Infra-red absorption spectra of some isomeric dodecanes

Abstract

IN INVESTIGATION [1], C12H26 paraffinic hydrocarbons of isomeric structure were synthesized and characterized. Since some of these compounds had been synthesized for the first time, it was of fundamental interest to investigate them by various methods, including the recording of their infra-red spectra and the establishment of the characteristics of these spectra. The infra-red spectra of the hydrocarbons were obtained on an IKS-14 double-beam spectrophotometer with LiF and NaCl prisms. In both cases linear evolution of the spectra by wavelengths was used. Dispersed light was not allowed for. The puri ty of the hydrocarbons was ca. 99%, except for the 5,6-dimethyldecane, the puri ty of which was somewhat lower at ca. 97-980/0 . The lithium fluoride prism resolved well the absorption bands in the region of the C--H stretching vibrations at about 3000 cm -~ well. The hydrocarbons investigated were mixed with CC14 in a ratio by volume of 1 : 50 and the spectra were taken with an absorbing layer thickness of 0.105 ram. The positions of the maxima of the absorption bands and their relative intensities agreed well with literature data: all the spectra clearly showed the bands at 2962 em -1 and 2827 cm -1 due to the asymmetrical and symmetrical vibrations of the (1H 3 group, respectively, and the same bands for the CH 2 group, at 2926 and 2853 em -1. As the number of methyl groups in the molecule increases, the intensity of the corresponding bands rises, with a simultaneous fall in the intensity of the bands of the methylene groups. In hydrocarbons with the same number of methyl and methylene groups the bands of the latter are more intense in those cases where all the methylene groups are present in the main chain of the molecule; it is sufficient to compare the spectra of 5,6-dimethyldecane with the spectra of 4,5-diethyloctane and 4-ethyl5-methylnonane, and those of 5-methylundecane and 4-ethyldecane. The position of the stretching vibrations of the C--H of the tertiary carbon atom distorts the low-frequency side of the 2926 cm -1 band, which explains its asymmetrical form. The absence of resolution is due to the high concentration of CHz groups