Terpenes Proper

TERPENES PROPER Most of the terpenes proper which occur naturally are colour less highly refractive fragrant liquids, of lower density than water; camphene and bornylene (the latter is a synthetic labora tory product) are crystalline solids of low melting points. They usually boil between 15o° and 18o° C and are readily volatile in steam. They are practically insoluble in water but dissolve freely in alcohol, ether and other organic solvents. Chemically very active, they tend to resinify by aerial oxidation and are easily attacked even by mild oxidizing agents, such as a dilute solution of potassium permanganate. They unite with hydrogen chloride, bromine, nitrosyl chloride, ozone, etc., forming additive com pounds, and some at least are easily converted into isomerides in presence of acids. (See ISoMERISM.) Most of the naturally oc curring terpenes are optically active, some being dextro- and others laevo-rotatory (d- and /-forms) ; optically inactive (dl-) modifications are also known.

The terpenes proper are classified in three groups: (I) mono cyclic terpenes, which unite with two molecules of halogens or halogen acids to form compounds such as and (2) bicyclic terpenes, which combine with only one molecule of a halogen or halogen acid to form compounds such as and and (3) tricyclic terpenes, which do not form additive compounds. Structurally, most of the monocyclic terpenes or menthadienes are related to the aromatic hydrocarbon cymene on the one hand and the cycloparaffin menthane on the other.

Thus such terpenes as limonene or a-phellandrene could be pro duced theoretically by the removal of two pairs of hydrogen atoms from menthane, and similarly the bicyclic and tricyclic terpenes could also be derived from menthane or an isomeride by with drawal of two pairs of hydrogen atoms and resultant formation of "bridged rings." These bridges are distinguished by numbers de noting the number of carbon atoms contained in them, the direct union of the two tertiary carbon atoms being designated as 0; if one carbon atom intervenes, then the number I is used, and so on. Thus three numbers serve as the "characteristic" of the

compound. Hydrocarbons of this class with five atoms of carbon are termed "bicyclopentanes," with six atoms of carbon "bicyclo hexanes," etc.

Monocyclic Terpenes.

Among the monocyclic terpenes one of the most widely distributed in essential oils is limonene, which appears to have been known in the 16th century, if not earlier. The dextro-rotatory form is best obtained by fractional distillation of bitter-orange oil, of which it forms the main constituent ; it also occurs in oils of lemon, bergamot, caraway, etc. ; the /-form is likewise found in nature, although not so commonly. The optically inactive or dl-form, likewise known as dipentene, is a constituent, frequently accompanied by other terpenes, of a large number of essential oils, and is also formed from other terpenes and polyterpenes at high temperatures. Thus the active limonenes and also pinene are converted into dipentene at C, and several oxygenated compounds of the terpene group, such as terpin hydrate and terpineol, easily yield it by loss of the elements of water when warmed with dehydrating agents. Moreover, it is one of the products of the destructive distillation of rubber, gutta percha and many resins. When warmed with alcoholic sulphuric acid it yields terpinene, whilst concentrated sulphuric acid or phosphorus pentasulphide convert it into paracymene. The con stitution of dipentene was definitely established in 1904 when W. H. Perkin, jun., effected its synthesis—the first instance of the synthesis of any of the naturally occurring terpenes.

Limonene, a colourless fragrant liquid, boils at 175-176° C; it forms a characteristic crystalline tetrabromide, and by the action of nitrosyl chloride is converted into a mixture of two stereoisomeric crystalline nitrosochlorides, (C.1113.NOCI), which can be separated by treatment with ether, in which the a-form is readily, and the 8-form only sparingly, soluble. (See