*2.2.1.2 Monoterpenes C10*

Monoterpenes are compounds made up of two units of isoprene, and of chemical formula C10H16, among the classes included in this group of terpenoids: iridoids and pyrethrins; they have anti-inflammatory pharmacological properties.

### *2.2.1.3 Sesquiterpenes C15*

This class of terpenes has a crude formula C15H24 (three units of isoprene), these molecules are found in linear or cyclic form; they have several applications but are rarely found in EOs because of their low volatility.

**33**

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

Diterpenes consist of a group of chemically heterogeneous compounds, their crude formula is C20H32 (four units of isoprene). Because of their higher boiling points, they are not considered to be essential oils; instead, they are classically considered to be resins, the material that remains after steam distillation of a plant

Diterpenes can be classified as linear, bicyclic, tricyclic, tetracyclic, pentacyclic,

Sesterterpenes consist of 5 units of isoprene (25 carbon backbone). They exist in a wide variety of forms, including linear, monocyclic, bicyclic, tricyclic, tetracyclic,

The triterpenes are made up of six isoprenes, they have relatively complex cyclic

There are also tetraterpenes which consist of 8 units of isoprene (C40H64); and when the number of isoprene units exceeds 8 units, terpenes are called polyterpenes

The phenylpropanoids are a family of organic compounds with an aromatic ring and a three-carbon propene tail [5]; they are synthesized via shikimic acid pathway

The different properties of OEs are related to the species and their chemical

Essential oils are characterized by several physicochemical properties:

in alcohol and most of the organic solvents.

• They have a lower density than water (0.8 to 1.08).

• The EOs are known by their volatile, odorous, and flammable character.

• They are colorless or have a yellowish color in the liquid state and at ambient

• They have a low polarity and a low solubility in water however they are soluble

Essential oils have various biological properties due to the variety of their

or macrocyclic Diterpenes depending on their skeletal core [4].

*2.2.1.4 Diterpenes C20*

*2.2.1.5 Sesterterpenes C25*

*2.2.1.6 Triterpenes C30*

*2.2.2 Phenylpropanoids*

**2.3 Essential oils properties**

*2.3.1 Physicochemical properties*

temperature.

*2.3.2 Biological properties*

chemical composition, they are using as:

(biosynthesis).

composition.

and macrocyclic frameworks [4].

extract [3].

structures.

(C > 40).

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

## *2.2.1.4 Diterpenes C20*

*Bioactive Compounds - Biosynthesis, Characterization and Applications*

Essential oils (EO) are complex mixtures of volatile, lipophilic, and liquid compounds, extracted from different parts of a plant by physical processes. These

According to ISO norm (ISO/D159235) "An essential oil is a product made by distillation with either water or steam or by mechanical processing of citrus rinds or by dry distillation of natural materiel. Following the distillation, the essential oil is

Essential oils are highly concentrated substances and therefore rarely used neat

they evaporate slowly when exposed to air at room temperature, and because of

The EOs are composed of terpenoids in large proportion, phenylpropanoids, and other compounds of diverse origin such as that resulting from the degradation of

The Terpenoids represent the major part of essential oil compounds; they are defined as substances composed of isoprene (2-methylbutadiene) units [2]; they are

The Terpenoids are classified according to the number of C5 isoprene units that

Hemiterpenes are the simplest type of terpenoids; they consist of a single unit of isoprene. There is evidence that these compounds may assist in plant defense by repelling herbivores or by attracting predators and parasites of herbivores [3].

Monoterpenes are compounds made up of two units of isoprene, and of chemical formula C10H16, among the classes included in this group of terpenoids: iridoids

This class of terpenes has a crude formula C15H24 (three units of isoprene), these molecules are found in linear or cyclic form; they have several applications but

and pyrethrins; they have anti-inflammatory pharmacological properties.

interesting natural products have the characteristic odor of the plant.

physically separated from the water phase."

EO's quality depends on many factors:

• Harvest way and extraction technique.

• Origin and parts of the plant used.

• Purification process.

they contain; the classes are:

*2.2.1.1 Hemiterpenes C5*

*2.2.1.2 Monoterpenes C10*

*2.2.1.3 Sesquiterpenes C15*

**2.2 Essential oils composition**

this, they are sometimes referred to as volatile oils [1].

synthesized by acetate via the mevalonic acid pathway.

are rarely found in EOs because of their low volatility.

**2. Essential oils**

**2.1 Definition**

[1],

fatty acids.

*2.2.1 Terpenoids*

**32**

Diterpenes consist of a group of chemically heterogeneous compounds, their crude formula is C20H32 (four units of isoprene). Because of their higher boiling points, they are not considered to be essential oils; instead, they are classically considered to be resins, the material that remains after steam distillation of a plant extract [3].

Diterpenes can be classified as linear, bicyclic, tricyclic, tetracyclic, pentacyclic, or macrocyclic Diterpenes depending on their skeletal core [4].
