A. Monocyclic Terpenes
Basically, the cyclic terpenes are the extended structural homologues of cyclohexane usually derived by varying extent of dehydrogenation. The parent molecule is methyl-isopropyl cyclohexane (or para-Menthane)
The structure of the monocyclic terpenes is expressed with reference to the saturated parent substance ‘menthane’ i.e.; hexahydrocymene. Consequently, the three isomeric menthanes viz; ortho-, meta- and para-, theoretically yield the monocyclic terpenes respectively.
A number of isomere that have been derived form various degree of dehydrogenation of p-menthane resulting into the formation of a series of p-menthenes are given on page 255:
Interestingly, all the six different species of menthenes have been systematically characterized and identified. However, the most important and abundantly found in various essential oils is ∆3 menthene, which is observed as a natural constituent of thymol oil and is very closely related to menthol, the main constituent of pippermint oil.
Furthermore, the subsequent dehydrogenation of para-menthane yields correspondingly the dihydro-p-cymenes, also termed as para-menthadienes.
There are five important members belonging to this particualr group, namely: α-terpene, β-terpene, α-phellandrene, β-phellandrene and limonene that are very frequently found in a variety of essential oils.
It is pertinent to mention here that the alicyclic (cyclic) hydrocarbons are invariably found to be more stable than the corresponding acyclic hydrocarbons. Nevertheless, the monocylic terpenes usually undergo isomerization, oxidation and polymerisation very rapidly especially when these are subjected to distillation at atmospheric pressure.
Bearing in mind the diagnostic and therapeutic efficacies of the monocyclic terpernes one has to consider the possibility that certain structural configurations like: geometrical isomerism, stereoisomerism, boat and chair form of isomers do exist amongst them as depicted below:
A few typical examples of the ‘monocyclic terpenes’ are described here under:
(i) Limonene Chemical Structure It is 1-methyl-4-(1-methyl ethynyl) cyclohexane (Synonym: Cinene, Cajeputene, Kautschin)
Occurrence It occurs in various ethereal oil, specially oils of lemon, orange, caraway, dill and bergamot. It is also found in grapefruit, bitter orange, mandarin, fennel, neroli and celery.
Isolation d-Limonene is isolated from the mandarin peel oil* (Citrus reticulata Blanco, Rutaceae).
It may also be isolated from the ethereal oils of lemon, orange, caraway and bergamot either by careful fractional distillation under reduced pressure (vauum) or via the preparation of adducts, such as: tetrabromides (mp 104-105oC) and the desired hydrocarbon may be regenerated with the help of pure zinc powder and acetic acid.
Characteristic Features It is colourless liquid having a pleasant lemon-like odour. It is practically insoluble in water but miscible with alcohol. Limonene when protected from light and air is reasonably stable, otherwise it undergoes oxidation rapidly. When it is heated with mineral acids, the former gets converted to terpentine and to some extent p-cymene. On the contrary , the action of mineral acids on limonene in cold yields terpin hydrate and terpineol (alcohols) due to hydration. However, limonene could be regenerated from these alcohols upon heating. The racemic mixture i.e. dl–limonene is also termed as dipentene (inactive limonene), which on being treated with HCl in the presence of moisture yields dipentene dihydrochloride (mp 50-51oC) from methanol.
Dehydrogenation of dipentene or limonene with sulphur rapidly yields p-cymene. Autoxidation of limonene gives rise to carveol and carvone which may be observed in poorly stored orange oils by a distinct and marked caraway like odour.
Identification
(a) Limonene on bromination yields tetrabromide derivative which is crystallized from ethyl acetate (mp 104-105oC).
(b) It forms monohalides with dry HCl or HBr, and the corresponding dihalides with aqueous HCl or HBr.
(c) Its nitrosochloride derivative** serves as an useful means of identification having mp ranging between 103-104oC.
Uses
(i) It is used in the manufacture of resins.
(ii) It is employed as a wetting and dispersing agent.
(iii) It is widely employed for scenting cosmetics, soaps as well as for flavouring pharmaceutical preparations.
-------------------------------------------------------------
* Kugler Kovate, Helv Chim Acta, 46, 1480, 1963
** Prepared by the action of amyl nitrite and hydrochloric acid
(ii ) Sylvestrene
Chemical Structure
Sylvestrene is generally found to be a mixture of two hydrocarbons (a) and (b) as shown above, wherein one of these forms predominates over the other. It is mostly available as its d-and l-isomers; whereas the racemic mixture is known as carvestrene.
Occurrence It is observed that sylvestrene does not occur as a natural product, but it is obtained from either of the two bicyclic monoterpene hydrocarbons, namely: 3-Carene and 4-Carene, during the course of its isolation from the respective dihyrochloride.
Isolation The turpentine obtained from Pinus sylveris L., may contain as much as 42% of 3-carene, whereas turpentine from Pinus longifolia Roxb. (Pinaceae) about 30% of 3-carene. Sylvestrene is isolated in a relatively pure form by preparing the corresponding dihydrochloride.
Characteristic Features It is a colourless oil with an agreeable limolene – like odour. It is considered to be one of the most stable terpenes. It is neither isomerized by heating nor by the interaction of alcoholic sulphuric acid. On being heated to 250oC it undergoes polymerization.
Identification
(a) Sylvestrene yields the following ‘dihalides’ by interaction with solutions of glacial acetic acid-hydrogen halides, for instance: dihydrochloride (mp 72oC); dihydrobromide (mp 72oC); and dihydroiodide (mp 66-67oC).
(b) The nitrosochloride derivative prepared by the action of amyl nitrite and hydrochloric acid has a mp 107°C.
(c) It is dextrorotatory.
Uses It does not find any substantial usage either in the perfume or flavour industries.
