Volatile oils

Volatile oils (Aetheroleum) are the odorous principles found in various plant parts. Because they evaporate when exposed to the air at ordinary temperatures, they are called volatile oils, ethereal oils or essential oils.
Practically all volatile oils consist of chemical mixtures that are often quite complex; they vary widely in chemical composition.
Chemical constituents of volatile oils may be divided into 2 broad classes, based on their biosynthetic origin:
terpene derivatives formed via the acetate-mevalonic acid pathway,
(2) aromatic compounds formed via the shikimic acid-phenylpropanoid route.

Terpenes are defined as natural products whose structures may be divided into isoprene units (C5H6). These units arise from acetate via mevalonic acid and are branched-chain, 5-carbon units containing 2 unsaturated bonds
Isoprene
Chemical structure of isoprene
During the formation of terpenoids, the isoprene units are linked in a head-to-tail fasion and the number of these units incorporated into a particular terpene serves as a basis for the classification of these compounds.
The link of isoprene units in a head-to-tail fasion in geraniol molecule.

Classification of terpenoids

Classification of volatile oils

Physical and chemical properties of volatile oils
Although volatile oils differ greatly in their chemical constitution, they have a number of physical properties in common:
Oils are colorless or colored transparent liquids (yellow, green, blue, brown),
characteristic odors and taste,
they are characterized by high refractive indices;
Essential oils are easy volatile liquids, but they do not leave a permanent grease spot on paper and cannot be saponified with alkalies;
Volatile oils can be distilled from their natural sources;
Most of them are optically active and their specific rotation are often a valuable diagnostic property;
Some terpens are easily isomerized (optical isomers, geometrical isomers, transferring of ethylene bonds);

Physical and chemical properties of volatile oils
As a rule, volatile oils are immiscible with water, but they are sufficiently soluble to impart their odor to water. The aromatic waters are dependent on this slight solubility;
Volatile oils, are soluble in ether, alcohol, and most organic solvents and fixed oils;
pH of oil: neutral or weak acid;
Oil density - from 0,700 g/cm3 tо 1,060 g/cm3, the majority of oil are lighter than water;
They don’t have fixed boiling point;
They are easily crystallized after freezing (camphora, menthol, thymol, anethol)
Volatile oils do not become rancid as do the fixed oils, but instead, on exposure to light and air, they oxidize and resinify,
Terpens react with halogens and phenol derivatives – with alkali solutions form phenolats.

Localization of volatile oil
Depending on the plant family, volatile oils may occur in specialized secretory structures:
1.Modified secretory parenhymal cells (Araceae,Piperaceae)
2. Lysigenous or schizogenous passages (Pinaceae, Rutaceae).
3. Glandular hairs
Asteraceae(Dandelion family)
4. Oil-tubes called vittae (family Apiaceae )
Lamiaceae (Mint family)

Peppermint: volatile oil-glands
Chamomile: volatile oil-glands

Eucalyptus secretory cavity
Eucalyptus secretory cavity
Curcuma, parenchyma with secretory cells
Pine: secretory passage

Methods of obtaining of volatile oils
Distillation:
Three types of distillation are used by industrial firms:
(1) water distillation,
(2) water and steam distillation,
(3) direct steam distillation
Dalton law or Raul law: The mixture of two not mixing substances (oils) begins to boil when the sum of their partial pressure reaches atmospheric pressure
2. Extraction:
2.1. with organic solvents (non –polar solvents);
2.2. with inert gas;
2.3. with fixed oils (maceration);
2.4. enfleurage is used in the production of perfumes and pomades (absorption of volatile oil by solid lipids).
3. Expression (for Citrus fruits)

Diagrammatic representation of water distillation unit where the plant material is suspended in water

Diagrammatic representation of a steam and water distillation unit with a baffle to prevent direct water contact with the plant material on the perforated grid.

Steam distillation is the process of distilling plant material with the steam generated outside the still in a stand-alone boiler.

Quantitative analysis of volatile oils by State Pharmacopoeia and European Pharmacopoeia
1 – round-bottom distillation flask
2 – graduated tube (receiver)
3 – stopper
4 – condenser
Ginsberg’s apparatus

Enfleurage
1.Construct enfleurage chassis: wooden frame filled with glass that fit tight together.
2. Solid fat are spread over glass (half centimeter thick).
3. Lay the fresh flowers over layer of fat.
4.Leave for about two days out of sun at room temperature.
5. After two days open the chassis and remove the flowers and repeat the process with freshly picked flowers. To get a good, strong-scented layer of fat, the process of renewing the flowers has to be done 4 times.
6. Remove the flowers and scrape the fat into a bowl.
7. In order to get essential oil the fat needs to be mixed with alcohol.
8. In order to remove the fat from alcohol, it can be put into container and frozen until the fat stays hard and alcohol remains in liquid form.

Assay of volatile oils
(European Pharmacopoeia)
European Pharmacopoeia demands to carry out the analysis:
Organoleptic analysis: to determine color, odor, taste, transparency.
Purity control: limit of alcohol, fixed and mineral oils.
Quality control: control of the contents of the main compounds of volatile oils by TLC and HPLC; identification reactions of volatile oils; residue on evaporation; limit test of foreign oil in volatile oil;
Determination of indices (value) and constants:
solubility in alcohol;
temperature of solidification (freezing point) ;
density of oil;
refractive index;
index of optical activity;
acid value,
ester value,
hydroxyl value

TLC-analysis of peppermint oil
Reference compounds:Tl menthol, T3 menthyl acetate, T2 menthone/isomenthone, T4 menthofuran A. Vanillin-sulphuric acid reagent, B. Phosphomolybdic acid reagent,
C. Anisaldehyde-sulphuric acid reagent

Analysis of volatile oil
Chemical values: acid value, ester value, hydroxyl value (ester value after acetylation).
Acid Value (Ph. Eur. )
The acid value is the number of mg of potassium hydroxide required to neutralise the free acid in 1 g of the substance when determined by the following method, unless otherwise specified in the monograph. This analysis is carried out by alkali titration.
Ester Value (Ph. Eur. )
The ester value is the number of mg of potassium hydroxide required to saponify the esters present in 1 g of the substance. This analysis is carried out by back titration.
Hydroxyl Value (Ph. Eur. )
The hydroxyl value of a substance is the number of mg of potassium hydroxide required to neutralize the acid combined by acylation in 1 g of the substance.

 bactericidal,
 anticeptical,
 fungicidal,
 antiinflammatory (antiphlogistic),
 antispasmodic,  expectoration,
 carminative,
 diuretic,
 antiworms,  sedatives,
 nephrolytic,
 phytoncidic,
 antioxydant,
 immunostimulative,
 as bitter principles (stimulate appetite)
NB! No one pure volatile oil should be taken orally! ( only diluted!)
Pharmacological effects of volatile oils