60 SCIENCE PROGRESS 



instead of operating upon ordinary dextro- or laevo-pinene, an 

 optically inactive mixture of the two hydrocarbons is use'd. 1 



The action of heat on the terpenes, long ago studied by 

 Bouchardat, Berthelot and other chemists, has led to results 

 which have become of some practical interest within recent 

 years in connection with the efforts which are being made to 

 produce caoutchouc or rubber artificially. When pinene is 

 heated to about 250 its rotatory power is reduced and ultimately 

 destroyed, a part of the hydrocarbon being converted into 

 dipentene and part into products of polymerisation. If the 

 vapour of pinene, limonene or dipentene is heated to a tem- 

 perature of about 400 it is partly resolved into a pentine, C 6 H 8 , 

 which is identical with one of the products of the action of heat 

 on indiarubber and is known as isoprene. It has been recog- 

 nised as /3-methylcrotonylene, CH 2 : CH . CMe : CH 2 . 2 



Isoprene heated alone in a sealed tube is converted into 

 dipentene mixed with viscous products of its polymerisation. But 

 if it is kept for some months or years under conditions which, 

 except exclusion of air, have not been exactly determined, it 

 passes spontaneously into a syrupy liquid which gradually 

 deposits solid masses of rubber. Rubber is also formed when 

 isoprene is heated with small quantities of acetic and other 

 acids. This observation, made more than twenty years ago, 

 has led to various attempts to employ isoprene as a practical 

 source of rubber on a manufacturing scale. 



When rubber or gutta-percha is destructively distilled it 

 yields a mixture of hydrocarbons from which can be separated 

 as chief products, isoprene and dipentene, originally called, 

 caoutchine. 3 It would appear from this that rubber is a com- 

 pound, the formula of which is either (C 10 H 16 ) n or (C 5 H 8 ) n . From 

 the direct conversion of isoprene into rubber the latter formula 

 seems the more probable, especially as the temperature at which 

 rubber decomposes is much below that at which dipentene is 

 broken down under the influence of heat. Indeed, it seems not 

 improbable that the dipentene which accompanies isoprene in 

 the distillation of rubber is a secondary result of the poly- 

 merisation of isoprene by heat The condensation of isoprene 

 into dipentene is easily represented as in the following formula 



1 Tilden, Trans. Chem. Soc. [1904], 759. 

 ' Mokiewsky, Chem. Cent. [1899], i. 589. 

 3 C. Greville Williams, Proc. R. Soc. [1856], 10, 516, 



