Lesson Introduction

Substitution reactions at an aliphatic carbon are generally nucleophilic. In aromatic systems the situation is reversed. Three types of reagents can perform substitution in aromatic compounds, that is, electrophiles, nucleophiles and free radicals. In this lesson, we will deal with electrophilic and nucleophilic substitutions; the substitution brought about by radicals has been discussed in free radicals. Benzene is a planar symmetrical hexagon with six trigonal (sp²) carbon atoms, each having one hydrogen atom in the plane of the ring. All the bond lengths are 1.39 Å and all the ¹³C shifts are the same. We know that the principal reaction of benzene and its derivatives is substitution rather than addition. It combines only with very reactive electrophiles. Indeed, electrophilic substitution in aromatic systems is one of the most important reactions in chemistry and has many commercial applications.

The π-electron cloud above and below the plane of the benzene ring is a source of electron density and confers nucleophilic properties on the system. Thus, reagents that are deficient in electron density (electrophiles) are likely to attack, but electron-rich nucleophiles should be repelled and, therefore, are unlikely to react. Furthermore, in electrophilic substitution the leaving group is a proton (H⁺), but in nucleophilic substitution it is a hydride ion (H⁻); the former process is energetically more favourable. In fact, nucleophilic aromatic substitution is not common, but it does occur in certain circumstances.

The carbocation generated by the addition of an electrophile to an alkene is destroyed in the second step by the addition of a nucleophilic species. Both carbon atoms become sp³ and the double bond is lost. A similar second step in aromatic molecules would result in destruction of the resonance-stabilized system and, therefore, does not occur.