IR Spectroscopy Tutorial: Aromatics

The =C–H stretch in aromatics is observed at 3100-3000 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. Compounds that do not have a C=C bond show C–H stretches only below 3000 cm-1.

Aromatic hydrocarbons show absorptions in the regions 1600-1585 cm-1 and 1500-1400 cm-1 due to carbon-carbon stretching vibrations in the aromatic ring.

Bands in the region 1250-1000 cm-1 are due to C–H in-plane bending, although these bands are too weak to be observed in most aromatic compounds.

Besides the C–H stretch above 3000 cm-1, two other regions of the infrared spectra of aromatics distinguish aromatics from organic compounds that do not have an aromatic ring:

Not only do these bands distinguish aromatics, but they can be useful if you want to determine the number and positions of substituents on the aromatic ring. The pattern of overtone bands in the region 2000-1665 cm-1 reflect the substitution pattern on the ring. The pattern of the oop C–H bending bands in the region 900-675 cm-1 are also characteristic of the aromatic substitution pattern. Details of the correlation between IR patterns in these two regions and ring substitution are available in the literature references linked in the left frame (especially the books by Shriner and Fuson, Silverstein et. al., and the Aldrich Library of IR Spectra).

In some instances, it is useful to remember that aromatics in general show a lot more bands than compounds that do not contain an aromatic ring. If you are presented with two spectra and told that one is aromatic and one is not, a quick glance at the sheer multitude of bands in one of the spectra can tell you that it is the aromatic compound.


The spectrum of toluene is shown below. Note the =C–H stretches of aromatics (3099, 3068, 3032) and the –C–H stretches of the alkyl (methyl) group (2925 is the only one marked). The characteristic overtones are seen from about 2000-1665. Also note the carbon-carbon stretches in the aromatic ring (1614, 1506, 1465), the in-plane C–H bending (1086, 1035), and the C–H oop (738).