Alicyclic Hydrocarbon

2.6.5.1 Alicyclic Hydrocarbon 

The alicyclic hydrocarbons are also termed as ‘monoterpenes’ or ‘true terpenes’ having the emperical formula C₁₀H₁₆. Generally, they may be classified into two categories, namely:

(i) Monocyclic Terpenes, and

(ii) Bicyclic Monoterpenes

These two types of alicyclic hydrocarbons shall be discussed individually with some typical examples as under:

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 terpenesusually 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 104-105^oC) 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 50-51^oC) 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 104-105^oC).

(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 103-104^oC.

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 250^oC it undergoes polymerization.

Identification

(a) Sylvestrene yields the following ‘dihalides’ by interaction with solutions of glacial acetic acid-hydrogen halides, for instance: dihydrochloride (mp 72^oC); dihydrobromide (mp 72^oC); and dihydroiodide (mp 66-67^oC).

(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.

B. Bicyclic Monoterpenes

The bicyclic monoterpenes, as the name suggests essentially possess two cyclic rings which are condensd together. This class of compound is relatively more complex in nature in comparison to the monocyclic species. The second ring system usually conatin 2, 3 or 4 C-atoms in common and the rings may be having 3, 4, 5 or 6 membered rings.

The bicyclic monoterpenes may be regarded as chemical entities derived from:

(a) para-Menthane – by direct fusion of 2–C atoms and the formation of a simple bridge, and

(b) Methylated Cyclohexanes– by having a bridge with either –CH₂– or C(CH₃)₂ – moieties.

In general, the ‘bicyclic monoterpenes’ are classified into five categories, namely:

(i) Thujane; (ii) Pinane; (iii) Carane

(iv) Camphane; and (v) Fenchane.

These five distinct categories shall be discussed briefly with typical examples as given below:

I. Thujane

4-Methyl-1-(1-methyl ethyl) bicyclo[3.1.0] hexane.

Eventually, thujane is derived from p-menthane with direct union between C-2 and C-4. It comprises of a 3-memberd and a 6-membered ring. The ‘bridge’ in this particular instance does not have the isopropyl group in it.

Example: Sabinene

A Sabinene

Chemical Structure

4-Isopropyl-p-methylene bicyclo-2, 4-hexane.

Occurrence It is the major constituent (≈30%) in oil of savin obtained from young shoots of Juniperus sabina L., Cupressaceae. It is also present in oils of cardamom and majoram.

Isolation It is obtained by the fractional distillation of oil of savin under reduced pressure.

Characteristic Features It is a liquid, lighter than water. It is found to be isomeric with α-thujane.

Identification Sabinene either on boiling with dilute sulphuric acid or on shakig with cold dilute sulphuric acid yields:

(i) different forms of terpinene, and

(ii) 1, 4-terpin.

II. Pinane It is formed from p-menthane by forming a bridge between C-3 and C-6 positions, thereby resulting into the formation of a 4-membered ring system and a parent 6-membered ring system.

Example α-Pinene.

Chemical Structure 2,6,6-Trimethyl bicyclo[3,1,1] hept-2-ene;

Occurrence It is obtained from oil of turpentine which contains 58-65% α-pinene along with 30% β-pinene. It is also widely distributed in essential oils belonging to the family Coniferae. It has been reported to be present in oils of American pippermint, corriander, cumin and lemon.

Isolation

(i) It is isolated from the essential oils stated above by the help of chromatographic techniques.

(ii) Mostly isolated by the fractional distillation from essential oils, preferable under reduced pressure followed by further purification. The fraction collected between 155-165^oC is converted to crystalline form of nitrosochloride (treated with amyl nitrite and hydrochloric acid) from which the desired product is liberted by treatment with aniline.

Characteristic Features It is a colourless oil which has a tendency to resinification on exposure to air. The various physical parameters of its isomers are given below:

dl-form : bp₇₆₀ 155-156^oC; d₄²⁰ 0.8592; n_D²⁰ 1.4664

d-form : bp₇₆₀ 155-156^oC; d₄²⁰ 0.8591; n_D²⁰ 1.4661;

l-form : bp₇₆₀ 155-156^oC; d₄²⁰ 0.8590; n_D²⁰ 1.4662.

The l-form is usually found in the French Turpentine Oil, whereas the d-form is found in the American, German and Swedish Turpentines.

Identification It may be characteristized by–

(a) Preparation of its nitrosochloride derivative mp115^oC, which is devoid of optical activity,

(b) Preparation of its hydrochloride derivative mp 132^oC, and [α ]_D²⁰ – 33.24 °C (in alcohol), and

(c) Preparation of its adduct with malic anhydride (crystalline ) mp 169^oC.

Uses

1. It is abundantly used as a starting material for the large-scale preparation of synthetic camphor

as given below:

2. Turpentine oil is cooled to –10^oC first and then hydrogen chloride gas is passed through it to obtain the pinene hydrochloride. The latter undergoes isomerization to yield bornyl chloride which on treatment with alkali gives rise to borneol. This on oxidation with nitric acid yields pure synthetic camphor.

3. It also finds its application in the production of insecticides, solvents, plasticizers, perfume bases and synthetic pine oil.

III. Carane para-Menthane with a bridge between C-3 and C-8 results into the formation of carane, which comprises of a 3-membered ring imbeded into the 6-membered parent ring as given below:

Example

A 3-Carene

Chemical Structure 3,7,7 Trimethylbicyclo [4,1,0] hept-3-ene (a); 4,7,7–Trimethyl-3 norcarene (b).

Occurrence It is a constituent of turpentine. The turpentine obtained from Pinus sylvestris L., contains upto 42%; turpentine from Pinus longifolia Roxb; Pinaceae about 30%.

Isolation It is isolated from the turpentine oil by the usage of chromatographic techniques.

Characteristic Features It is a sweet and pungent odour essential oil having a more agreeable odour than that of turpentine. It is practically insoluble in water but miscible with most fat solvents

and oils. The d-form possess physical characteristics, e.g.; d₁₅¹⁵ 0.8668; d₃₀³⁰0.8586; bp₇₀₅ 168-169^oC; [α]_D²⁰ + 17.69; n_D³⁰ 1.468.

Identification The d-form gives rise to the nitrosoate derivative (C₁₀H₁₆ N₂O₄), which may be prepared by treating d-Carene with amyl nitrile, acetic acid and nitric acid. Its prism decomposes at 147.5°C.

Uses It is used as an antiseptic, carminative, stimulant, stomachic and diuretic.

IV. Camphane It is formed with a direct bondage between C-1 and C-8 in the structure of p-menthane. It essentially comprise of two five-membered rings besides a six-membered ring.

Example

A Camphene

Chemical Structure 2,2, Dimethyl-3-methylenebicyclo-[2,2,1] heptane;

Occurrence It mostly occurs in a large variety of essential oils, for instance:

(i) Turpentine oil (levo and dextro forms),

(ii) Cypress oil (dextro form),

(iii) Camphor oil (dextro form in species of Lauraceae),

(iv) Bergamot oil, and

(v) Oils of Citronella, Neroli, Ginger, and Valerian).

Camphene occurs in a number of species, namely: Achillea, Milefolium, Acorus calamus, Anethum graveolens, Artemisia, Cinnamonum, Foeniculum vulgare, Juniperus, Kaempferia galanga, Myristica fragans, Peumus boldus, Pinus ellottii, Piper nigrum, Pistacia lentiscus, Rosamarins officinalis, Satureja, Schinus molle, Thymus, a and Valeriana officinalis.

Isolation Camphene is isolated by the chromatographic techniques from rectified turpentine oil.

Characteristic Features Camphene obtained from alcohol found in cubic crystals (dl-form) having an insipid odour.

dl-form: mp 51 to 52^oC; bp₇₆₀- 158.5 to 159.5^oC; d⁵⁴₄ 0.8422; n⁵⁴_D1.45514.

Solubility Soluble in ether, dioxane, cyclohexane, cyclohexene and chloroform. Practically insoluble in water and moderately soluble in alcohol.

d-form : mp 52°C; [α]¹⁷_D+ 103.5°; (C=9.67 in ether); d⁵⁰₄ 0.8486; n⁵⁰_D1.4605;

l-form : mp 52°C; [α]²¹_D– 119.11°; (C=2.33 in benzene); d⁵⁴₄ 0.8422; n⁴⁰_D1.4620.

Identification It forms large dodecahedra on being subjected to slow sublimation

Uses

1. As an important constituent of eucalyptus oil which is used as a counter-irritant, antiseptic and expectorant.

V. Fenchane It is a trimethyl cyclohexane with a methylene (—CH₂—) bridge. It consists of two five-membered and a six-membered ring.

Example d-Fenchone

Chemical Structure

(1S) – 1,3,3,-Trimethylbicyclo [2,2,1]-heptan-2-one.

Occurrence It occurs in fennel oil and in the essential oil of Lavondula stochas L., Libitatae.

Isolation It is isolated from the fennel oil by column chromatography which mostly contains this ketone to the extent of 20%.

Characteristic Features It is a colourless oily liquid having a camphor like odour. It attributes the bitter taste to the drug. It is very soluble in absolute alcohol and ether; but practically insoluble in water.

D¹⁸₄ 0.948; mp 6.1°C; bp₇₆₀193.5°C; [α]²⁰_D + 66.9°; n¹⁸_D1.4636.

Identification The pH of its saturated solution is 6.82.

Uses

1. It is employed extensively in foods and in perfumes.

2. It also finds its application as counterirritant.