An essential oil is a concentrated hydrophobic liquid containing volatile aroma compounds from plants. Essential oils are also known as volatile oils, ethereal oils or aetherolea, or simply as the "oil of" the plant from which they were extracted, such as oil of clove. Volatile oils are the odorous and volatile products of various plant and animal species. As they have a tendency to undergo evaporation on being exposed to the air even at an ambient temperature, they are invariably termed as volatile oils, essential oils or ethereal oils. They mostly contribute to the odoriferous constituents or “essences” of the aromatic plants that are used abundantly in enhancing the aroma by seasoning of eatables. Oil is "essential" in the sense that it carries a distinctive scent, or essence, of the plant. Essential oils do not form a distinctive category for any medical, pharmacological, or culinary purpose. Essential oils are generally extracted by distillation. Steam distillation is often used. Other processes include expression or solvent extraction. They are used in perfumes, cosmetics, soaps and other products, for flavouring food and drink, and for adding scents to incense and household cleaning products. In general, it has been observed that a single volatile oil invariably comprises even more than 200 different chemical components, and mostly the trace constituents are solely responsible for attributing its characteristics flavour and odour. Essential oils have been used medicinally in history. Medical applications proposed by those who sell medicinal oils range from skin treatments to remedies for cancer and often are based solely on historical accounts of use of essential oils for these purposes. Claims for the efficacy of medical treatments, and treatment of cancers in particular, are now subject to regulation in most countries. Interest in essential oils has revived in recent decades with the popularity of aromatherapy, a branch of alternative medicine that claims that essential oils and other aromatic compounds have curative effects. Oils are volatilized or diluted in carrier oil and used in massage, diffused in the air by a nebulizer, heated over a candle flame, or burned as incense.
Classification of Essential Oils
The most acceptable classification whereby volatile oils and volatile-oil containing drugs may be grouped together as follows, namely: i. Hydrocarbon volatile oils
ii. Alcohol volatile oils
iii. Aldehyde volatile oils
iv. Ketone volatile oils
v. Phenol volatile oils
vi. Phenolic ether volatile oils
vii. Oxide volatile oils and
viii. Ester volatile oils
(A) Hydrocarbon Volatile Oils
It has been observed that terpene hydrocarbons usually occur in most of the volatile oils obtained from natural sources. They may be further classified into three categories, namely: (a) Unsaturated acyclic hydrocarbons,
(b) Aromatic hydrocarbons, and
(c) Alicyclic hydrocarbons.
(a) Examples of chemical constituents belonging to the category of unsaturated acyclic hydrocarbons are β-Myrcene.
Chemical Structure It is 7-methyl-3-methylene-1, 6-octadiene. Occurance It is found in several essential oils, such as Oil of Bay ( or Myrcia oil)- Myrcia acris ( Family : Myricaceae) ; Oil of Hops – Humulus lupulus Linn., ( Family : Moraceae); and Oil of Turpentine –Pinus logifolia., (Family : Pinaceae). Isolation The oil of bay is treated with sodium hydroxide solution and the remaining undissolved portion which mostly contains myrcene, is repeatedly subjected to fractional distillation under vacuo (it is also obtained by pyrolysis of β-pinene)
Characteristic Features It has a pleasant odour. It is lighter than water. It is practically insoluble in water, but soluble in alcohol, chloroform, ether and glacial acetic acid.
(a) β-Myrcene on reduction with sodium and alcohol (absolute) gives rise to dihydromyrcene (C10H18) which on subsequent...