Determination of Methyl, Methylene, and Alkylbenzene Group Types by Infrared Absorption

Abstract

The first part of this paper describes the anomalies that appear in the determination of methyl, cyclopentyl and cyclohexyl methylene groups by previously published infrared group type methods. Serious discrepancies were found when these analyses were applied to polymers such as polypropylene and polybutene and also to compounds with different molecular weights than those used in the calibration. Previous workers have shown that methyl groups and the methyl part of ethyl groups on the side of a high molecular weight chain absorb more strongly at 7.25μ, the wavelength for the methyl determination, than do the end methyl groups on normal paraffins. An examination of extensive data on low molecular weight compounds shows the same behavior, pendant methyl groups absorbing more strongly at 7.25μ than do end methyl groups. By assuming that pendant methyl groups absorb approximately three times as strongly as do end methyl groups, the absorptions by polypropylene and high molecular weight compounds containing pendant methyl groups are reconciled with those of low molecular weight compounds. Pendant methyl groups also give anomalies in the short wavelength region. They absorb less at 3.38μ than do end groups, but they contribute more to the methylene absorption at 3.42μ than do end methyl groups. This anomalous behavior creates difficulties in the cyclopentyl and cyclohexyl methylene determinations at these short wavelengths. The second part of the paper discusses an investigation into the possibility of a group type analysis for the determination of the different types of alkyl substituted benzenes in a mixture of the types using the 10.0–15.0μ portion of the spectrum. A method has been developed whereby mono-substituted and ortho-, meta-, and para-di-substituted benzenes can be determined, but only with severe restrictions as to over-all sample composition. Tri- and higher substituted benzenes as well as polynuclear aromatics interfere with this scheme of analysis. These interferences severely limit the usefulness of the method, especially when applied to petroleum fractions. A table top integrator for use with previously recorded spectra from the Perkin-Elmer model 21 spectrometer was used in developing the group type analysis. It is described in some detail.