1.7.2 Liberation of Free Alkaloidal Base
It has been observed that the alkaloids invariably occur in the plant sources as the salt of acids, such as: oxalates, tannates etc. Therefore, when the plant substance is exposed to an alkaline medium, the alkaloidal salts are readily converted to the corresponding alkaloid bases.
Choice of Alkali Indeed, the choice of a suitable mineral base (alkali) for the ease of liberation of the alkaloid from the salts is not only very vital but also equally significant and largely depend on the following factors, namely:
(a) Natural state of the alkaloids: It has been observed that the salt of a strongly basic alkaloid with a mineral acid usually tends to undergo cleavage under the influences of a stronger base. Likewise, the corresponding salt of a weakly basic alkaloid and a relatively weak organic acid shall require a rather weaker base for its cleavage.
(b) Chemical characteristics of the alkaloidal base: The usage of strong alkali e.g., NaOH or KOH should be avoided as far as possible by virtue of the fact that certain alkaloids undergo hydrolysis on prolonged contact with a strong base.
Example
(i) Hydrolysis of ester-alkaloids, e.g., cocaine, hyoscyamine;
(ii) Phenolic alkaloids e.g., cephaeline, morphine. These alkaloids normally get solubilized while in contact with a strong alkali and, therefore milder alkaline reagents e.g., dilute ammonia solution are necessary for their liberation.
(c) Presence of fatty substances: The usage of strong alkali is strictly prohibited in the case of fat containing plant materials because of the formation of saponified products causing troublesome emulsions. In such cases, it is always preferred to defat the plant substance before proceeding for the liberation of free alkaloids.
Ammonium Hydroxide Solution Dilute aqueous ammonium hydroxide solution is one of the choicest alkali most frequently used for the liberation of alkaloids from the plant sources. It enjoys a two-fold advantage. First, being its adequate alkalinity to liberate most of the common alkaloids, and second by, its volatile nature so that it may be removed by evaporation of the solvent. As it has a tendency to be extracted by solvent ether from the aqueous solution, therefore, it is almost necessary to get rid of it by evaporation and subsequent washing repeatedly. In normal practice, usually even the last traces of ammonia are removed when the combined ethereal extract is reduced to half of its original volume under vacuum.
NaOH or KOH Solution The alkaloids that occur naturally as their tannate salts specially require either NaOH or KOH solution for their subsequent liberation. In certain typical instance even the use of KOH or NaOH fails to cleave the tannate salts because of their intimately strong bondage with the alkaloid and extremely insoluble nature.
Example
(i) Cinchona Bark: It has got to be treated first by heating with dilute HCl so as to decompose the salts and liberate the alkaloids in the form of water soluble hydrochlorides, and
(ii) Pomegranate Bark: It does not have the tannin so tenaciously bound to the alkaloids as in the case of cinchona bark. Hence, NaOH solution is strong enough to cause on effective split of the alkaloidal salts. It also acts to control the solubility of the water-soluble pomegranate alkaloids by preventing their dissociation.
