*2.1.1 Simple phenolics*

Simple substituted phenol compounds can be hydroxyphenols or dihydroxybenzenes. Examples are catechol (1,2-dihydroxybenzene), resorcinol (1,3-dihydroxybenzene), and hydroquinone (1,4-dihydroxybenzene) (**Figure 3**).

**Figure 2.** *General structure of simple substituted phenols.* *Phenolic Compounds: Classification, Chemistry, and Updated Techniques of Analysis… DOI: http://dx.doi.org/10.5772/intechopen.98958*

#### **Figure 3.**

*The structure of hydroxyl-substituted phenols.*

Other simple substituted phenol compounds can also be dihydroxyphenols or trihydroxybenzenes. Examples are pyrogallol (1,2,3-trihydroxybenzene), hydroxyquinol (1,2,4-trihydroxybenzene), and phloroglucinol (1,3,5-trihydroxybenzene) (**Figure 4**).

#### *2.1.2 Phenolic acids*

Phenols that contain a carboxylic acid are termed as phenolic acids. If the carboxylic acid functional group is directly bonded to the phenol ring, the phenolic compound is termed as hydroxybenzoic acid. When carboxylic acid functional group and the phenol ring are separated by two doubly bonded carbons (a C=C bond), phenolic compounds are termed as hydroxycinnamic acids.

#### *2.1.2.1 Hydroxybenzoic acids*

Hydroxybenzoic acids are benzoic acids substituted with a hydroxyl group. Alternatively, they can be viewed as phenols that are substituted with a carboxylic acid functional group that is directly bonded to the phenol ring (**Figure 5**).

The hydroxyl group in hydroxybenzoic acids can be *ortho* (*o*) (salicylic acid), *meta* (*m*), or *para* (*p*). The structures are shown below (**Figure 6**).

**Figure 4.** *The structure of dihydroxyl-substituted phenols.*

**Figure 5.** *General structures of hydroxyl-substituted benzoic acids.*

Dihdyroxybenzoic acids are benzoic acids that are substituted with two hydroxyl groups. The two hydroxyl groups can mainly be in 2,3-, 2,4-, 2,5-, 2,6-, 3,4-, and 3,5 relative positions (**Figure 7**).

Trihdyroxybenzoic acids are benzoic acids that are substituted with three hydroxyl groups. Examples include 2,4,6-trihydroxybenzoic acid and 3,4,5- trihydroxybenzoic acid (gallic acid) (**Figure 8**).
