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MECHANISM FOR AROMATIC SUBSTITUTION

6 years ago

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Answers : (2)

										

Dear Abhishek


Electrophilic aromatic substitution


Electrophilic aromatic substitution or EAS is an organic reaction in which an atom, usually hydrogen, appended to an aromatic system is replaced by an electrophile. The most important reactions of this type that take place are aromatic nitration, aromatic halogenation, aromatic sulfonation, and acylation and alkylating Friedel-Crafts reactions.


Basic reaction mechanism


In the first step of the reaction mechanism for this reaction, the electron-rich aromatic ring which in the simplest case is benzene attacks the electrophile A. This leads to the formation of a positively-charged cyclohexadienyl cation, also known as an arenium ion. This carbocation is unstable, owing both to the positive charge on the molecule and to the temporary loss of aromaticity. However, the cyclohexadienyl cation is partially stabilized by resonance, which allows the positive charge to be distributed over three carbon atoms.


In this diagram, A+ is an arbitrary electrophile


In the second stage of the reaction, a Lewis base B donates electrons to the hydrogen atom at the point of electrophilic attack, and the electrons shared by the hydrogen return to the pi system, restoring aromaticity.


An electrophilic substitution reaction on benzene does not always result in monosubstitution. While electrophilic substituents usually withdraw electrons from the aromatic ring and thus deactivate it against further reaction, a sufficiently strong electrophile can perform a second or even a third substitution. This is especially the case with the use of catalysts.


 


Nucleophilic aromatic substitution


 


A nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring.


reaction mechanism


The following is the reaction mechanism of a nucleophilic aromatic substitution of 2,4-dinitrochlorobenzene in a basic aqueous solution.



 Nucleophilic aromatic substitution


In this sequence the carbons are numbered clockwise from 1-6 starting with the 1 carbon at 12 o'clock, which is bonded to the chloride. Since the nitro group is an activator toward nucleophilic substitution, and an ortho/para director, it allows the benzene carbon to which it is bonded to have a negative charge. In the Meisenheimer complex, the nonbonded electrons of the carbanion become bonded to the aromatic pi system which allows the ipso carbon to temporarily bond with the hydroxyl group (-OH). In order to return to a lower energy state, either the hydroxyl group leaves, or the chloride leaves. In solution both processes happen. A small percentage of the intermediate loses the chloride to become the product (2,4-dinitrophenol), while the rest return to the reactant. Since 2,4-dinitrophenol is in a lower energy state it will not return to form the reactant, so after some time has passed, the reaction reaches chemical equilibrium.


 


All the best.


AKASH GOYAL


AskiitiansExpert-IITD


 


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6 years ago
										

In the first step of the reaction mechanism for this reaction, the electron-rich aromatic ring which in the simplest case is benzene attacks the electrophile A. This leads to the formation of a positively-charged cyclohexadienyl cation, also known as an arenium ion. This carbocation is unstable, owing both to the positive charge on the molecule and to the temporary loss of aromaticity. However, the cyclohexadienyl cation is partially stabilized by resonance, which allows the positive charge to be distributed over three carbon atoms.


In this diagram, A+ is an arbitrary electrophile


In the second stage of the reaction, a Lewis base B donates electrons to the hydrogen atom at the point of electrophilic attack, and the electrons shared by the hydrogen return to the pi system, restoring aromaticity.


An electrophilic substitution reaction on benzene does not always result in monosubstitution. While electrophilic substituents usually withdraw electrons from the aromatic ring and thus deactivate it against further reaction, a sufficiently strong electrophile can perform a second or even a third substitution. This is especially the case with the use of catalysts.

6 years ago

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