Alkaloids (meaning alkali-like) are an important class of basic organic compounds that occur in higher plants. They are known to contain one or more nitrogen heterocyclic rings as an integral part of their structure. They produce striking physiological effects when administered to humans. Thus, some alkaloids stimulate the central nervous system, while others cause paralysis; some alkaloids act as pain-relievers, others as local anesthetics; and still others act against infectious microorganisms. Most alkaloids are toxic and yet many find use in medicine.
What are Alkaloids?
A precise definition of the term alkaloid covering such a wide variety of compounds of this class is difficult. However, a broad definition may be stated as : Alkaloids are plant-produced basic (alkaline) compounds which contain one or more nitrogen heterocyclic rings and can have profound physiological effect an humans.
The above definition covers a large majority of alkaloids but it is by no means perfect. Its inexactness in evident from facts such as: (1) Piperine, the alkaloid of pepper, is neither basic nor does it produce and physiological effect ; (2) Caffeine, a basic nitrogen heterocycle, a heart stimulant and a diuretic, is usually not considered an alkaloid ; (3) The alkaloid Marijuana contains no nitrogen ; and (4) Ephiderene is an open chain alkaloid.
Classification of Alkaloids.
The individual alkaloids are frequently called by common names. These end in -ine, indicating that are alkaloids are amines. Alkaloids are divided into classes depending on the prominent heterocyclic ring system present in the molecule. Thus we have:
- Pyridine alkaloids e.g., Nicotine
- Piperidine alkaloids e.g., coniine. Piperine
- Quinoline alkaloids e.g., Quinine
- Isoquinoline alkaloids e.g., Papaverine, morphine
- Indole alkaloids e.g., Gramine, strychnine
Note : In the line formulas corners or junctions, or end of lines represent carbons with appropriate number of H atoms.