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Organic Chemistry

15.7 Spectroscopy of Aromatic Compounds

Organic Chemistry15.7 Spectroscopy of Aromatic Compounds

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).

Monosubstituted: 690–710 cm–1         1,2,4-Trisubstituted: 780–830 cm–1
  730–770 cm–1           870–900 cm–1
o-Disubstituted: 735–770 cm–1         1,2,3-Trisubstituted: 670–720 cm–1
m-Disubstituted: 690–710 cm–1           750–790 cm–1
  810–850 cm–1         1,3,5-Trisubstituted: 660–700 cm–1
p-Disubstituted: 810–840 cm–1           830–900 cm–1

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

The infrared spectrum of toluene shows peaks as follows: Monosubstituted ring C single bond H stretches at 690 and 710 wavenumbers, ring bonds at 1500 and 1600 wavenumbers and C single bond H stretches of methyl group at 3000 wavenumbers.
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.

The ultraviolet spectrum of benzene shows peaks at 184 and 202 nanometers. Seven small peaks are between 225 to 268 nanometers
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.

In an applied magnetic field, aromatic ring is oriented at 90 degrees. Labels read, circulating pi electrons, proton deshielded by induced field, and induced magnetic field because of the ring current.
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 structure and ball-and-stick model along with the electrostatic potential map of [18]annulene. In annulene, inside H, and outside H are labeled minus 3.0 delta and plus 9.3 delta, respectively.

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.

Aromatic ring with carbon atom at C 1. The carbon is bonded to two hydrogen atoms and R group, labeled benzylic protons. Hydrogens bonded to the ring are aryl protons.

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.

The 1 H N M R spectrum of para-bromotoluene shows peaks at 0 (T M S), 2.3, 7.0, and 7.4 parts per million.
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.

The structures of benzene, toluene, chlorobenzene, and naphthalene with 13 C N M R spectrum values indicated.
Figure 15.16 Some 13C NMR absorptions of aromatic compounds (δ units).
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