ESSENTIAL &IL AND RESINS 147 



THE ESSENTIAL OILS 



These may be divided, according to their chemical composition, 

 into two major groups; (1) the hydrocarbon oils, or terpenes, and 

 (2) the oxygenated and sulphuretted oils. 



The terpenes are of three different types, namely: (a) the hemi- 

 terpenes, CsHg, unsaturated compounds of the valerylene series, of 

 which isoprene (found in crude rubber) is the best-known example; 

 (6) the terpenes proper, CioHie, which constitute the major pro- 

 portion of the whole group; and (c) the polyterpenes (CsHg)^ of 

 which colophene and caoutchouc are the most common examples. 



Eleven different terpenes having the formula CioHie have 

 been isolated from various plant juices, and their molecular arrange- 

 ment carefully worked out. The following three examples will 

 serve as typical of the general structural arrangement of these 

 hydrocarbons : 



Limonene Camphene PIn ^ ne 



CH 3 H | 



H 3 C- <f C1U 



CH f 



CH 3 



A discussion of the evidence which supports these formulas as 

 properly represented the molecular arrangements of the various 

 isomeric forms would be out of place here, as its only particular 

 interest is in connection with the medicinal effects of the different 

 compounds. It is clear, however, that they are six-membered 

 hydrocarbon rings, with additional hydrocarbon groups attached 

 to one or more of the carbon atoms in the ring. 



Different modifications, or varieties, of the terpenes constitute 

 the main proportions of the oils of turpentine, bergamot, lemon, 

 fir needles, eucalyptus, fennel, pennyroyal, etc. 



The oxygenated essential oils may be either alcohols, aldehydes, 

 ketones, acids, esters, or phenols, derived from either five-mem- 

 bered or six-membered closed-ring hydrocarbons. They are 

 usually present in the plant oil in mixtures with each other or 

 with a terpene. Since most of them have pronounced physiological 



