Long-chain Alkyl Substituents in Native Asphaltic Molecules

Abstract

THE term petroleum asphaltic usually refers to either the resin fraction (propane insoluble, n-pentane soluble), or the asphal-tene fraction (n-pentane insoluble, benzene soluble), or the combination of the two. Infrared spectra between 10 and 16 µm of these materials contain four bands at approximately 865, 815, 760 and 730 cm−1 (see Fig. 1). The first three bands have hitherto been assigned to 1-isolated, 2-adjacent and 3-adjacent C–H aromatic out-of-plane bending vibrations and the fourth to polymethylene type rocking1. Because, according to Bellamy2, the ranges for 1-isolated, 2, 3 and 4-adjacent bending vibrations are 900–860, 860–800, 810–750 and 752–718 cm−1 respectively, it is conceivable that the fourth band in the asphaltic spectrum may well be that of 4-adjacent vibrations. Whether the assignment of the fourth band should be to (CH2)n rocking or to the 4-adjacent aromatic bending is a controversial question. The fact that the (CH2)n rocking vibration is unsymmetrical and weak in these samples is consistent with X-ray observations that asphaltenes do not normally contain long-chain alkyl substituents3. Hence the fourth band in the asphaltic spectrum may not be due to long-chain (CH2)n rocking.