Polyterpenes

POLYTERPENES Diterpenes, C20H32, are found in resins and balsams, but hitherto have not been much investigated. They are generally obtained in the form of viscous oils, which usually boil at temperatures above 300° C, and although unsaturated do not readily form crystalline additive compounds. Such substances are copaivene, which has been separated from copaiba balsam, and colophene, obtained by distillation of colophony and by treatment of pinene with con centrated sulphuric acid; the latter is a thick yellow oil, boiling at 318-320° C. A liquid diterpene (b.p. 168°-169° C under Io mm. pressure) has been obtained along with camphene by heating borneol with zinc chloride.

Triterpenes, C301148.—The arnyrilenes, obtained by dehydration of the crystalline alcohols a- and /3-amyrin, which are found in elemi resin, crystallize well from ether or benzene, but are sparingly soluble in alcohol. They cannot be distilled at the ordi nary pressure without decomposition.

A tetraterpene, C40H64, has been produced by shaking oil of tur pentine with antimony trichloride. This tetraterebenthene is an amorphous solid, as also is its dihydrochloride, C40H64.2HC1, and it decomposes when distilled.

Synthetic Rubber.

In 1892 Sir Wm. Tilden observed that a specimen of isoprene which had been kept for some time in a sealed tube had polymerized to form a substance almost indistin guishable, so far as physical properties are concerned, from rub ber, and like the natural product capable of vulcanization. It was

later discovered that this production of a synthetic rubber from isoprene took place much more rapidly in presence of metallic sodium, which appeared to act as a catalyst. Heating isoprene with acetic acid and other chemical agents also has the effect of producing a form of rubber. Moreover other unsaturated hydro carbons of the isoprene series polymerize to yield rubber-like sub stances. These synthetic rubbers have been classified in two groups—the "normal" rubbers obtained by heating isoprene and other related hydrocarbons alone or with acetic acid, and the "sodium" rubbers prepared by the action of that metal on the hydrocarbons. The two series exhibit various differences, and ap parently are not chemically identical.

These observations have led to the development of methods for the production of synthetic rubber, which may ultimately com pete with the natural product, if its physical and mechanical qual ities prove satisfactory and if it can be produced at a sufficiently low cost.

BiBuocRApHY.-Article "Terpenes," Thorpe's Dictionary of Applied Chemistry, vol. vii. (1926) ; 0. Aschan, Chemie der alicyklischen Ver bindungen (1905) ; 0. Wallach, Terpene and Camphor (1909) Gilde meister and Hoffman, The Volatile Oils (1913) H. Finnemore, The Essential Oils (1926). (G. G. H.) TERRACE CULTIVATION: see AGRICULTURE, PRIMI TIVE.