244 REPORTS ON THE STATE OF SCIENCE. 



both of which on distillation with solid potash split up and give isoprene, 

 trimethylamiue, potassium iodide, and water :— 



I 



ca 



II 



+ N(CH3)3 + KI + H.,0. 



The isoprene thus prepared was found to be identical with the 

 isoprene obtained from caoutchouc. It was identified by means of the 

 hypochlorous acid addition product, melting-point 81°, isoprene dichlor- 

 hydrin, obtained by Mokiewsky. 



In 1898 Mokiewsky ' prepared an isoprene dibromhydrin similar to 

 the above dichlorhydrin, and having probably the constitution— 



Cff, . CBr . CH.,OH or CHa-CBr-CH.OH 



I " I ' 



<JHBv-CH,OH CH(OH) . CEr,Br 



and in 1899 - this same investigator showed that on attaching a molecule 

 of hydrogen bromide to isoprene the same unsaturated tertiary bromide 

 is obtained as from dimethylallene when similarly treated. This furnishes 

 additional confirmation of the correctness of the isoprene formula : — 



\C-CH = CH, 

 Ch/ ^ CH3. 



isojyrene '>CBr . CH = CH^. 



CH3. .^ CH/ 



>C=C = CH., 

 CH3/ 



dimethyl allene 



Composition of Caoutchine, Di-isoprene, or Dipentene. 



We have seen that Wallach had proved conclusively the identity 

 of di-isoprene, the polymerisation product of isoprene, obtained by 

 Bouchardat and Tilden, with the hydrocarbon known as cinene or 

 dipentene, and that the caoutchine of Himly and Williams was also 

 identical with this substance. The constitution of dipentene, however, 

 for a long time remained a matter of much uncertainty. 



Dipentene is one of the most commonly occurring terpenes, being 

 found free in nature, and being also easily prepared from many other 

 naturally occurring products, such as terpineol and pinene. It is an 

 inactive hydrocarbon, but is really an externally compensated mixture 

 of two active liydrocarbons, the d and I limonenes. Tilden ^ showed that 

 it was closely related to the alcohol terpineol CioHigO, and Wallach'' 

 found that on heating terpineol with potassium bisulphate at 200° water 

 was eliminated and dipentene was formed. Wagner, in 1894,'* after a 



' Jaurn. Rv.ss. Phys. Chem. Soc, 30 (1898), pp. 885-900. 

 2 Ibid., 32 (1899). pp. 207, 21ti. ' £er., 1879, 12, 848. 



Ann. (1885), 230, p. 258, ' £er., 27, 1636. 



