**1. Introduction to phenolic compounds**

#### **1.1 Structures and classification**

There are more than ten thousand phenolic structures identified in nature, ranging from simple aromatic rings to complex polymerized compounds, making the phenolic compounds one of the main and largest groups of secondary metabolites of plants [1, 2].

The polyphenol structure (composed of several hydroxyl groups on aromatic rings) has been identified in higher plants in abundance, and to a lesser degree in edible plants [3]. Depending on the extent of their distribution in nature, phenolic compounds have been classified as being shortly distributed, widely distributed, or as polymers [4]. The types of phenolic compounds that are present or available in all plants are considered widely distributed. Examples include flavonoids and/or flavonoid derivatives, coumarins, and a wide range of phenolic acids including benzoic acid and cinnamic acid (**Figure 1**).

Phenolic compounds that are shortly or less widely distributed have limited presence in plants and include simple phenols, pyrocatechol, hydroquinone, and resorcinol (**Figure 2**).

The polymer class of phenolic compounds contains macromolecules such as tannin and lignin, illustrated in **Figure 3**.

#### **Figure 1.**

*Examples of widely distributed phenolics.*

#### **Figure 2.** *Examples of shortly distributed phenolics.*

Another method of classification is according to the size of a phenolic compound's carbon chain, dividing the compounds into 16 major classes: simple phenols (C₆), benzoquinones (C₆), phenolic acids (C₆-C₁), acetophenones (C₆-C₂), phenylacetic acids (C₆-C₂), hydroxycinnamic acids (C₆-C₃), phenylpropenes (C₆-C₃), coumarins and isocoumarins (C₆-C₃), chromones (C₆-C₃), naphthoquinones (C₆-C₄), xanthones (C₆-C1- C₆), stilbenes (C₆-C₂-C₆), anthraquinones (C₆-C₂- C₆), flavonoids (C₆-C₃-C₆), lignins ((C₆-C₃)ₙ), lignans and neolignans ((C₆-C₃)₂) [5].

A broader designation into flavonoids and non-flavonoids has traditionally been used; it was brought on based on the plethora of natural flavonoids and the diversity of C₆-C₃-C₆ structural offshoots [6]. The nonflavonoids group is classified according to the number of carbons that they have and comprises the following subgroups: phenolic acids, stilbenes, lignans, and others [5, 6]. **Figure 4** shows the main groups of plant phenolics.

### **1.2 Production and functions**

The results of plants' photosynthesis can be classified as primary or secondary metabolites. Primary metabolites are usually described as substances that are essential chemical units of living plant cells. These fundamental substances are cellulose, hemicelluloses, polysaccharide, and lignin [7]. Plants synthesize a vast number of smaller molecules that are secondary metabolites. The secondary metabolites are formed by evolution to defend plants against harmful attacks by herbivores, pathogens, insects, and parasitic species [8].

Polyphenols, as secondary metabolites, are involved in functions related to reproduction, growth, defense, and pigmentation in plants, acting against pathogens, parasites, and predators. They also exhibit a great capacity to minimize the harmful effects of UV radiation, which may alter the regular metabolism in plants [2, 3]. Phenolic compounds are second only to cellulose in making up the bulk of organic matter, with phenolics (mainly lignin) accounting for about 40% of the organic carbon in the

#### **Figure 4.**

*Main groups of polyphenolic compounds based on plants' biological functions (adapted from [5]).*

biosphere. The phenolic secondary metabolites are produced through the shikimic and malonic acid pathways, as shown in **Figure 5** (adapted from [9]).

Since the synthesis of phenols can proceed by different pathways, phenols are a diverse metabolic group, and their chemical diversity is matched by their varied roles in plants (**Table 1**). Phenols' roles include these actions: function in mechanical support; protect the plant from harmful ultraviolet solar radiation and excessive water loss; attract pollinators and seed dispersers; serve as signals that induce defensive

#### **Figure 5.**

*Main plant pathways for production of phenolic compounds (adapted from [9]).*


#### **Table 1.**

*The most prominent biological functions of phenolic groups.*

*Phenolic Compounds in the Built Environment DOI: http://dx.doi.org/10.5772/intechopen.98757*

reactions to biotic or abiotic stresses; suppress the growth of nearby competing plants (i.e., allelopathy); be attractive substances to accelerate pollination; provide coloring for camouflage and defense against herbivores; act as antibacterial and antifungal agents against pathogens; and provide protection against herbivores by repulsive taste or smell.

Phenolic acids constitute about one-third of the phenolic compounds in the human diet and are characterized by a remarkable antioxidant activity [10]. Phenolic acids can be divided into two groups: benzoic acids and their derivatives; and cinnamic acids and their derivatives. The benzoic acids are the simplest phenolic acids found in nature. Cinnamic acids are rarely found in their free form in plants and are generally in the form of esters.

#### **1.3 Characteristics and reactions**

Phenolics function as antioxidants in a number of ways. Phenolic hydroxyl groups are good hydrogen donors: hydrogen-donating antioxidants can react with reactive oxygen and reactive nitrogen species [11, 12] in a termination reaction, which breaks the cycle of generation of new radicals (reactions 1–5, adapted from [13] where, φ: phenolic antioxidant, •: free radical species, R-C-R: organic molecule).

$$\text{Fe}\_4\text{q}^- + \text{N}\_2\text{O} + \text{H}\_2\text{O} \rightarrow \text{HO}\bullet + \text{N}\_2 + \text{HO}^- \tag{1}$$

$$\text{MgO} + \text{HO}\bullet \rightarrow \text{q}\bullet + \text{H}\_2\text{O} \tag{2}$$

$$
\mathfrak{q}\mathfrak{q} + \mathfrak{H}\mathfrak{q} \to \mathfrak{H}\mathfrak{q}\mathfrak{q} \tag{3}
$$

$$\text{R}-\text{C}-\text{R} + \text{HO}\bullet \rightarrow \text{R}-\text{C}\bullet -\text{R} + \text{H}\_2\text{O} \tag{4}$$

$$\mathbf{R} - \mathbf{C}\bullet - \mathbf{R} + \mathfrak{q} \Longleftrightarrow [\mathbf{R} - \mathbf{C} - \mathbf{R} \cdot \cdots \mathbf{} \mathfrak{q}]\bullet \tag{5}$$

$$[\mathbf{R} - \mathbf{C} - \mathbf{R} \cdot \cdots \cdot \mathbf{q}]\bullet \to \mathbf{R} - \mathbf{C} - \mathbf{R} + \mathbf{q}\bullet \tag{6}$$

The antioxidant molecule (φ) reacts with the initial reactive species and forms the antioxidant radical (φ•). The interaction between φ• and organic molecules produces an intermediate radical specie which has much greater chemical stability than the initial radical (reaction 5). The phenolics have the unique ability to produce stabilized free radicals due to delocalization of electrons between hydroxyl groups and the πelectrons of the benzene ring. These long-lasting radicals modify the oxidation processes and interrupt the free radical attack on other organic molecules (reaction 6) [14]. Similarly, the phenolic compounds can chelate metal ions and effectively stop the metal ions from producing free radicals. Another property which attributes to phenolic compounds antioxidant capability [10].

The phenolic compounds of plant origin act as antioxidants due to their redox properties, allowing them to act as reducing agents, hydrogen donors, free-radical quenchers, and metal chelators [15, 16]. In an organism, an oxidative process can be responsible for the generation of free radicals that attack the cells; the oxidative and nitrosative stress leads to serious diseases such as cancer, cardiovascular diseases, atherosclerosis, neurological disorders, hypertension, and diabetes mellitus [17, 18]. The principal function of antioxidants is to delay the oxidation of other molecules by inhibiting the initiation or propagation of oxidizing chain reactions by free radicals, consequently reducing oxidative damage [19]. Phenolic compounds can slow the oxidative degradation of lipids due to their antioxidant properties. Accordingly, the

food industry is showing increasing interest in application of phenolic-rich plant materials, such as crude extracts and oils of herbs, fruits and spices, to improve the quality and nutritional value of foods [20].

Phenolic compounds have also been associated with other bioactivities important for maintaining good health, such as anti-inflammatory, antimicrobial, and antiproliferative activities [21, 22]. In addition to the pharmacological interest in these compounds, their biological activities have also been explored in other industry sectors such as food [23, 24], cosmetics [22, 25], packaging, and textiles [18, 26, 27].
