**2. Chemistry and occurrence**

Plant biomass is a major potential sustainable source of organic carbon. Terpenes, terpenoids and resin acids are a group of non-polar molecules and share a building block, isoprene or isoprenoid [3], as a common elementary unit (**Figure 1**).

Isoprene, the epitomic terpene substance, is one of the most plentifully ethereal hydrocarbon compounds on Earth attributable to the worldwide plenitude of terpenoid biosynthesis, not the other way around. Around 40% of the biogenic volatile natural compounds transmitted by plants are isoprene, and isoprene is the key hydrocarbon distinguished in human breath. Film crowds breathe out more isoprene when watching scenes of anticipation [5].

Based on ancient scientifically verified data, it is found that the expressions "isoprenoid" and "terpenoid" are applied conversely and that there is still no worldwide accord on terminology.

For instance, a few researchers have named "terpene" to allude just to hydrocarbons dependent on an indispensable number of C5 units, and "terpenoid" or "isoprenoid" to assign the entire class of compound dependent on an intrinsic number of C5 units [6].

Such compounds can be in every way called "terpenes," and the expression "terpenoid" ought to be held for compounds, for example, the steroids, which have differing quantities of carbon molecules, however, are originated from a (C5)n structure [7]. Ruzicka considered this multitude of compounds aggregately to be "terpenic" [8]. Nes and McKean employed the expression "isoprenoid" to portray the entire group [9].

**Figure 1.** *Structure of isoprene unit [4].*

*Revisiting the Medicinal Value of Terpenes and Terpenoids DOI: http://dx.doi.org/10.5772/intechopen.102612*

#### **Figure 2.**

*Classification of terpenes and terpenoids.*

Different classes along with carbon units of terpenes and terpenoids are depicted in **Figure 2**.

Terpenes are significant elements of natural oils, which are secondary metabolites synthesized for battling infectious or secreted because of stress conditions. These are extricated from different fragrant plants commonly limited in mild to warm climatic regions like the Mediterranean and tropical nations where they represent a significant piece of the conventional pharmacopeia. They are ethereal, fluid, transparent, and seldom colored, lipid-dissolvable, and dissolvable in natural solvents with a by and large lower thickness than water. They can be isolated from all plant organs, for example, blossoms, buds, leaves, twigs, stems, seeds, roots, wood, fruits or bark, and are accumulated in secretory cells, holes, trenches, glandular trichomes, or epidermic cells [10].

### **2.1 Monoterpene**

The smallest of terpenes are monoterpenes (**Figure 3**). They contain the compound C10H16, come from different flowers, fruits and leaves and are known as the main component of essential oils, fragrances and many structural isomers [11]. Monoterpenes are found in natural scents for example α-pinene, which imparts scent to pine trees [12], and limonene from citrus plants [13]. One of the main purposes of monoterpenes is to attract pollinators or to serve the purpose of repelling other organisms from feeding off of plants [14].

### **2.2 Sesquiterpene**

Sesquiterpenes, containing the chemical formula C15H24 (**Figure 4**), are much larger compounds than monoterpenes and are much more stable in comparison [15]**.** Sesquiterpenes are naturally occurring and found in plants, fungi, and insects and act as a defensive mechanism or attract mates with pheromones in insects [16].

**Figure 3.**

*Sub-classes of monoterpenes with structural example.*

#### **Figure 4.**

*Classification of sesquiterpenes with structural example.*

Gossypol is a sesquiterpene that is present in cotton plants. It has anti-neoplastic properties and might hinder fertility in male people that is the reason it should be taken out from natural oils and different items before human application [17]. Avarol, a sesquiterpenoid that has been displayed to have antifungal and antimicrobial effects, is compelling against AIDS infection [18].

*Revisiting the Medicinal Value of Terpenes and Terpenoids DOI: http://dx.doi.org/10.5772/intechopen.102612*

#### **2.3 Diterpene**

Diterpenes are natural substances that contain the atomic skeleton, C20H32 (**Figure 5**) [19]. Diterpenes have physiologically dynamic compounds, for example, plant development chemicals that manage germination, blooming, switch regenerative cycles (from abiogenetic to sexual multiplication) of plants, and vitamin A activity [20]. Cafestol and kahweol are diterpene alcohols that are found in the oil derived from coffee beans [21].

#### **2.4 Triterpene**

Triterpenes are composed of three or six isoprene units and have the chemical formula C30H48 (**Figure 6**) which includes steroids and sterols with squalene being the biological precursor of all Triterpenes [22]. Triterpenes are produced by animals, plants, and fungi. They play a role as precursors to steroids in animal and plant organisms, and are derived from mevalonic acid [5].

Their properties have been studied for anticancer, antioxidant, antiviral, and anti-atherosclerotic activities [23]. Although, the medicinal uses of tri-terpenes are not quite as recognized as other different types of terpenes but their uses are being continuously investigated by researchers.

#### **2.5 Tetraterpene**

Tetraterpenes are also known as carotenoids (**Figure 7**) that have the molecular formula C40H56 and can be in the category of terpenes because they are made from isoprene units [24]. They are found in all different types of fungi, bacteria, and plants and are mainly responsible for red, yellow, or orange fat-soluble plant and animal pigments [25]. One of the most crucial and common tetraterpenes is betacarotene [26].

**Figure 6.** *Classification of Triterpenes.*

**Figure 7.** *Structural example of tetraterpene.*
