Overview of Electrophilic Aromatic Substitution in Organic Chemistry

Electrophilic aromatic substitution refers to a reaction of the benzene ring in which an electrophile such as a nitro group is added to the ring with subsequent elimination of a hydrogen atom.  This type of reaction is typical for aromatic compounds and in particular for the benzene ring. 

Addition of an electrophile to the benzene ring is more difficult than benzene molecule with substituent that are electron donating in nature are attached to the ring such as a methyl group or an amino group.  This is so due to the activation of the benzene ring by the electron donating group which pushes electron density into the benzene ring. 

Benzene is unusually stable and it requires energy in the form of heat to break the aromaticity of the ring.  When an electrophile adds to a benzene ring there is usually a high energy barrier for this reaction to take place due to breaking the aromatic nature of the ring.  A carbocation intermediate is formed as a result of the addition of the electrophile which is relatively a stable intermediate due to the delocalization or resonance of the double bonds of the ring. 

This stable intermediate is still higher in energy than the starting material.  The intermediate nonaromatic structure then restores the aromaticity by expelling a hydrogen atom from the intermediate.  The driving force for this process is the restoration of the aromaticity.  This last step is exothermic because it releases energy due to the formation of the highly stable substituted benzene ring. 

TNT or tri-nitro-toluene is an explosive which can be synthesized from toluene with addition of three nitro groups to the ortho and para positions.  Toluene in particular and electron donating groups in general prefer the substitution of electrophiles to the ortho and para positions in the ring.  This is so due to the increase in electron density at these positions caused by the electron donating groups. 

Phenol is another compound which can undergo electrophilic aromatic substitution of electrophilic groups such as nitro groups.  One of these compounds the tri-nitro-phenol or picric acid was used in world war II as an explosive in amunitions.

The degree of reactivity of the benzene ring for electrophilic aromatic substitution increases with the attachment of electron donating groups on the ring such as a hydroxyl group or an amino group which push electron density into the ring by conjugation of its lone pair pf electrons through the ring.  Methyl groups push electron density into the ring by the mechanism of hyperconjugation.  Hyperconjugation refers to the participation of C-H bonds in the methyl group as an alternative to lone pair of electrons such as those on a hydroxyl or amino groups.        

Electron withdrawing groups have the opposite effect to that of the electron donating group of substituents.  They withdraw electron density from the ring also by conjugation with a pi* orbital on the electron withdrawing group that is attached to the ring.  For the reason that they pull electron density from the ring they deactivate it for an additional addition of an electrophile. 

A positive charge next to an electron donating group is a favourable and energetically low process.  On the other hand, putting a positive charge next to an electron withdrawing group is an energetically unfavourable process which costs a lot of energy.  For this reason the manner of substitution on the aromatic ring in electron donating groups substituted rings differ than that with electron withdrawing groups substituted rings. 

Due to the fact that electron withdrawing groups put a positive charge next to an electron withdrawing group in ortho and para substitutions which is an energetically unfavoured process, the manner of electrophilic aromatic substitution with electron withdrawing groups occur in the meta position in which case there is avoidance of putting positive charge next to the electron withdrawing group.