76 INFRA-RED ABSORPTION SPECTRA. 



Still be present, a film was made by melting solid asphaltum on rock 

 salt. The whole seemed of interest in connection with the absorption 

 and anomalous dispersion of asphaltum investigated by Nichols^ in the 

 optical region. The curves show several marked bands. As is to be 

 expected, the bands are found in the petroleum distillates. 



Class II : Carbocycuc Compounds. 



In this class the carbon atoms are joined in a closed chain or ring. 

 To it belong the methylene hydrocarbons of the petroleum distillates, 

 already discussed, the pyridine group, thiophene and pyrrol, and, most 

 important of all, benzene and its derivatives. As will be pointed out 

 elsewhere, the benzene spectrum is so wholly unlike that of the petro- 

 leum distillates that, if we had no knowledge of the latter, gained from 

 organic chemistry, the evidence presented in their absorption spectra 

 would be sufficient to show that we are dealing with two distinct classes 

 of compounds. In a more restricted sense, pyridine, thiophene, and 

 pyrrol belong to the so-called heterocyclic compounds of carbon. 



Benzene. CeHe. (Fig. 77.) 



Benzene is the parent hydrocarbon of a large number 

 of compounds. The idea that the constitution of benzene 

 is a closed chain or ring of carbon atoms was first pro- 

 pounded by Kekule,^ in his " Benzoltheorie," in 1865. It 

 is based on numerous facts, such as the power to form 

 three isomeric biderivatives, which is not possible in an open chain 

 like stearic acid, but which is possible if each C atom is joined to an 

 H atom and the six CH-groups are joined together in a ring. This 

 forms a fairly complicated molecule, not easily reduced to simpler com- 

 pounds, like CO2. These facts should be remembered in considering 

 the following curves, in which certain benzene bands persist even in 

 very complicated derivatives. The relations of the benzene derivatives 

 to benzene are very limited,^ although the derivatives are intimately 

 connected by many reactions. This fact is not out of place in consid- 

 ering the following curves. The great dissimilarity between them and 

 the curve of benzene also shows that the relations are limited. Nothing 

 has been found more marked than the change in the maxima by substi- 

 tuting a CI or Br atom for an H atom in benzene. On the other hand, 

 the curves of the CI and Br derivatives show many bands in common. 



^Nichols, E. L. : Phys. Rev., xiv, p. 204, 1902. 



^Kekule : Liebig's Annalen der Chemie, 137, p. 129 (1865.) 



^Bernthsen : Organ. Chemie, p. 330; also, p. 327. 



