- 128 — 



Isoprene, which is regarded as the fundamental substance of the 

 terpenes, is well-known to play a leading part in the manufacture of 

 artificial rubber. It can be prepared synthetically by various processes. 

 According to Harries 1 ), pure isoprene is obtainable from /3-methyftetra- 

 methylene glycol H 3 CCH • CH 2 OH, which is prepared by heating pyro- 



CH 2 CH 2 OH 

 tartaric acid diethylate with sodium. When the glycol is heated with 

 hydrobromic acid of 60% in a sealed tube to 100°, /?-methyltetramethylene 

 bromide is generated, and this, when heated with trimethylamine in fused 

 glass-tubes to 100°, yields an isoprene possessing the following constants: 

 b. p. 36 to 37°, d 2 io 0,6804, n D21Q 1,42267. 



We cannot spare the space here to describe in detail the synthesis 

 of isoprene into rubber. It is polymerised into rubber, or into a sub- 

 stance closely resembling natural rubber, by merely heating it for a longer 

 or shorter period, according to the temperature applied, in a closed vessel. 



It is extremely difficult to differentiate between artificial and natural 

 rubber: only two compounds can be used for this purpose, viz., the nitro- 

 site and the tetrabromide, both of which are amorphous, have no definite 

 m. p. and either cannot be purified at all, or only with difficulty. 



Linaloolene. Apart from recent work by means of metals acting as 

 catalysers, Semmler is the only investigator who has carried out experiments 

 in reduction of the aliphatic terpene-alcohol series. Semmler 2 ) has ascer- 

 tained that geraniol, in contradistinction to linalool, is very stable against 

 nascent hydrogen derived from chemical sources, whereas linalool yields 

 a hydrocarbon, linaloolene, in the generation of which the hydroxyl-group 

 is replaced by hydrogen ("elimination of the oxygen"). The abstraction 

 of oxygen may be accomplished, in the case of linalool, either by reduction 

 with sodium in a solution of ethyl or amyl alcohol or in excess of linalool, 

 or by heating the linalool with zinc-dust in a sealed tube. 



CH 2 ^ CH 2 x 



>C-CH 2 CH 2 CH 2 .CCH:CH 2 — > >CXH 2 CH 2 CH 2 CHCH: CH 2 3 ) 



CHa ^OH CH3 CH 3 



In Semmler's opinion 4 ), linaloolene is only an isomeride ofdi hydromyrcene, 

 the reduction-product of the aliphatic terpene hydrocarbons myrcene and 



*) Liebigs Annalen 383 (1911), 157. 



2 ) Berl. Berichte 27 (1894), 2520. 



3 ) All the schemes of molecular structure are set out in the limonene formula; but it 

 would of course be equally proper to adopt the terpinolene formula favoured by Enklaar. It 

 is evident that oxidative resolution alone could supply definite proof whether only one of two 

 formulae or a mixture of both is present. 



*) Berl. Berichte 34 (1901), 3122. 



