Aromatic chemistry: Preparation and properties

PREPARATION AND PROPERTIES

Key Notes

Preparation

Simple aromatic structures such as benzene, toluene, or naphthalene are isolated from natural sources and converted to more complex aromatic structures.

Properties

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.

Spectroscopic analysis

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.

Preparation

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.

Properties

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.

Spectroscopic analysis

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.