Benzene and the Concept of Aromaticity

There are certain limitations to the usefulness of nitration in aqueous sulphuric acid. Because of the behaviour of the rate profile for benzene, comparisons should strictly be made below 68% sulphuric acid ( 2.5 fig. 2.5) rates relative to benzene vary in the range 68-80% sulphuric acid, and at the higher end of this range are not entirely measures of relative reactivity. For deactivated compounds this limitation is not very important, but for activated compounds it is linked with a fundamental limit to the significance of the concept of aromatic reactivity as already discussed ( 2.5), nitration in sulphuric acid cannot differentiate amongst compounds not less than about 38 times more reactive than benzene. At this point differentiation disappears because reactions occur at the encounter rate. 

Despite its unsaturated nature, benzene with its sweet aroma, isolated by Michael Faraday in 1825 [1], demonstrates low chemical reactivity. This feature gave rise to the entire class of unsaturated organic substances called aromatic compounds. Thus, the aromaticity and low reactivity were connected from the very beginning. The aromaticity and reactivity in organic chemistry is thoroughly reviewed in the book by Matito et al. [2]. The concepts of aromaticity and antiaromaticity have been recendy extended into main group and transition metal clusters [3-10], The current chapter will discuss relationship among aromaticity, stability, and reactivity in clusters. 

Aromatic heterocycles are described as being heteroaromatic, and we shall concentrate on these systems in this book at the expense of more saturated systems. Let us now consider the concept of aromaticity with regard to benzene. 

The concept of aromaticity is vety important in understanding the chemical behavior of cyclic, conjugated compounds. It is most important with benzene and its derivatives, but it also has applications to many other types of compounds. Whenever a reactant, product, or intermediate contains a planar cycle of p orbitals, the effect of aromaticity (or antiaromaticity) on the reaction must be considered. 

The concept of aromaticity is not restricted to hydrocarbons. Heterocyclic systems, whether of the pyrrole type 1.38 with trigonal nitrogen in place of one of the C=C double bonds, or of the pyridine type 1.39 with a trigonal nitrogen in place of a carbon atom, are well known. The n orbitals of pyrrole are like those of the cyclopentadienyl anion, and those of pyridine like benzene, but skewed by the presence of the electronegative heteroatom. The energies and coefficients of heteroatom-containing systems like these cannot be worked out with the simple... 

The first chapter discusses the concept of aromaticity, after which there is a description of aromatic substitution reactions. Chapters covering the chemistry of the major functionalized derivatives of benzene follow. A chapter on the use of metals in aromatic chemistry discusses not only the chemistry of Grignard reagents and aryllithium compounds but also the more recent uses of transition metals in the synthesis of aromatic compounds. The penultimate chapter discusses the oxidation and reduction of the benzene ring and the text concludes with the chemistry of some polycyclic compounds. 

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