. Carnegie Institution of Washington publication. 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 disc
. Carnegie Institution of Washington publication. 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
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