**3.1. Natural sources**

**2.2. Classification based on the number of phenol units present in the molecule**

nins, stilbenes and lignans (**Figure 2**) [8].

422 Phenolic Compounds - Natural Sources, Importance and Applications

**2.3. Classification based on distribution in nature**

**Figure 2.** Structure of simple phenols.

**2.4. Classification based on location in plants**

**3. Sources of phenolic compounds in water**

molecular weight), and proteins [10].

the polymer class of phenolic compounds are tannin and lignin [9].

Phenolic compounds can be grouped as simple, bi and polyphenols depending on the number of phenol groups present in a particular molecule. Simple phenols are phenols with only one substituted phenolic ring. In other words, they have substituted phenols. Examples of simple phenols include phenolic acid, hydroquinone, resorcinol, thymol, etc. [7]. Biphenols contain two phenolic units while polyphenols consist of multiple units of the phenolic structure. Based on the number of phenol rings present, and the type of elements binding rings together, polyphenols can be subdivided into various groups such as flavonoids, phenolic acids, tan‐

Based on their extent of distribution in nature, phenolic compounds have been classified as being shortly distributed, widely distributed and as polymers. Widely distributed phenols are the types that are present or available in all plants, or are of high significance in specific plants. Examples include flavonoids and/or flavonoid derivatives, coumarins, phenolic acid including benzoic acid and cinnamic acid. Those that are shortly or less widely distributed are limited and include simple phenols, pyrocatechol, hydroquinone and resorcinol. Examples of

Phenolic compounds that are free in soluble forms in cells are categorised as being in solution while those bound as complexes in the cell wall, as insoluble. Soluble phenolic compounds include low and medium molecular weight phenolics such as simple phenol, flavonoids and tannins. Insoluble phenolics include high molecular weight tannins and phenolic acids. Other insoluble phenolics include those bound to polysaccharides in the cell walls (usually with low

The existence of phenolic compounds in water can be attributed to natural and anthropogenic activities. Natural sources of phenolic compounds in water pollution include decomposition
