Alkaloids - Lecture Note

Alkaloids are compounds that have a heterocyclic structure containing an N atom in their core (heterocyclic ring) and alkaloids are basic, therefore they can dissolve in acids and form salts. In general, the alkaloid group has the following properties: alkaline, bitter taste, generally comes from plants and has pharmacological properties. A. General properties of alkaloids: 1. Alkaloids are insoluble or difficult to dissolve in water but alkaloids in salt form usually dissolve easily in water. 2. Free alkaloids (which are basic) usually dissolve in ether CHCl3or other organic solvents, but the salt is insoluble. 3. Alkaloid salts do not have the same crystal form and this crystal form is useful for microscopic identification. 4. The N bonds in alkaloids are usually in the form of primary, secondary, tertiary, quaternary amines, ammonium hydroxide and all of these N bonds are basic. B. Physical properties of alkaloids 1. Generally it has 1 N atom, although there are some that have more than 1 N atom, such as Ergotamine which has 5 N atoms. 2. Most of the alkaloids that have been isolated are insoluble crystalline solids with a certain melting point or have a range of decomposition. Some alkaloids are amorphous. 3. Most alkaloids are colorless but some complex compounds, aromatic species are colored (for example, berberine is yellow and betanin is red). 4. Free base alkaloids are soluble only in organic solvent although some pseudo alkaloids and proto alkaloids are soluble in water. Quaternary alkaloid salts are highly soluble in water. C. Chemical properties of alkaloids 1. Most alkaloids are basic. This property depends on the presence of an electron pair on the nitrogen. If the functional group adjacent to nitrogen is electronremoving, for example; alkyl group, then the availability of electrons in nitrogen increases and the compound is more basic. 2. On the other hand, if the adjacent functional group is electron-withdrawing (for example, the carbonyl group), then the availability of electron pairs is reduced and the effect of the alkaloid can be neutral or even slightly acidic. Example ; compounds containing an amide group. 3. The alkalinity of alkaloids causes these compounds to decompose very easily, especially by heat and light in the presence of oxygen. The product of this reaction is often N-oxide. Decomposition of alkaloids during or after isolation can cause various problems if storage lasts for a long time. The formation of salts with organic (tartarates, citrates) or inorganic (hydrochloric or sulfuric acids) compounds often prevents decomposition. That is why in trade alkaloids are commonly found in the form of salts C. Qualitative Test of the alkaloid group 1. General identification: a. Organoleptic tests: shape, color, taste, smell b. Solubility c. pH of the solution d. Fluorescence 2. Specific properties a. Soluble in HCl by Mayer and Bouchardat → ↓ soluble in alcohol b. Isolation: with NH base4OH → Alkaloid/chloroform. D. Color reaction 1. H2SO4concentrated 2. HNO3concentrated 3. Erdmann reaction: 12 ml H2SO4concentrated + 8 drops of HNO3Concentrated 4. Marquis: 2 drops of formalin + H2SO4concentrated 5. Frohde reaction: (Amm. Molybdate 0.5% in water) + H2SO4concentrated. 6. Hoshida: Mixture of Frohde and Marquis (Amm. Molybdate 0.3 g + Formalin 40% 0.5 ml + H2SO4concentrated 60 cc). 7. Mandelin (Amm. Vanadat 10% H2SO4concentrated). 8. FeCl3