[boswell] the constitution OF RUBBER 29 



Thus rubber came to be looked upon as a polymerization product 

 of isoprene (C5H8)x. especially after Gladstone and Hibbert showed 

 that rubber does in reality possess the empirical formula CsHs. It 

 may be mentioned in passing that these polymerizations products of 

 isoprene are somewhat closely related chemically to rubber, but are 

 not identical with it. 



Commercial rubbers were also known to contain varying amounts 

 of acetone soluble compounds called resins. These were found to be 

 oxygen containing compounds, and led to the investigation by Herbst 

 (13) and others of the action of free oxygen on a benzol solution of 

 rubber. Herbst definitely isolated a compound of the empirical formula 

 CioHieO. I shall return to the consideration of these compounds 

 presently, in connection with the products of the oxidation of rubber 

 by hydrogen peroxide, by potassium permanganate and by free 

 oxygen, obtained in this laboratory. I only wish, at this point, to 

 indicate that this CioHieO compound was among the first prepared of 

 those derivatives of rubber indicating the existence of CioHie in the 

 rubber molecule. 



We thus see how the two ideas which have dominated rubber 

 chemistry have arisen, viz.: (1) that rubber is a polymerization pro- 

 duct of isoprene and (2) that the rubber molecule consists of several 

 CioHie groups in combination. 



The action of halogens upon rubber was studied by Gladstone 

 and Hibbert (14). On passing chlorine through a chloroform solution 

 of rubber they obtained a compound, the analyses of which left them 

 undecided between the formulas CioHuCls and CioHioClg. Hydro- 

 chloric acid was evolved in the reaction, indicating substitution of 

 hydrogen by chlorine, although the major part of the reaction was one 

 of direct addition of chlorine. Using bromine in place of chlorine 

 they obtained as chief product a mixture of compounds of the com- 

 position CioHi6Br4. Weber stated that the so-called tetrabromide of 

 rubber, after precipitation and washing, had a constant composition. 

 Budde (15) developed an analytical method for the determination of 

 rubber in mixtures based upon this reaction. 



The reaction of hydrochloric acid upon a benzol solution of 

 rubber was studied by Weber (16). He isolated a product having 

 the composition CioHie 2 HCl. Harries and Fonrobert (17) confirmed 

 this observation and also found that upon repeated washings and 

 precipitations the halogen content decreased considerably. Weber 

 found that upon gentle warming the chlorine content fell from 32% 

 to 18%. Harries found that this could be still further decreased to 

 12.3% on warming to 100° in vacuo and on heating with pyridine 



