1.7.5 Fractionation of Crude Alkaloids
It has been observed largely that most of the alkaloid-bearing plant materials usually contain a mixture of closely-related alkaloids. Therefore, it has become almost necessary to carry out an effective fractionation of crude alkaloids from the extract or solution of total crude alkaloids.
However, the traditional and orthodox methods of separation are not only difficult but also tedious and cumbersome. The commonly employed techniques of separation that were found to the reliable and dependable may be short-listed as follows:
(i) Fractional crystallization,
(ii) Fractional distillation, and
(iii) Derivatization with low solubility products.
The latest methods employed for the separation of alkaloids are the preparative high performance liquid chromatography (HPLC), high performance thin-layer chromatography (HPTLC), chromatotron, counter-current distribution and other chromatographic techniques including column chromatography, ion-exchange chromatography.
Following are some of the typical situations whereby the mixture of alkaloids may be separated effectively, such as:
(a) A larger section of the alkaloids are easily soluble in chloroform and relatively less soluble in other organic solvents. In general, the order of solubility is as stated below chloroform > acetone > ethanol > methanol > ethyl acetate > ether > n-hexane. Keeping in view the above solubility profile of alkaloids in organic solvents, if one of the alkaloids is much less soluble in ethanol than chloroform, the fractional crystallization of this alkaloid is possible. In this particular instance the chloroform-fraction is concentrated to an appropriate level, and hot ethanol added in small proportions at intervals. Thus, upon cooling the alkaloid, which is less soluble in ethanol, separates out conveniently.
(b) In case, the fractional crystallization of the mixture of closely related alkaloids become tedious and ineffective, one may try to form their respective salts,** and then carry out the separation indicated above.
(c) The various acids, namely: HCl, HBr, HI, HClO4, HNO3, C2H2O4, and C6H3N3O7, may either be employed in aqueous or methanolic solution. Thus, from the resulting methanolic solution, the salts of the respective alkaloids may be precipitated by the addition of ether. The precipitated crude alkaloidal salts may be further recrystallized from hot acetone containing a small proportion of methanol.
(d) In certain other specific instances, the salts of the respective oxalates, picrates and perchlorates may be precipitated from their solutions in acetone, by the addition of ethyl acetate.
