Electrophilic aromatic substitution (EAS) reactions – the general picture

Although the delocalized pi electrons in aromatic rings are much less reactive than those in isolated or conjugated alkenes, they can undergo electrophilic reactions given a powerful enough electrophile.  Electrophilic addition to aromatic double bonds, however, is not generally observed – this would be energetically unfavorable because it would result in a loss of aromaticity in the product.

Rather, aromatic double bonds undergo electrophilic substitution reactions, (abbreviated S_EAr), which are mechanistically very similar to the substitutive addition-elimination reactions of alkenes. The product of an S_EAr reaction, like the starting compound, is aromatic.

Because the starting point in an S_EAr reaction is inherently low in energy (due to aromaticity), the activation energy for the first electrophilic step – in which aromaticity is temporarily lost in the carbocation intermediate- is correspondingly high.  This is why aromatic pi bonds are significantly less reactive in electrophilic steps compared to the π bonds of nonaromatic alkenes.

As a consequence, S_EAr reactions  tend to share two basic characteristics.  First, the electrophilic partner needs to be highly reactive – this serves to increase the energy of the starting point and decrease the activation energy.  A second common characteristic that we will see in many S_EAr reactions is that the reaction is often affected by the presence of substituents on the aromatic ring.  These substituents will affect the rate of reaction and the position of electrophilic attack.