Electrophilic Aromatic Reactions

The electrophilic reactions that occur with aromatic compounds are predominantly substitution reactions, in contrast to the addition reactions that occur with alkenes. The enhanced stability of aromatic systems makes it energetically favourable for reactions of the electron-rich p-system with electrophiles to involve first an addition step, followed by an elimination leading to restoration of the aromaticity of the ring.

Because of the stability of aromatic systems, such substitution reactions require very reactive electrophiles, usually cationic species. The generalised reaction is a two-stage one involving first the formation of a cationic intermediate that is stabilised by delocalisation, followed by loss of a proton to restore the aromaticity. This energy profile diagram illustrates the course of a typical aromatic substitution reaction.

Curly arrows are particularly useful because they can give a quite accurate prediction of the electron distribution, and therefore the distribution of positive charge, over the ring; and this in turn leads to predictions and explanations of the directing and activating effects of substituents on aromatic rings that are summarised in this table.

Drawing Benzene Rings

Because benzene is symmetrical, with all six bonds strictly equivalent, the circle structure shown on the left here is often regarded as the the best representation of the structure. However, it is not possible to draw curly arrow mechanisms with that representation, so instead one or other of the Kekulé forms on the right is always used instead. Note that this does not imply any implication that the bonds are localised, but it does fit with the fact that in most aromatic molecules, the bond lengths are not exactly the same, because of the effect of substituents on the ring.