158 THE PLANT BASES [cH. 



The alkaloids themselves belong to various classes of compounds, 

 though the basic character always preponderates. Thus, for example, 

 piperine is an amide and can be hydrolyzed into the base piperidine 

 and piperic acid : atropine is an ester made up of the base tropine and 

 tropic acid. 



Various methods are employed for the extraction of alkaloids but 

 the exact course of events depends on the alkaloid in question. On the 

 whole the method is either to treat the plant material with alkali and 

 then extract the free alkaloid with ether or chloroform and finally purify 

 by making a salt again ; or to extract the alkaloid from the plant with 

 dilute acid, set free the insoluble, or difficultly soluble, base with alkali, 

 and then prepare a salt of the base. 



Though individual alkaloids have distinctive reactions, the group as 

 a whole has certain reactions in common, namely the precipitation by 

 the so-called " alkaloidal reagents." These reagents are tannic, phospho- 

 tungstic, phosphomolybdic and picric acids, also potassium-mercurio- 

 iodide solution and iodine in potassium iodide solution. 



Expt. 151. General reactions of alkaloids. Make a 0'5 7o solution of quinine 

 sulphate in warm water and add a few drops of each of the following reagents: 



(a) Tannic acid solution, A white precipitate is formed. 



(6) Mercuric iodide in potassium iodide solution [Briicke's reagent : 50 gms. of 

 potassium iodide in 500 c.c. water are saturated with mercuric iodide (120 gms.) and 

 made up to 1 litre]. A white precipitate is formed. 



(c) Phosphotuugstic acid (50 gms. of phosphotungstic acid and 30 c.c. of cone, sul- 

 phuric acid are dissolved in water and made up to a litre). A white precipitate is formed. 



{d) Iodine in potassium iodide solution. A brown precipitate is formed. 



(e) Picric acid solution. A yellow precipitate is formed. 



Expt. 152. Extraction of the free base from quinine sulphate. Add strong sodium 

 carbonate solution drop by drop to some of the quinine sulphate solution until a 

 white precipitate of quinine is formed. Then add ether and shake up in a separating 

 funnel. The precipitate will disappear as the quinine passes into solution in the ether. 

 Separate off the ethereal solution and let it evaporate in a shallow dish. The quinine 

 is deposited. Take up the quinine again in dilute sulphuric acid and test the solution 

 with the alkaloidal reagents. 



The alkaloids are classified into five groups according to the nucleus 

 which constitutes the main structure of the molecule. These five groups 

 are: 



1. The pyridine group. 



2. The pyrrolidine group. 



3. The tropane group. 



4. The quinoline group. 



5. The isoquinoline group. 



