200 PLANT BIOCHEMISTRY 



The group includes unsaturated hydrocarbons or monoterpenes 

 (CioHis), sesquiterpenes, diterpenes, triterpenes, tetraterpenes, and 



CHs 



I 

 CH2=C— CH=CH.> 



isoprene 



polyterpenes. The carotenoids are examples of tetraterpenes, and rub- 

 ber and gutta-percha are examples of polyterpenes. Isoprene itself is 

 not found in plant material. 



Essential oils are produced in quantity in only a small number of 

 plants compared with the whole plant spectrum, 2,000 of about 400,000 

 total species. The oil is formed in special cells or in glandular hairs of 

 the leaf and stem. In pine species the terpene and accompanying 

 resin are formed by cells which line the resin canals of the bark. The 

 oil is obtained by tapping these canals. Over 500 different chemical 

 constituents have been identified in the essential oils, and these com- 

 pounds have been classified on the basis of the number of isoprene 

 equivalents per molecule. Monoterpenes (two isoprene equivalents) 

 represent a large number of essential oil components. The simplest 

 are the unsaturated acyclic hydrocarbons myrcene and ocimene. The 



CH3 CH2 



\ II 



C=CH- CH2— CH2C— CH=CH2 



/ 

 CH3 



myrcene 



CHo CH3 



\ I 



C— CHo— CH2— CH=C— CH=CH2 



/ 

 CH3 



ocimene 



well-known essential oils citronellol and geraniol are closely related 

 to myrcene. Similarly, by elimination or shifting of a double bond or 



CH3 CH3 



\ 1 



C=CH— CH2— CH2CH— CH2CH2OH 



/ 

 CH3 



citronellol 



CH3 CH3 



\ I 



CH— CH2— CH2C=GH— GH2OH 



/ 



CH3 



geraniol 



