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Electrophilic Aromatic Substitution

Benzene is the simplest aromatic compound with C6H6 molecular formula. There are three pi bonds in alternate manner which are in conjugation with other. The conjugation of pi electrons provides the stability to the molecule. No doubt there are three pi bonds, but still the molecule cannot be considered as unsaturated molecule because the pi bonds are in conjugation. That is the reason, benzene and other aromatic compounds easily give substitution reaction instead of addition reaction. The high electron density makes the molecule susceptible for electrophilic substitution reactions. Benzene and other aromatic compounds can also give addition reaction but it requires drastic conditions such as high temperature and catalyst.

Presence of any functional group on the aromatic ring affects the reactivity of aromatic ring due to electronic effects of these functional groups. For example, phenol is more reactive compared to benzene due to the presence of hydroxyl group on the benzene ring. In an electrophilic substitution reaction, an electrophile attacks on aromatic ring and form an intermediate which is stabilised by resonance. The intermediate converts into substituted product with the release of hydrogen ion. Let’s discuss the electrophilic aromatic substitution reaction with appropriate examples.

Sigma SkeletonBenzene

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Electrophilic Aromatic Substitution Mechanism

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As stated earlier the reaction is a two step process. One step mechanism is hypothetical.

Aromatic Electrophilic Mechanism

The rate law of the reaction should be the same as the SN2 process, in which the proton elimination-cum- transition state formation step is considered to be the rate determining step. Hence,

Rate = k [Benzene][E +]

On the other hand, if we consider the two step mechanism.

Step-1:

Sigma

Step-2:

Electrophilic Aromatic Substitution

In this case also the rate equation is the same as the previous case.

Rate = k [Benzene][E +]

There are direct and indirect evidences supporting the above kinetics of the two step process.

Energy Profile Diagram For Aromatic Electrophilic Substitution


Reaction Coordinates Energy


Sigma And Pi Complexes

  1. The intermediates in aromatic electrophilic substitution described are commonly referred to as "sigma" and "pi" complexes.
  2. In these complexes, there is actual bonding of the reagent to a ring carbon atom and since the pi orbitals now involve only five carbon atoms, there is loss of aromaticity.
  3. Experimental evidences, however, suggests that the formation of a sigma complex follows the initial formation of a complex known as a pi complex.
  4. In contrast t the sigma complex, the pi complex does not involve actual bonding but the electrophile is held near the pi electron cloud of the aromatic ring.
  5. The electron transfer does not occur during the pi complex formation. This induction type bonding is of higher energy and lesser stability than sigma complex and thus eventually leads to the formation of the later in order to attain stability by transferring charges(electrons) to the electrophile.

Electrophilic Aromatic Substitution Reactions

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There are several reaction belongs to this category. Some commons are

  1. Nitration of benzene
  2. Sulphonation of benzene
  3. Friedel-Crafts alkylation and
  4. Acylation of benzene
  5. Iron or iron(III)chloride catalyzed halogenation of benzene


In Friedel-Crafts alkylation and acylation, we use aluminium chloride (or some other Lewis acids like FeCl3 or BF3 etc.) for the generation of electrophile.

AlCl3 + CH3Cl $\rightarrow$ AlCl4 - + CH3+ (electrophile)


Generation of cationic chlorine in the chlorination (iron catalyzed) is believed to take place in the similar fashion. But the generation of electrophile follows a different sequence. In that case concentrated sulfuric acid acts as Bronsted-Lowry acid or proton donor and concentrated nitric acid acts as a Bronsted-Lowry base or proton acceptor:


H2SO4 $\rightarrow$ H+ + HSO4-

HNO3 + H+ $\rightarrow$ H2O + NO2+ (electrophile)


The above facts can be represented diagrammatically.

Friedal Crafts

Aromatic Electrophilic Examples

Name of the product of the Friedel-Crafts acylation of benzene : - Acetophenone

Name of the product of the nitration of benzene: - Nitrobenzene

Name of the product of the halogenation of benzene :- Chlorobenzene

Heterocyclic aromatic compounds also undergo aromatic electrophilic substitution. A few examples are

Furan Mechanism

Thiopen

Evidences for Aromatic Electrophilic Substitution

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1. Isolation of intermediates

The intermediate has been isolated and eventually found to be the sigma complex mentioned in the two-step mechanism. When benzotrifluoride was treated with nitryl fluoride and boron trifluoride at about - 800C temperature, an yellow crystalline sigma complex was detected and isolated.

A + B = $\bar{a} \cdot \bar{b}$

2. Kinetic isotope effect

Usually aromatic substitutions do not exhibit kinetic isotope effects. The rates of nitration of deutero benzene has been found to be the same as that of normal benzene. This is a clear indication that the synchronous making and breaking of C - E and C - H bond is not happening in this reaction. Therefore, the one step mechanism is inconsistent with the experimental observation.

The complete mechanism of the aromatic electrophilic substitution is then

Complete Mechanism Sigma

More topics in Electrophillic Aromatic Substitution
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