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Mechanism of Aromatic Substitution

7 years ago


Answers : (1)


Mechanism of Aromatic Substitution:- there are three possible mechanisms for aromatic substitution.

A)  Electrophilic aromatic substitution:- it is believed to proceed by a bimolecular (SE2) mechanism involving the formation of an intermediate carbonium ion which is a rate determining step. Examples are halogenations nitration, sulphonation and Friedel Crafts reaction, etc.,

B)  Nucleophilic aromatic substitution:- The replacement of hydrogen or any other substitutent by a nucleophilic reagent is termed nucleophilic substitution. This replacement of hydrogen by nucleophile does not occur in benzene itself, but the presence of an electron withdrawing group is sufficient to activate the o and p position. In general, there are three types of mechanism unimolecular bimolecular and the elimination addition mechanism.

i)            Unimolecular mechanism:- this mechanism is most uncommon in aromatic substitution. There is only one example and that is the decomposition of aryl diazonium cations in aqueous solution to yield phenol or anyl halides

ii)          Biomolecular mechanism:- Nucleophilic aromatic substitutions are commonly bimolecular and proceeds through a negative ion intermediate, i.e., cyclo hexadienyl anion, e.g., anyl halides undergo nucleophilic substitution only in vigorous conditions. Thus, the reaction between chlorobenzene and aqueous alkali at 3000 C to form phenol may be represented. This replacement when facilitated by the presence of an electron withdrawing group in the o-and/or the p-position, is termed activated nucleophilic aromatic substitution and an example is the conversion of o-chloronitro benzene into 0-nitrophenol when heated with aq. Alkali at 2000C.

7 years ago

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