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# Electrophillic Aromatic Substitution

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 Sub Topics Most substitutions at an aliphatic carbon are nucleophilic. In aromatic systems the situation is reversed, because the high electron density at the aromatic ring attracts positive species and not negative ones. In electrophilic substituents the attacking species is a positive ion or the positive end of the a dipole or induced dipole. The leaving group that is electrofuge must necessarily depart without its electron pair.  An electrophilic substitution involves the substitution of one electrophile from the aromatic ring with another electrophile. Electrophilic aromatic substitution is the most important reaction of aromatic compounds because it has broad applications for a wide variety of aromatic compounds. The study of aromatic heterocyclic reactivity can be said to have started with the results of electrophilic substitution processes.

## Electrophilic Aromatic Substitution Mechanism

The mechanism of electrophilic substitution involves two stages. The aromatic ring uses two of its $\pi$ electrons to form a bond to the electrophile which results in a positively charged intermediat. Electrophilic substitution of aromatic molecules proceeds by two step sequence, initial addition giving a positively charged intermediate the elimination of which the former is usually the slower means rate determining step.

Electrophilic aromatic substitutions are unlike nucleophilic substitutions in that the large majority proceed by just one mechanism with respect to the substrate. In this mechanism which we call the arenium ion mechanism, the electrophile attacks in the first step, giving rise to a positively charged intermediate and the leaving group departs in the second step.

The rate determining step in electrophilic substitution is the formation of the positively charged intermediate and so the rate of the reaction is determined by the energy level of the transition state leading to that intermediate. The transition state resembles the intermediate in character and so  any factor stabilizing also stabilizes the transition state and lowers the activation energy required for the reaction. Therefore electrophilic substitution is more likely to take place if the positively charged intermediate can be stabilized. Stabilization is possible if the positive charge can be spread amongest different atoms that is a process called delocalization. The process by which this can takes place is known as resonance.