The most important absorptions in the double bond region are those of the carbonyl (C=O), alkene or arene (C=C), and nitro (NO₂), groups. All give rise to sharp bands, C=O gives one strong (i.e. intense) band anywhere between 1900 and 1500 cm⁻¹; alkene C=C gives one weak band at about 1640 cm⁻¹, and NO₂ gives two strong (intense) bands in the mid-1500s and mid 1300s cm⁻¹. Arenes usually give two or three bands in the region 1600–1500 cm⁻¹. We can illustrate several of these features in the spectrum shown below, which is that of 4-nitrocin namaldehyde, shown in the margin.

Why the nitro group gives two bands is easily understood. Just as with OH and NH₂, it is a matter of how many identical bonds are present in the same functional group. Carbonyl and alkene clearly have one double bond each. The nitro group at fi rst sight appears to contain two different groups, N⁺–O⁻ and N=O, but delocalization means they are identical and we see absorption for symmetric and antisymmetric stretching vibrations. As with NH₂, more energy is associated with the antisymmetric vibration and it occurs at higher frequency (>1500 cm⁻¹).

Arenes, being rings, have a much more complex pattern of vibration that cannot be ana lysed simply, However, it’s worth noting that arene C=C bonds come at lower frequency (1600 cm⁻¹). Why? Well, the individual C–C bonds in benzene are of course not full C=C double bonds—all six bonds are the same, and have the averaged character of one-and-a-half bonds each. Not surprisingly, the absorptions of these bonds fall right on the boundary between the single and double bond regions. You’ve already seen the IR spectra of the three carbonyl compounds below in this chapter. It’s easy to identify the C=O peak in each spectrum—C=O peaks are always intense (you will see why in a minute) and come somewhere near 1700 cm⁻¹.

Why the positions of the peaks vary, and what we can make of this information, will be discussed in Chapter 18.

● Important absorptions in the double bond region