Volatile oils: 

Volatile Oils are the odorous oily principles which are entirely, or almost entirely, volatile without decomposition obtained from the sap and tissues of certain plants. A few, e.g. oil of anise, are solid at 15.5° c., but melt to form a liquid at slightly higher temperatures. They are called essential oils because they are the essences of plants. Chemically, they are compounds of Mono-, Sesqui- or Di- terpenes which are hydrocarbon of the general formula [C5H8-, Isoprene Unit] and their oxygenated, hydrogenated and dehydrogenated derivatives. Physically they are colorless odoriferous principles found in various plant parts and they evaporate when exposed to air at ordinary temperature.

According to the French Pharmacopoeia (1965), “Essential oils are products, generally of rather complex composition, comprising the volatile principles contained in the plants, and more or less modified during the preparation process. To extract these volatile principles, there are various procedures. Of these, only two may be used to prepare official oils: steam distillation of oil containing plants or of selected plant parts, and expression.” 

Volatile oils are called essential oils because volatile oils represent the “essences” or odoriferous constituents of the plants. Volatile oils are colorless as a rule, particularly when they are fresh, but on long standing they may oxidize and resinify, thus darkening in color. To prevent this darkening, they should be stored in a cool, dry, place in tightly stopper, preferably full (not half emptied) amber glass containers.

To give some examples, cinnamon oil from cinnamon, peppermint oil from mint, clove oils from clove flower buds are some commonly used volatile oils.

Properties of volatile oils: 

1. Source: All official volatile oils are of vegetable origin. Normally they pre-exist in the plant – stored in a special secretory tissue (e.g. Citrus peel oil cells or oil ducts in umbelliferous fruits).Oil of bitter almond is an exception; it is formed by hydrolysis of the glycosides.

2. Color: Colorless Liquid

3. Evaporation: Volatile oils are evaporated on open exposure at ordinary temperature, on exposure to air, volatile Oils oxidize on open air and Resins are formed.  

4. Thermal decomposition: Thermal decomposition of Volatile Oils gives Isoprene  Units [Isoprene Monomer (C5H8-)].

5. Odor: Having characteristic odor.

6.  Density: Lighter than water.

7. Miscibility: Immiscible with water but freely miscible with organic solvents- alcohol, ether, chloroforms etc.

8.  Optically Activity: Optically active.

9. Method of obtaining: Obtained by distillation and expression.

10. Chemical Nature:  Volatile oils are not glyceryl esters of fatty acids. So, they do not be saponified with alkalis and do not be rancified like fixed oils.

Essential oils, due to their high concentration of volatile compounds, can ignite and even start a fire when heated with an open flame or added in diffusers with exposed flames, like oil burners. Not all volatile oils are flammable, but most essential oils are. Volatility (evaporation rate) and flammability are related but different properties; a substance can be volatile without being easily flammable, and a substance can be flammable without being very volatile. For example, water is volatile but not flammable, while motor oil is flammable but not volatile.

Distribution of volatile oils in plant kingdom: 

Depending on the plant Family in the whole Plant Kingdom, Volatile Oils may occur in-

1. Glandular Hairs of the stems and leaves – Labiateae.

2. Modified parenchymal cells – Piperaceae

3. Oil tube or Vittae – Umbelliferae (e.g. Citrus peel oil cells or oil ducts in umbelliferous fruits).

4. Lysigenous Passage – Rutaceae, Pinaceae

5. They may also be formed from protoplasm, resinogenous layer of cell and         hydrolysis of glycosides (oil of bitter almond is formed by hydrolysis of the glycosides).

6. They can also extracted from wood (Sandal wood), seed (Nutmeg), leaves (Eucalyptus oil), barks (Cinnamon), petals (Rose) and stigma (Clove).

4. 7. Volatile oils may occur in all tissues- in the conifers.

5. 8. They appear in appreciable quantities only in the petals- in the rose.

6. 9. Volatile oils can found only in the bark and leaves- in cinnamon.

7. 10. It can found in the umbelliferous fruits.

8. 11. In the orange, one kind of oil occurs only in the flower petals and another kind only in the rind.

Chemistry of volatile oils: 

Each volatile oil differs widely in chemical compositions. Usually hydrocarbons- alcohols, phenols, ether, aldehyde, ketone, oxides and their esters are present in volatile oils. The major chemical constituent is Terpene-a natural product. Terpene is composed of Isoprene units which is the structural component of volatile oils and widely occurring in natural compounds. This unit comprises of C5H8-  a conjugated diene  (2-methyl-1,3 diene) containing 2 unsaturated bonds.

During the formation of Terpenes, the isoprene units are linked a head-to-tail fashion and the number of units incorporated into a particular terpene serves as a basis for the classification of these compounds.

Volatile Oils are the Mono-, Sesqui or Di-Terpenes.

Monoterpenes are composed of 2 Isoprene units and have molecular formula C10H16- or C10H18. The Terpenes found most often in volatile oils are monoterpene. Example: Geraniol, Limonene

Sesquiterpenes are composed of 3 Isoprene Units and have molecular formula C15H24-. Example: Cardinene

Diterpenes are composed of 4 Isoprene Units and have molecular formula C20H32-. Example: Abietic acid, steviol etc.

Diterpenes form the basis for biologically important compounds such as retinol, retinal, and phytol.

Triterpenes are composed of 6 Isoprene Units and have molecular formula C30H48-. Example: Lanosterol

Another major group of volatile oil constituent are the Phenyl-propanoids. These compounds contain the 6-phenyl ring with attached C3  Propene side chain. Many of them are Phenol or Phenol Esters. Example: Methyl Salicylate, Methyl Eugenol, Anethole, Cinnamic Aldehide

In some cases, the propane side chain has been abridged to give a C1-C6 structure.

Difference between Volatile Oils and Fixed Oils: 

Classification of Volatile Oils: 

Depending on the chemical constituents or chemical  nature volatile oils may classified as: