PREPARATION AND PROPERTIES
Simple aromatic structures such as benzene, toluene, or naphthalene are isolated from natural sources and converted to more complex aromatic structures.
Many aromatic compounds have a characteristic aroma and burn with a smoky flame. They are nonpolar, hydrophobic molecules which dissolve in organic solvents rather than water. Aromatic molecules can interact by van der Waals interactions or with a cation through an induced dipole interac-tion. Aromatic compounds undergo reactions where the aromatic ring is retained. Electrophilic substitution is the most common type of reaction. However, reduction is also possible.
Aromatic compounds show characteristic absorptions in the IR spectrum due to ring vibrations. Signals due to Ar-H stretching and bending may also be observed. Signals for aromatic protons and carbons appear at character-istic positions in nmr spectra. Fragmentation ions can be observed in mass spectra which are characteristic of aromatic compounds.
It is not practical to synthesize aromatic structures in the laboratory from scratch and most aromatic compounds are prepared from benzene or other simple aromatic compounds (e.g. toluene and naphthalene). These in turn are isolated from natural sources such as coal or petroleum.
Many aromatic compounds have a characteristic aroma and will burn with a smoky flame. They are hydrophobic, nonpolar molecules which will dissolve in organic solvents and are poorly soluble in water. Aromatic molecules can interact with each other through intermolecular bonding by van der Waals interactions (Fig. 1a). However, induced dipole interactions are also possible with alkyl ammonium ions or metal ions where the positive charge of the cation induces a dipole in the aromatic ring such that the face of the ring is slightly negative and the edges are slightly positive (Fig. 1b). This results in the cation being sandwiched between two aromatic rings.
Aromatic compounds are unusually stable and do not react in the same way as alkenes. They prefer to undergo reactions where the stable aromatic ring is retained. The most common type of reaction for aromatic rings is electrophilic substitution, but reduction is also possible.
The presence of an aromatic ring can be indicated by IR, nmr and mass spectroscopy.
In the IR spectrum, the stretching absorption of an Ar-H bond occurs at 3040–3010 cm−1 which is higher than the range for an aliphatic C–H stretch (3000–2800 cm−1). However, the absorption is usually weak and may be hidden.
Absorptions due to ring vibrations are more reliable and can account for up to four absorptions (typically about 1600, 1580, 1500 and 1450 cm−1). These occur at lower wavenumbers to the C=C stretching absorptions of alkenes (1680– 1620 cm−1). Ar-H out of plane bending can give absorptions in the region 860–680 cm−1. The number and position of these absorptions can indicate the sub- stitution pattern of the aromatic ring. For example, an ortho disubstituted ring typ- ically has an absorption at 770–735 cm−1 whereas a para disubstituted ring has an absorption in the region 860–800 cm−1. A monosubstituted aromatic ring has two absorptions in the regions 690–710 and 730–770 cm−1 while a meta-disubstituted aromatic ring has two absorptions in the regions 690–710 and 810–850 cm−1. These bending absorptions are not always reliable and may be difficult to distinguish from other absorptions in the region.
The nmr signals for aromatic protons and carbons occur in characteristic regions of the nmr spectra (typically 6.5–8.0 ppm for 1H nmr; 110–160 ppm for 13C nmr). Moreover, it is possible to identify the substitution pattern of the aro- matic ring based on the chemical shifts of the signals. Tables exist which allow the calculation of the expected chemical shifts based on the types of substituents that are present and their relative positions on the ring. In the 1H spectrum, coupling patterns can often be useful in determining substitution patterns for the aromatic ring. For example, para-disubstituted aromatic rings often show two signals, both of which are doublets.
There are characteristic fragmentation patterns in the mass spectra of aromatic compounds. For example, compounds containing monosubstituted aromatic rings typically show fragmentation ions at m/e 39, 50, 51, 52, & 77. Benzylic compounds usually have a strong signal at m/e 91 due to cleavage of a benzylic fragmentation ion.
Since aromatic rings contain a conjugated π electron system, they can be detected by uv spectroscopy. They generally show an intense absorption near 205 nm and a less intense absorption in the range 255–275 nm.