82 THE CHEMISTRY OF RUBBER 



vacuum, the chief products are high-boiliiig hydrocarbons 

 (boiling-point 180° to 300° C. in vacuo), and that only small 

 quantities of isoprene and dipentene are formed. It 

 would appear, therefore, that the isoprene and dipentene 

 obtained when the distillation is conducted at the ordmary 

 pressure are the result of secondary decompositions. 



If strongly cooled, rubber loses its elasticity and 

 becomes hard and brittle, but it regains its normal 

 characters on warming. 



Chemical Reactions of Caoutchouc. — Caoutchouc is not 

 affected by aqueous or alcoholic solutions of the all^alis. 



Hydrochloric acid combines with the hydrocarbon 

 under certam conditions to form addition compounds ; 

 by passing moist hydrochloric acid gas into a solution of 

 rubber at the ordinary temperature Weber obtained a 

 product of the empirical formula doHisCla. Hydro- 

 bromic acid exerts a similar action, but is not so energetic 

 as hydrochloric acid. Strong nitric acid readily attacks 

 rubber with the production of a yellow colour, and Dit- 

 mar, who has carefully studied this reaction, has shown 

 that a definite product is obtamed which he has identified 

 as a dinitro-monobasic acid, probably dinitro-hydrocumic 

 acid. 



With the halogens (chlorine, bromine, and iodme) 

 caoutchouc forms addition products, some substitution 

 frequently occurring at the same time. The best known of 

 the addition compounds is the tetrabromide (CioHisBri) ; 

 this compound is prepared by slowly adding bromine 

 to a well-cooled solution of caoutchouc in chloroform until 

 the bromine is present in excess, and then pouring the 

 liquid into alcohol when the tetrabromide separates in 

 the form of white flakes. 



By passing nitrous fumes, obtained by the action of 

 nitric acid on arsenious oxide, into a solution of rubber 

 in light petroleum, Harries succeeded in isolating three 

 different compounds, according to the conditions of the 

 experiment. To these products he gave the name nitro- 

 sites. A similar substance was also obtained by Weber 

 on passing dry nitrogen dioxide gas into a solution of 

 rubber in benzene. The formation of these nitrosites 

 lias been recommended as a method for the direct estima- 

 tion of caoutchouc. 



Harries has shown that caoutchouc combines with 



