THE CHEMISTRY OF 1NDIARUBBER 499 



having isolated as a definite separate compound a volatile 

 hydrocarbon, boiling, after careful rectification, between 37 and 

 38°C. To this he gave the name " isoprene." He observed that 

 its vapour density was 2-44 times that of air, and gave its specific 

 gravity as '6823. Analysis showed it to have the formula C 5 H 8 . 

 On standing in the air isoprene rapidly absorbed oxygen and 

 became ozonised ; the ozonised compound thus formed was 

 found to be a strong oxidising agent. Williams also observed 

 that isoprene readily absorbed bromine. 



An interesting and important stage in the history of rubber 

 chemistry was reached in 1879, when M. G. Bouchardat l took 

 up the investigation of isoprene. 



Polymerisation of Isoprene into Dipentene and into Caoutchouc 



On heating isoprene in a sealed tube to a temperature of 280 — 

 290° C. for ten hours, Bouchardat found that there resulted a 

 viscous, sticky mass, which on distillation was shown to contain 

 (1) some unchanged isoprene, (2) an inactive hydrocarbon, boiling 

 at 170° — 185° C. with an odour like lemons — a substance now 

 known as dipentene — and (3) other high boiling hydrocarbons. 

 The tendency to ready polymerisation by heat on the part of 

 isoprene was thus demonstrated. The same investigator after- 

 wards observed that polymerisation could also be effected by 

 other means, viz. by the action of a cold saturated solution of 

 hydrochloric acid upon isoprene. In this case, however, the 

 polymerised product was found to be elastic, and in other respects 

 possessed the properties of ordinary indiarubber. 



These observations of Bouchardat were in 1882 confirmed by 

 Prof. W. A. Tilden, 2 and similar results were obtained with 

 isoprene derived from other sources, such as the decomposition 

 of turpentine oil by heat. Tilden also showed that this poly- 

 merisation could be brought about by the action of other reagents 

 than hydrochloric acid — for example, by nitrosyl chloride. A 

 further important communication on this subject was made by 

 Prof. Tilden in 1 892 s to the effect that on standing for a long 

 time in bottles, isoprene is converted spontaneously into 

 indiarubber. The following is an abstract from Prof. Tilden's 



'& 



1 Comptcs Rendus, lxxix. p. 1117 (1879). 



2 Chent. News, 1882, pp. 120, 121 (vol. xlvi.). 



3 Ibid. 1892, p. 265. 



