Charateristic IR Absorption of Benzene Derivatives

Arenes have absorption bands in the 650-900 cm⁻¹ region due to bending of the C–H bond out of the plane of the ring. The exact placement of these absorptions can indicate the pattern of substitution on a benzene ring. However, this is beyond the scope of introductory organic chemistry. Arenes also possess a characteristic absorption at about 3030-3100 cm⁻¹ as a result of the aromatic C–H stretch. It is somewhat higher than the alkyl C–H stretch (2850–2960 cm⁻¹), but falls in the same region as olefinic compounds. Two bands (1500 and 1660 cm⁻¹) caused by C=C in plane vibrations are the most useful for characterization as they are intense and are likely observed.

In aromatic compounds, each band in the spectrum can be assigned:

• C–H stretch from 3100-3000 cm^-1
• overtones, weak, from 2000-1665 cm^-1
• C–C stretch (in-ring) from 1600-1585 cm^-1
• C–C stretch (in-ring) from 1500-1400 cm^-1
• C–H "oop" from 900-675 cm^-1

Note that this is at slightly higher frequency than is the –C–H stretch in alkanes. This is a very useful tool for interpreting IR spectra. Only alkenes and aromatics show a C–H stretch slightly higher than 3000 cm^-1.

Figure 1. Infrared Spectrum of Toluene