4.1.4 Classification
A plethora of pharmacologically active naturally occurring substances derived from ‘medicinal plants’ essentially comprise of rather large and complex molecules that invariably possess one or more than one of the chemical functional moieties which are responsible for attributing characteristic features for alcohol, phenols, esters, aldehydes, ketones, oxides and organic acids. In fact, the aforesaid chemical groups are often attached to the molecular skeletons (e.g. aromatic, heterocyclic compounds) of noticeable diversified nature and complexity.
In the light of the following two observations, the phytochemical classification is eventually
done on a more rational and broader perspective:
(a) Morphine and salicylic acid has one phenolic—OH group in their molecule but structurally they are world-apart, and
(b) Essential (or volatile) oils mostly contain a mixture of substances, such as: hydrocarbons, ketones, aldehydes, and terpenes.
Therefore, ideally the phytochemical classification is solely based on the types of plant constituents present in the natural products, namely:
The above phytochemical classification will be further expatiated with the help of some typical examples from the domain ‘pharmacognosy’ along with their structures, wherever possible, as under:

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