312 PRACTICAL ORGANIC AND BIO-CHEMISTRY. 



THE TERPENE GROUP. 



Nearly all parts of plants contain volatile substances with a highly 

 characteristic and pleasant smell. These substances are the essential 

 oils, e.g., oil of turpentine, oil of lemons, etc. The various kinds of 

 camphor, which are crystalline solids, the resins and india-rubber are 

 closely related substances. 



They are prepared from plants by steam distillation, by pressing, 

 or by extraction with organic solvents. Besides their use in per- 

 fumery, in making essences, they are used in the preparation of oil 

 paints, varnishes, etc. Several are used in medicine. 



The essential oils are generally complex mixtures, the main constitu- 

 ent imparting the characteristic properties ; several es sential oils may 

 contain the same constituent and yet differ in smell on account of the 

 presence of different highly odoriferous substances. Oil of turpentine 

 exists in the greatest quantity. It flows from the stems of pine trees 

 when incisions are made in the surface and consists of solids dissolved 

 in the liquid. Crude oil of turpentine is separated by steam distillation, 

 the solids remaining behind and constituting colophony or resin. 



The chief constituent of oil of turpentine is pinene. Limonene is 

 present in oil of lemon. They are colourless, very refractive liquids 

 boiling between 1 50 and 1 80. Camphene is solid. They are insol- 

 uble in water, but soluble in most organic liquids. They are good 

 solvents, dissolving resin, caoutchouc, iodine, phosphorus and sulphur. 

 The majority are optically active ; sometimes both the dextro and 

 laevo forms are found in nature, and the inactive mixture of some of 

 tfiem has been prepared. 



Reactions. 



(1) They easily polymerise to form resinous substances. 



(2) On exposure to air or oxygen, they are oxidised and yield 

 resins. 



(3) On oxidation with permanganate, etc., they are converted into 

 benzene derivatives. 



(4) On treatment with ozone, they form ozonides. 



(5) On reduction, they are converted into hydroterpenes. 



(6) They combine with bromine and halogen acids to form addi- 

 tion compounds, which are frequently crystalline solids. 



(7) They react with nitrosyl chloride, NOC1. 



Constitution of the Members of the Terpene Group. 



Most of the members of the terpene group are unsaturated hydrocarbons 

 of the formula Ci H 16 ; others, such as camphor, are alcoholic or ketonic 

 derivatives and possess the empirical formulae C 10 H 16 O, C 10 H 18 O, C 10 H 20 O. 



Though most of the hydrocarbons have the empirical formula C 10 H 16 

 several have the formulae C 15 H 24 , C 20 H 82 and (C 6 H 8 )n. The unsaturated 

 hydrocarbon isoprene, C 5 H 8 , has the same percentage composition and is 

 obtained by the distillation of caoutchouc ; it polymerises to a hydrocarbon 

 Cj H ]6 and can be made to polymerise to caoutchouc, the constituent of 

 rubber. The group may therefore be divided into 



