英文Alkaloids.ppt

WHAT IS THE ALKALOIDSS GENERAL PROPERTIES? Alkaloids are generally bitter in taste and are optically active. Most alkaloids are crystalline,colourless,non-volatile solids,insoluble in water , but soluble in polar organic solvents. Some alkaloids are water-soluble liquids (for exemple coniine,hygrine,nicotine). Few alkaloids are coloured (for exemple berberine is yellow). Nearly all basic alkaloids form salts with inorganic acids,which are soluble in water and insoluble in organic solvents. HOW ARE ALKALOIDS DETECTED? Alkaloids yield precipitates or characteristic colours when reacted with solutions of molybdic acid,picric acid,chloroplatinic acid,potassium mercuric iodide etc.Therefore the presence of an alkaloids in a plant is detected by using various reagents such as Mayers reagent,Wagners reagent,Dragendorffs reagent,Hagers reagent,chloroplatinic acid, Frohdes reagent etc. HOW ARE ALKALOIDS DETECTED? These precipitates may be amorphous or crystalline and are of various colours: cream(Mayers) yellow(Hagers) reddish-pinkish-brown(Wagner‘s and Dragendorff’s ) ※Caffeine and some other alkaloids do not give these precipitates. ISOLATION AND  PURIFICATION The differences in solubility between alkaloids and their salts provide methods for the isolation of alkaloids from the plant and their separation from the non-alkaloid substances. For many alkaloids are basic and occur as salts of 2-hydroxybutane-1,4--dioic acid(malic acid),or of 1,3,4,5-tetrahydroxycyclohexane(quinic acid).They can thus be extracted into acid solution using aqueous HCl, tartaric, or citric acids.Essentially neutral alkaloids like colchicine or piperine remain in the organic phase ,while most other alkaloids are isolated after basification and extraction into ethyl acetate. The almost invariably subsequent purification of the crude alkaloid mixtures is effected by chromatography using silica or alumina,and then recrystallization of the part