**5.1 Classification**

Due to the wide variety of phenolic compounds ranging from simple to polymeric, there exist multiple classifications:

• Based on the carbon chain:

From simple phenols to complicated lignins, 16 different classes can be distinguished. The main classes are shown in **Figure 6**.

• Based on solubility:

Depending on the form phenolics are found in plans (either free from any bound to other compounds or attached to cell wall polysaccharides or proteins), their solubility varies. Therefore, two categories can be defined:


Phenolic compounds are found in nature in different proportions. They thus can be divided to shortly distributed (simple phenols, pyrocatechol, hydroquinone, resorcinol ...), widely distributed (flavonoids and their derivatives, coumarins, and phenolic acids) and polymers (lignins and tannins) [21].

### **5.2 Main classes**

### *5.2.1 Simple phenols*

They are monomers constituted of various substitutions and represent the forming blocs of polymeric phenolic and acid compounds that make up the plant tissue. Some of them (p-hydroxybenzoic acid, protocatechuic acid, vanillic, syringic,

**39**

polymers (**Figure 7**) [11].

*Classes of phenolic compounds [26].*

*5.2.2 Flavonoids*

**Figure 6.**

*Plants' Bioactive Metabolites and Extraction Methods DOI: http://dx.doi.org/10.5772/intechopen.96698*

salicylic, and gallic acids) can be obtained by the hydrolysis of the plant tissue whereas some other free simple phenols do not require the destruction the cell wall

They are compounds containing two aromatic rings attached by a bridge of triple carbon atoms (C6-C3-C6). The bridge often contains double carbon bonds and hetero- elements, which forms another mid-cycle. Therefore, flavonoids can be classified into 13 subgroups among which flavanols, flavones, isoflavones, anthocy-

anidins or anthocyanins, and flavanones (**Figure 8**) [21].

*Bioactive Compounds - Biosynthesis, Characterization and Applications*

such as berries and spinach, is increasingly recommended [20].

polymeric, there exist multiple classifications:

guished. The main classes are shown in **Figure 6**.

as phenols, flavonoids, and tannins.

solubility varies. Therefore, two categories can be defined:

phenolic acids) and polymers (lignins and tannins) [21].

• Based on the carbon chain:

• Based on solubility:

lic acids.

**5.2 Main classes**

*5.2.1 Simple phenols*

• Based on distribution:

Phenolics are organic compounds containing one aromatic ring to which is attached one or more hydroxyl groups [11]. Phenolic compounds represent one of the most extensive groups of plants' secondary metabolites [20]. They are contained in a great share in the daily diet due to their countless benefits for human health and well- being. Henceforth, the consumption of phenolic-rich fruits and vegetables,

They are synthesized either through the shikimic acid or the acetic acid pathways. Phenylpropanoids are the result compounds of the shikimic acid pathway while simple phenols result from the acetic acid pathway acid. However, both pathways result in the formation of flavonoids, abundant phenolic compounds [21]. In addition to the presence of aromatic rings and hydroxyl substitutions, phenolic compounds are less likely to be found in the free form, rather, they are most frequently bound to other complex compounds as sugars to reduce their toxicity [21].

Due to the wide variety of phenolic compounds ranging from simple to

From simple phenols to complicated lignins, 16 different classes can be distin-

Depending on the form phenolics are found in plans (either free from any bound

• Soluble phenolics: Free phenolics found in the soluble fraction of the cell such

• Insoluble phenolics: Bound to other more complicated molecules to orm stable insoluble compounds. They majorly include condensed tannins and pheno-

Phenolic compounds are found in nature in different proportions. They thus can be divided to shortly distributed (simple phenols, pyrocatechol, hydroquinone, resorcinol ...), widely distributed (flavonoids and their derivatives, coumarins, and

They are monomers constituted of various substitutions and represent the forming blocs of polymeric phenolic and acid compounds that make up the plant tissue. Some of them (p-hydroxybenzoic acid, protocatechuic acid, vanillic, syringic,

to other compounds or attached to cell wall polysaccharides or proteins), their

**5. Phenolic compounds**

**5.1 Classification**

**38**


#### **Figure 6.**

*Classes of phenolic compounds [26].*

salicylic, and gallic acids) can be obtained by the hydrolysis of the plant tissue whereas some other free simple phenols do not require the destruction the cell wall polymers (**Figure 7**) [11].

### *5.2.2 Flavonoids*

They are compounds containing two aromatic rings attached by a bridge of triple carbon atoms (C6-C3-C6). The bridge often contains double carbon bonds and hetero- elements, which forms another mid-cycle. Therefore, flavonoids can be classified into 13 subgroups among which flavanols, flavones, isoflavones, anthocyanidins or anthocyanins, and flavanones (**Figure 8**) [21].

**Figure 7.** *Simple phenols [11].*

#### *5.2.3 Tannins*

They are compounds with a medium to high molecular weight that comprise two major classes: hydrolysable and condensed tannins. Hydrolysable tannins are glucose or a polyhydric alcohol centered compounds esterified with gallic acid or hexahydroxydiphenic acid to form gallotannin and ellagitannins, respectively. They are hydrolyzed by enzymatic, acidic or base treatment. On the other hand, condensed tannins, also known as non-hydrolyzable tannins or recently proanthocyanidins, are polymers of catechin and leucoanthocyanidin. Their antioxidant activity is dependent on their degree of polymerization. High molecular weight tannins have been demonstrated to be fifteen to thirty times more active than simple phenols (**Figure 9**) [21].

#### *5.2.4 Phenolic acids*

As their name implies, phenolic acids are characterized by the presence of a phenolic ring and a carboxylic group. They comprise two categories:

• Benzoic acids:

They are the simplest phenolic acids in nature. They contain seven carbon atoms C6-C1 with hydroxyl or methoxyl roots.

**41**

ring [21].

**Figure 9.**

**Figure 8.**

*Main classes of flavonoids [21].*

• Cinnamic acids:

They contain nine carbon atoms C6-C3 with hydroxyl or methoxyl roots. Their nomenclature varies according to the nature of roots attached to the main

*Chemical structures of (A) hydrolysable tannins and (B) condensed tannins [21].*

*Plants' Bioactive Metabolites and Extraction Methods DOI: http://dx.doi.org/10.5772/intechopen.96698*

*Plants' Bioactive Metabolites and Extraction Methods DOI: http://dx.doi.org/10.5772/intechopen.96698*

*Bioactive Compounds - Biosynthesis, Characterization and Applications*

They are compounds with a medium to high molecular weight that comprise two major classes: hydrolysable and condensed tannins. Hydrolysable tannins are glucose or a polyhydric alcohol centered compounds esterified with gallic acid or hexahydroxydiphenic acid to form gallotannin and ellagitannins, respectively. They are hydrolyzed by enzymatic, acidic or base treatment. On the other hand, condensed tannins, also known as non-hydrolyzable tannins or recently proanthocyanidins, are polymers of catechin and leucoanthocyanidin. Their antioxidant activity is dependent on their degree of polymerization. High molecular weight tannins have been demonstrated to be fifteen to thirty times more active than simple

As their name implies, phenolic acids are characterized by the presence of a

They are the simplest phenolic acids in nature. They contain seven carbon atoms

phenolic ring and a carboxylic group. They comprise two categories:

**40**

*5.2.3 Tannins*

**Figure 7.** *Simple phenols [11].*

phenols (**Figure 9**) [21].

*5.2.4 Phenolic acids*

• Benzoic acids:

C6-C1 with hydroxyl or methoxyl roots.

#### **Figure 8.** *Main classes of flavonoids [21].*

### **Figure 9.**

*Chemical structures of (A) hydrolysable tannins and (B) condensed tannins [21].*

• Cinnamic acids:

They contain nine carbon atoms C6-C3 with hydroxyl or methoxyl roots. Their nomenclature varies according to the nature of roots attached to the main ring [21].
