**3. Terpenes and terpenoids**

Terpenes are defined as secondary metabolites with molecular structures containing carbon backbones of isoprene (2-methylbuta-1,3-diene) units [124]. Terpenes are synthetized in the cytoplasm of plant cells through the mevalonic acid pathway. Biochemical modification such as oxidation or rearrangement of terpenes produces the related terpenoids. Terpenoids are then oxygenated derivatives of hydrocarbon terpenes such as aldehydes, ketones, alcohols, acids, ethers, and esters [34]. Terpenoids are the largest classes of plants' natural products accounting for more than 40,000 individual compounds of both primary and secondary metabolisms been identified; to date, new terpenoids are being discovered every year [12, 124].

In general, terpenoids can be divided into at least four groups of compounds that include true terpenes, steroids, saponins, and cardiac glycosides.

These types of natural lipids can be found in every class of living things, mainly in plants as constituents of EOs, and are therefore considered as the largest and structurally diverse group of natural products [125]. In general, only the hemiterpenoids, the monoterpenoids, and sesquiterpenoids are sufficiently volatile to be components of EOs. As widely acknowledged, the composition of EOs is mainly represented by mono-, sesqui-, and even diterpene hydrocarbons and their respective oxygenated derivatives [30, 126–128].

**Figure 13.**

*Biosynthesis pathways of monoterpenes, sesquiterpenes, and diterpenes.*

**89**

**Figure 15.**

*Structures of some monoterpenes and monoterpenoids.*

*Terpenoids as Important Bioactive Constituents of Essential Oils DOI: http://dx.doi.org/10.5772/10.5772/intechopen.91426*

Structurally, EO constituents typically have low molecular weights, which contribute to their high volatility. Terpenes are the most common constituents found in EOs [128]. They are made from isoprene units (several five carbon base units). Each group of terpenes arises from the head-to-tail condensation of a variable number of isoprene units. Variations in the number of isoprene unit repetitions, cyclisation reactions, and rearrangements are primarily responsible for their chemical and structural diversity. EOs consist of mainly monoterpenes (C10) and sesquiterpenes (C15) but also have diterpenes (C20), triterpenes (C30), and tetraterpenes (C40) at very low concentration with their oxygenated derivatives, respectively (**Figure 13**)

Hemiterpenes are part of minor terpenes of EOs. They are usually alcohols, aldehydes, and esters, with a 2-methylbutane skeleton [131]. The number of

**3.1 Classification**

[15, 102, 130].

*3.1.1 Hemiterpenes*

**Figure 14.** *Structure of few isolated hemiterpenes and hemiterpenoids.*
