502 SCIENCE PROGRESS 



Later, Greville Williams l isolated a similar fraction (b.p. 170 

 — 180 ) from the distillation products of both indiarubber and 

 gutta-percha. On rectification this yielded a liquid b.p. 170 — 

 173 C, identical with the caoutchine obtained by Himly, and 

 which had a vapour density double that of isoprene. Williams 

 observed that it belonged to a group of substances isomeric with 

 turpentine oil, having the formula C 10 Hi 6 . Afterwards, in 1879, 

 M. G. Bouchardat, by heating isoprene in a closed tube to 280 C. 

 for ten hours, also obtained a hydrocarbon, of which he found 

 the boiling point to be 174:6° 0. This also had an odour of lemons, 

 and its formula was C 10 H 10 . The name di-isoprene was given to 

 this hydrocarbon, and Bouchardat suggested that it was identical 

 with caoutchine and also with the inactive hydrocarbon of tur- 

 pentine oil, because he was able to obtain the same derivatives, 

 e.g. terpene hydrate, from all three. The same hydrocarbon was 

 again obtained by Bouchardat when he distilled the elastic com- 

 pound which he had obtained by the polymerisation of isoprene. 



The identity of these hydrocarbons with one another, and 

 with the terpene hydrocarbon cinene (now known as dipentene), 

 was, afterwards confirmed by Wallach. 2 It is thus seen that in 

 some way or other dipentene is intimately connected with the 

 rubber hydrocarbon. Its constitution, therefore, is a matter of 

 considerable importance from this point of view. Fortunately, 

 as in the case of isoprene, the constitution of dipentene has been 

 thoroughly worked out. 



Constitution of Dipentene 



Dipentene belongs to that class of hydrocarbons known as 

 terpenes. Most of the members of this class may be regarded 

 as derivates of the hydrocarbon cymene, methyl-isopropyl- 

 benzene — 



CH.-QCH<^; 



Dipentene is one of the most commonly occurring terpenes ; it 

 is found free in nature, and may also be easily obtained from 

 certain other natural products such as terpineol and pinene. 

 Tilden 3 in 1879 showed that it was closely related to the alcohol, 



1 Proc. Royal Soc. i860, p. 517. 



2 Ann. der Chem. 1884 (225), p. 311 ; 1885 (227), pp. 292-6. 



3 Ber. 1879 (12), p. 848. 



