Alkanes and Cycloalkanes

The infrared spectra of the alkanes show clearly absorptions corresponding to the C−HC−H stretching frequencies at 2850cm−1 to 3000cm−1. The C−C stretching absorptions have variable frequencies and are usually weak. Methyl (CH3−) and methylene (−CH2−) groups normally have characteristic C−H bending vibrations at 1400cm−1 to 1470cm−1. Methyl groups also show a weaker band near 1380cm−1. Two sample infrared spectra that illustrate these features are given in Figure 9-11.

The infrared spectra of the cycloalkanes are similar to those of the alkanes, except that when there are no alkyl substituents the characteristic

Figure 9-11: Infrared spectra of (a) octane and (b) 2,2,4-trimethylpentane as pure liquids. Notice the C−H stretching around 2900cm−1 and C−H bending frequency around 1460cm−1. The bands near 1370cm−1 for 2,2,4-trimethylpentane are characteristic of methyl C−H bending frequencies.

bending frequencies of methyl groups at 1380cm−1 are absent. A moderately strong CH2 "scissoring" frequency is observed between 1440cm−1 and 1470cm−1, the position depending somewhat on the size of the ring. These features of the infrared spectra of cycloalkanes are illustrated in Figure 9-12 using cyclooctane and methylcyclohexane as examples.

Figure 9-11.