1. Hemiterpenes:
Hemiterpenes are the compounds having 5-carbon atoms i.e.
a. Isoamyl alcohol:
Found in mentha spp.
b. Isovaler aldehyde:
Found in eucalyptus.
2. Monoterpenes:
These are composed of two isoprene units and have the formula C10H16. Monoterpenes are further classified as
a. Open chain or Acylic hydrocarbons.
b. Monocyclic hydrocarbons.
c. Dicyclic hydrocarbons.
d. Tricyclic monoterpenes.
3. Sesquiterpenes:
These contain three isoprene units having the molecular formula C15H24. Sesquiterpenes may be;
a. Acyclic or open chain e.g. Farnesol.
b. Monocyclic e.g. Zingeberene.
c. Dicyclic e.g. Cadinine.
4. Diterpenes:
Diterpenes have four isoprene units and have the molecular formula C20H32. Diterpenoid compounds include such resin acids as + and – pimaric acid and their isomer, the abietic acid of pine resin. Many diterpenoids (e.g. vit A and gibberellic acid) do not belong to the volatile oil-resin group.
Acyclic Diterpenes :
Phytol is an unsaturated alcohol, a component of the chlorophyll molecule. Phytol has following formula.
Monocyclic Diterpenes:
a-camphorene
Dicyclic Diterpenes:
e.g. Agathic acid found in Agathis alba
Tricyclic Diterpenes.
e.g. Abietic acid and Pimaric acid
Tetracyclic Diterpenes.
e.g. Phylleladene
5. Triterpenes:
These are abundant in nature particularly in resins and may occur as either esters or glycosides. They may be aliphatic e.g. squalene, tetracyclic or pentacyclic. Tetracyclic’s examples are Limonoids, sterols and pentacyclics examples are b-amyrin, 4,4-dimethyl sterol.
6.Tetraterpenes: (C= 40)
These are mainly cartenoids pigments. The important pigments are orange red and yellow. These are found along with chlorophyll in photosynthetic tissues and are located in chloroplast lamella.
The colour of red tomato and the orange are due to the lycopene.
Polyterpenoids:
These are compounds of many isoprene units e.g. rubber.
Rubber is a polyterpenoid product in latex of about 300 genera of angiosperm but only Hevea brasiliensis is used. Rubber is manufactured in
Mixed Terpenoids:
Mixed Terpenoids compounds consist of terpenoid and non-terpenoid components other than sugar and fatty acids e.g. Terpenoid glycosides and terpenoid esters.
Stereopenes:
Another major group of volatile oils is stereopenes or phenyl propanoids. These compounds contain C=6 Phenyl ring with an attached c=3 propane side chain. Many of the phenyl propanoids found in volatile oils are phenols or phenol esters. Some of the famous examples are Cinnamaldehyde, anethole, eugenol, and phenyl ethyl alcohol, anisaldehyde and methyl salicylate.
These compounds form of oil in some spp. While in other spp. The hydrocarbon liquid portion predominates. The odour and the taste of ess. Oil is mainly determined by the oxygenated components which are usually soluble in water but more soluble in alcohol. They can further be classified as.
1. Open chain or Acentric.
2. Monocyclic.
3- Dicyclic:
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