CHAPTER 2 - Alkanes

Abstract

Publisher Summary This chapter examines the characteristic infrared (IR) and Raman group frequencies of alkanes. The characteristic vibrations of alkanes can be categorized as CH motions and skeletal (C—C) vibrations. The vibrations involving CH2 and CH3 stretchings give rise to bands that are strong and characteristic in both IR and Raman spectra, whereas bending modes give rise to strong IR bands but variable Raman bands. The CH3 antisymmetrical stretchings and symmetrical stretchings occur near 2962 cm−1 and 2872 cm-1, respectively. The intensities of these vibrations are strong and their frequencies are characteristic. The four bending vibrations of CH2 groups are scissoring, rocking, wagging, and twisting. The scissoring mode of the CH2 group gives rise to a characteristic band near 1465 cm−1 in IR and Raman spectra. The rocking, wagging, and twisting appear in the region of 1422–719 cm−1. The skeletal vibrations involve the carbon-chain stretchings and deformations. IR absorption of these vibrational modes is weak for n-alkanes. Although, more IR details are observed for branched alkanes in the skeletal vibration region, determination of skeletal structures by IR spectroscopy is always challenging.