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General Methods of Extraction and Isolation of Alkaloids:-

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الكلية كلية الصيدلة     القسم فرع العقاقير والنباتات الطبية     المرحلة 3
أستاذ المادة ايناس نجم عبيد عاشور       28/03/2018 22:02:23
Pharmacognosy
1
Third stage Dr. Enass Najem
2nd semester Lec:2
General Methods of Extraction and Isolation of Alkaloids:-
The general methods of extraction and isolation of the alkaloids from the plant sources one has to take into consideration the following steps in a sequential manner, namely:
1. Separation of the alkaloid(s) from the main bulk of the non-alkaloidal substances,
2. Most of the alkaloid-containing plants, several alkaloids having closely related chemical structures are normally present, such as: the cinchona alkaloids consist of more than twenty five alkaloids. Separation of each individual alkaloid from the mixture of alkaloids obtained from a particular plant source (e.g., cinchona bark) using latest separation techniques, for instance, preparative high-performances liquid chromatography, (HPLC) .
Nevertheless, the general methods of isolation of alkaloids largely depend upon several vital factors, for instance: (a) the alkaline nature of most alkaloids, (b) the ability and ease of formation of alkaloidal salts with acids, and (c) the relative solubilities of the resulting alkaloidal salts either in polar organic solvents e.g., ethanol, chloroform, isopropanol etc., or in aqueous medium.
The general methods of extraction of alkaloids from the plant sources followed five steps namely
(i) Sample preparation
(ii) Liberation of free alkaloidal base
(iii) Extraction of alkaloidal base with organic solvent
(iv) Purification of crude alkaloidal extract
(v) Fractionation of crude alkaloids
All these five steps shall be discussed individually as under:
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Sample Preparation
The first and foremost step is the sample preparation. The plant material is reduced to a moderately coarse powder by appropriate means using grinders and sieves, to facilitate maximum effective contact of the solvent with the ruptured alkaloid bearing tissues and cells.
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 depend on the following factors, namely:
(a) Natural state of the alkaloids: It has been observed that the salt of a strongly basic alkaloid 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 (yield, their corresponding soluble sodium or potassium salts) and, therefore milder alkaline reagents e.g., dilute ammonia solution are necessary for their liberation.
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.
NaOH or KOH Solution The alkaloids that occur naturally as their tannate salts specially require either NaOH or KOH solution for their subsequent liberation
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because of their intimately strong bondage with the alkaloid and extremely insoluble nature.
Extraction of Alkaloidal Base
The extraction of alkaloidal base may be accomplished by three different types of solvents that are discussed below, namely:
[A] Extraction with Water-Miscible Solvents:- A plethora of alkaloids and their respective salts are soluble in alcohols, such as: methanol, ethanol, isopropanol; therefore, these very solvents may also be employed for the extraction of the plant substances. The usual pretreatment of the crude drug with alkali may be avoided completely, because alcohol appears to affect dissolution of not only the alkaloidal salts but also the free bases found in the plant substances. It is, however, believed that alcohol predominantly exerts a hydrolyzing effect upon the alkaloidal tannates and other salts. In actual practice, neither pretreatment of the crude drug with an alkali nor acidification of the alcohol with a small amount of a mineral acid or an organic acid is required.
[B] Extraction with Water-Immiscible Solvents:- In reality, the most widely used water-immiscible solvents for the extraction of alkaloids are: chloroform, diethyl ether (solvent ether) and isopropyl ether. Interestingly, chloroform is regarded as the choicest water-immiscible solvent for a broad-spectrum of alkaloids present in the plant kingdom and extracts them with varying degrees of ease.
Note: Chloroform is not suitable for the extraction of quaternary alkaloids e.g., tubocurarine.
[C] Extraction with Water The crude drug is subjected to extraction with water previously acidified with dilute solution of HCl, H2SO4 or CH3COOH, which is subsequently rendered alkaline, preferably with dilute NH4OH solution and finally extracted with a water-immiscible solvent as stated in [B] above.
Undoubtedly, water being an excellent and absolutely inexpensive polar solvent for the extraction of alkaloids, but if offers an enormous volume of disadvantages because it carries along with it a large number of other plant components, for instance: sugar, pigments (e.g., chlorophylls), starches, tannins, proteins etc., which ultimately puts across a collosal waste of time, energy and chemicals. Hence, its usage has been resulting to a bear minimum level.
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Purification of Alkaloidal Extract
The main bulk of the crude alkaloidal extract is invariably subjected to further purification by means of either anyone or combination of the following methods:
(a) Extraction with Acid Solution The extraction of the alkaloid from the bulk of the crude alkaloid solution in immiscible organic solvent is invariably carried out by shaking with an acid solution like HCl and H2SO4.
Subsequently, the acid solution is rendered alkaline with dilute NH4OH solution to liberate the alkaloids which is then extracted with an organic solvent. The solvent is removed under reduced
pressure and the traces of moisture is removed with anhydrous sodium sulphate.
(b) Precipitation of Alkaloid with Precipitating Reagent The usual precipitation of the alkaloid as a complex compound is accomplished by the addition of a suitable precipitating reagent. The resulting alkaloidal complex is further purified by filtration, recrystallization and ultimately decomposed to obtain the desired free alkaloid(s).
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. 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) and other chromatographic techniques including column chromatography, ion-exchange chromatography.

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