15.7 Spectroscopy of Aromatic Compounds

Infrared Spectroscopy

As we saw in the brief introduction to infrared spectroscopy (Section 12.8), aromatic rings show a characteristic C–H stretching absorption at 3030 cm^–1 and a series of peaks in the 1450 to 1600 cm^–1 range of the infrared spectrum. The aromatic C–H band at 3030 cm^–1 generally has low intensity and occurs just to the left of a typical saturated C–H band.

As many as four absorptions are sometimes observed in the 1450 to 1600 cm^–1 region because of the complex molecular motions of the ring itself. Two bands, one at 1500 cm^–1 and one at 1600 cm^–1, are usually the most intense. In addition, aromatic compounds show weak absorptions in the 1660 to 2000 cm^–1 region and strong absorptions in the 690 to 900 cm^–1 range due to C–H out-of-plane bending. The exact position of both sets of absorptions is diagnostic of the substitution pattern of the aromatic ring (Figure 12.24 in Section 12.8).

The IR spectrum of toluene in Figure 15.12 shows these characteristic absorptions.

Figure 15.12 The infrared spectrum of toluene.

Ultraviolet Spectroscopy

Aromatic rings are detectable by ultraviolet spectroscopy because they contain a conjugated π electron system. In general, aromatic compounds show a series of bands, with a fairly intense absorption near 205 nm and a less intense absorption in the 255 to 275 nm range. The presence of these bands in the ultraviolet spectrum of a molecule is a sure indication of an aromatic ring. Figure 15.13 shows the ultraviolet spectrum of benzene.

Figure 15.13 Ultraviolet spectrum of benzene. There are primary bands at 184 and 202 nm and secondary (fine-structure) bands at 255 nm.

Nuclear Magnetic Resonance Spectroscopy

Aromatic hydrogens are strongly deshielded by the ring and absorb between 6.5 and 8.0 δ. The spins of nonequivalent aromatic protons on substituted rings often couple with each other, giving rise to spin–spin splitting patterns that can identify the substitution of the ring.

Much of the difference in chemical shift between aromatic protons (6.5–8.0 δ) and vinylic protons (4.5–6.5 δ) is due to a property of aromatic rings called ring-current. When an aromatic ring is oriented perpendicular to a strong magnetic field, the electrons circulate around the ring, producing a small local magnetic field. This induced field opposes the applied field in the middle of the ring but reinforces the applied field outside the ring (Figure 15.14). Aromatic protons therefore experience an effective magnetic field greater than the applied field and come into resonance at a lower applied field.

Figure 15.14 The origin of aromatic ring-current. Aromatic protons are deshielded by the induced magnetic field caused by delocalized π electrons circulating around the aromatic ring.

Note that the aromatic ring-current produces different effects inside and outside the ring. If a ring were large enough to have both “inside” and “outside” protons, the protons on the outside would be deshielded and absorb at a field lower than normal but those on the inside would be shielded and absorb at a field higher than normal. This prediction has been strikingly confirmed by studies on [18]annulene, an 18-π-electron cyclic conjugated polyene that contains a Hückel number of electrons (4n + 2 = 18 when n = 4). The six inside protons of [18]annulene are strongly shielded by the aromatic ring-current and absorb at –3.0 δ (that is, 3.0 ppm upfield from TMS, off the normal chart), while the 12 outside protons are strongly deshielded and absorb in the typical aromatic region at 9.3 ppm downfield from TMS.

The presence of a ring-current is characteristic of all Hückel aromatic molecules and is a good test of aromaticity. For example, benzene, a six-π-electron aromatic molecule, absorbs at 7.37 δ because of its ring-current, but cyclooctatetraene, an eight-π-electron nonaromatic molecule, absorbs at 5.78 δ.

Hydrogens on carbon next to aromatic rings—benzylic hydrogens—also show distinctive absorptions in the NMR spectrum. Benzylic protons normally absorb downfield from other alkane protons in the region from 2.3 to 3.0 δ.

The ¹H NMR spectrum of p-bromotoluene, shown in Figure 15.15, displays many of the features just discussed. The aromatic protons appear as two doublets at 7.04 and 7.37 δ, and the benzylic methyl protons absorb as a sharp singlet at 2.26 δ. Integration of the spectrum shows the expected 2 : 2 : 3 ratio of peak areas.

Figure 15.15 The ¹H NMR spectrum of p-bromotoluene.

The carbon atoms in an aromatic ring typically absorb in the range 110 to 140 δ in the ¹³C NMR spectrum, as indicated by the examples in Figure 15.16. These resonances are easily distinguished from those of alkane carbons but occur in the same range as alkene carbons. Thus, the presence of ¹³C absorptions at 110 to 140 δ does not by itself establish the presence of an aromatic ring. Supporting evidence from infrared, ultraviolet, or ¹H NMR spectra is needed.

Figure 15.16 Some ¹³C NMR absorptions of aromatic compounds (δ units).