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Electrophillic Aromatic Substitution


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. 

Electrophilic Substitution Recation
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

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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 Substitution Recation Mechanism

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.

Electrophilic Aromatic Substitution Reactions

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For example, the presence of more electronegative nitrogen atom in the pyridine ring decreases the availability of electrons thus makes it more difficult for an electrophile to attack on carbon atoms in the ring. The nitrogen atom deactivates the 2- and 4- positions more than meta position. 

In electrophilic aromatic substitution reactions a proton on an aromatic ring is replaced by a potent electrophile. In all electrophilic aromatic substitution reactions, the aromatic ring acts as a nucleophile. In electrophilic substitution halogenation a chlorine or bromine atom is installed on the ring. A Lewis acid catalyst, such as iron trichloride, aluminum trichloride or iron trobromide is also used.

It is an organic reaction in which an atom that is attached to an aromatic system is replaced by an electrophile. Some of the most important electrophilic aromatic substitutions are aromtic nitration,, aromatic halogenation, aromatic sulfonation and acylation and alkylating Friedel-Crafts reactions.