Rubber. Part XIV. The Behavior of Rubber with Iodine Chloride and with Dithiocyanogen

Abstract

Abstract 1. The Determination of the Iodine Number of Rubber The investigation of carotinoids has shown us that a large excess of iodine chloride must be employed if conjugated systems of double bonds are to be completely attacked. If, for example, with isoprene 150% of the calculated quantity of iodine chloride is used, then the reaction reaches after one day and after one week only 1.77 and 1.80 double bonds, respectively. 200% of iodine chloride must be used in order to obtain the correct number of double bonds. A still greater excess of iodine chloride does not then change the results any further. Such isoprene systems, which have added a halogen atom on every carbon atom in the chain, are obviously stable to substitution by iodine chloride. The frequently discussed question of whether in rubber a pair of conjugated double bonds is present as a terminal group, therefore a true isoprene system, has been proved by Pummerer and Mann by means of iodine chloride. At that time, however, the results did not apply to isoprene. For that reason it was necessary again to titrate the rubber with a great excess of iodine chloride. In this way it was shown that trustworthy results were obtained only by using 110–120% of iodine chloride (100% = 1 mol. of iodine chloride per C5H8 group), accordingly with an excess of from 10–20% iodine chloride. Within this range the results of the titration did not vary. Also only very few (often none at all) acids appeared with the titration, and these could be disregarded. On the contrary, if a greater excess of iodine chloride is used, the iodine numbers and acid values are essentially higher, which, as is well known, indicate substitution. Thus with 200% iodine chloride a value of 147 was obtained. The same phenomenon is true of gutta-percha. The earlier titrations of rubber were accidentally carried out within the favorable range of excess iodine chloride, so that the values for sol-rubber have undergone scarcely any correction. We have now carried out iodine chloride titrations with six fractions of crepe sol-rubber extracted with cold acetone, then fractionally dissolved in cold ether, and in this way we have found, as for sol-rubber, values for alkali-purified latex. They are very close to 100 (Fraction I: 100.1; II: 100.3; III: 99.9, IV: 99.6; V: 100.0; VI: 99.9). No difference in the titre was established by the various fractions.