold solvent . the suction is continued few minutes longer, and the hose on the suction flask is removed before the water is turned off. ( set your apparatus as is shown on the board ).
The crystalline mass is spread upon a porcelain plate covered with a filter paper, and a allowed to stand until perfectly dry .
Some organic substances do not crystallize readily, even from supersaturated solutions. A small crystal of the pure material may be added to induce the crystallization process- a technique known as “ seeding” . crystallization can be induced also by scratching the sides or bottom of the container with a glass rod.
The microscopic scratches in the glass surface provide sharp edges upon which crystal growth may start