Hydrocarbon Volatile Oils

2.6.5.1 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.

2.6.5.1A Unsaturated Acyclic Hydrocarbons Two typical examples of chemical constituents belonging to the category of unsaturated acyclic hydrocarbons are given below:

(i) 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 Roxb., (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 d⁴₂₀ 0.794, n_D²⁰ 1.4709 and UVmax (ethanol): 226 nm (ε16, 100). It is practically insoluble in water, but soluble in alcohol, chloroform, ether and glacial acetic acid.

Identification

(a) β-Myrcene on reduction with sodium and alcohol (absolute) gives rise to dihydromyrcene (C₁₀H₁₈) which on subsequent bromination yields tetrabromodihydromyrcene (mp 88^oC), and

(b) It readily forms addition compounds with α-naphthoquinone (mp 80-81.5^oC) and with maleic anhydride (mp 34-35^oC).

Use It is used as an intermediate in the manufacture of perfumery chemicals.

(ii ) Ocimene (or trans-β-Ocimene)

Chemical Structure It is 2,6 dimethyl 2,5,7 octatriene.

Use It is used as an intermediate in the manufacture of perfumery chemicals.

(ii ) Ocimene (or trans- β-Ocimene)

Chemical Structure It is 2,6 dimethyl 2,5,7 octatriene.

Occurrence trans-β Ocimene is found in the volatile oil obtained from the leaves of Ocimum basilium L., (Labiatae); Baronia dentigeroides Cheel (Rutaceae); Litsea zeylanica C & T Nees (Lauraceae) and Homoranthus flavescens A. Cunn., (Myrtaceae).

Isolation The volatile oil obtained from the fresh leaves of O. basilicum is treated first with NaOH solution to get rid of the phenolic constituents i.e; eugenol present in the range of 30-40% of the oil. The undissolved fraction of the oil is taken up in an appropriate solvent, solvent removed under vacuum and the resulting volatile oil is subjected to fractionation under vacuum so as to obtain the desired main constituent.

Characteristic Features

trans-β-Ocimene : d₄²⁰ 0.799; n_D²⁰ 1.4893; UVmax (ethanol)

                                232 nm (ε 27, 600)

cis-β-Ocimene : d₄²⁰ 0.799; n_D²⁰ 1.4877; UVmax (ethanol)

                                 237.5nm (ε 21, 000)

The Ocimene (trans- or cis-) undergo oxidation most readily and with relatively shorter exposure to air to form a yellow resin. However, in an atmosphere free from oxygen ocimene may be preserved unaltered. Its bp ranges between 176-178^oC (decomposes).

Identification

(a) Reduction with sodium and absolute alcohol yields dihydromyrcene which on bromination yields tetrabromodihydromyrcene (mp 88^oC),

(b) It yields ocimenol – an alcohol on hydration with sulphuric acid (50%) in glacial acetic acid solution,

(c) Its phenylurethane derivative has mp 72 ^oC, and

(d) Ocimene upon oxidation with KMnO₄ in alkaline solution affects complete degradation to form acids, the lead salts of which has a rhombic crystalline form, whereas the corresponding lead salts of myrcene treated in a similar fashion has a needle form thereby differentiating between ocimene and myrcene distinctly.

Uses It is used in perfumery.

5.2.6.5.1B Aromatic Hydrocarbons A typical example of aromatic hydrocarbon is that of paracymene as detailed below:

(i) para-Cymene

Chemical Structure It is 1-methyl-4 (1-methyl ethyl) benzene (Syn: Dolcymene)

Occurrence It occurs in a number of essential oils, such as: oils of lemon, nutmeg, corriander, cinnamon, sage and thyme. p-Cymene has been reported in a number of volatile oils either due to conversion from cyclic terpenes e.g., pinene or terpinene or from various terpene analogues e.g., citral, carvone, sabinol etc.

Isolation The p-cymene fraction obtained by the fractional distillation of volatile oils may be freed from terpenes having identical boiling points by subjecting it to oxidation with cold KMnO₄ solution, whereby the former being resistant to the oxidising agent is recovered in its pure form. However, pure p-cymene may be prepared from thymol.

Characteristic Features para-Cymene is a colourless liquid and is found to be inactive optically. Its fragrance resembles to that of the aromatic hydrocarbons closely.

Identification

(a) Its boiling point is 177.10^oC.

(b) It melts at –67.94^oC.

(c) Its specific density d²⁰₄ 0.8573 and d²⁵₄ 0.8533.

(d) Its specific rotation n²⁰_D 1.4909 and n²⁵_D 1.4885.

(e) p-Cymene on oxidation with hot concentrated potassium permanganate solution gives rise to p-hydroxy- isopropylbenzoic acid having a melting point 155-156^oC.

Uses

(i) It is employed in the formulation of certain imitation (artificial) essential oils.

(ii) It is used profusely for the preparation of scented soaps and toileteries.

(iii) It also finds its application in the masking of undesirable odours.

Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar