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Reactions of Aromatic Compounds (overall)

Mechanism of Electrophilic Aromatic Substitution (Ar-SE)

Addition-elimination mechanism
Electrophilic aromatic substitution is a multistep process. In the first step, the addition of an electrophile yields a high-energy carbocation in which the aromatic π system has been broken. Subsequently, the aromatic system is recovered by splitting off a proton. Therefore, the mechanism of an electrophilic aromatic substitution (Ar-SE) may be classified as an addition-elimination mechanism.
Fig.1
Mechanism of electrophilic aromatic substitution (Ar-SE).

Initially, the approaching electrophile interacts with the nucleophilic, aromatic π electron cloud. The so-called π complex may be identified through minor alterations in the absorption spectrum. However, the π complex is usually only slightly relevant in the understanding of the electrophilic aromatic substitution's reaction mechanism.

The π complex that is initially generated by the aromatic π system's nucleophilic attack on the electrophile is then rapidly converted into a cyclohexadienyl cation knownas either the Wheland complex, the σ complex, or the arenium ion. In the σ complex, the electrophile is connected to one of the aromatic ring carbons by a σ bond.

In the second step, a proton is eliminated from the σ complex and then accepted by a base. The base is frequently the counterion of the electrophile; occasionally, the solvent acts as the base. The first step of Ar-SE reaction, that is, the formation of the σ complex, requires a particularly high amount of energy, as the aromatic π system is broken. In the second step, that is, in the deprotonation, the aromatic π system is recovered.

In some cases, it is not a proton but another cation that is eliminated from the σ complex. As a result, the aromatic compound is defunctionalized - it loses a functional group. Such a reaction is called an ipso attack.

In the cyclohexadienyl cation (σ complex), the positive charge is delocalized - that is, it is shared by several carbon atoms.

Fig.2
σ complex.

In Lewis formulas, the positive charge's delocalization is indicated by several resonance structures. The various resonance structures of a σ complex are often illustrated by only one simplified structural formula in which four partial π bonds are represented by a broken line with the plus sign in the center of the ring. It must be kept in mind in this case that the positive charge is unevenly distributed among the four carbons involved, as it is primarily concentrated in the ortho and para position (view also nomenclature of aromatic compounds).

The distribution of electrons and charge in aromatic systems and in σ complexes may also be illustrated by the semiempirical-quantum-mechanically calculated electrostatic potential surfaces and LUMOs of benzene, as well as a σ complex (here the chlorobenzyl cation, for example). Benzen's electrostatic potential surface depicts a positive excess charge (blue) that is evenly distributed among the ring atoms, as well as a negative excess charge (red) that is located above the aromatic ring's center and is the result of the high electron density of the aromatic π electron system. The electrostatic potential surface of the chlorobenzyl cation (σ complex) corresponds well with the positive charge's distribution that has already been indicated by the Lewis resonance structures. That is, it has a positive excess charge (blue) in ortho and para positions. In contrast to benzene, the chlorobenzyl cation does not have a negative excess charge above the ring's center because it does not contain an aromatic π system. As expected, the LUMO's orbital lobes of benzene are evenly distributed among the ring, while the corresponding orbtial lobes of the σ complex are mainly located at the more strongly positively charged ortho and para positions.

Fig.3

From the left: a) LUMO of benzene.    b) Electrostatic potential surface of benzene (red = negative excess charge, blue = positive excess charge).    c) Electrostatic potential surface of the chlorobenzyl cation (σ complex; chlorine atom is on the top right).    d) LUMO of the chlorobenzyl cation.

If the three resonance structures of the σ complex are simply superposed, the positive charge in each ortho and para position would amount to +0.33. In an improved model, the meta position also carries a small positive charge.

Fig.4
Charge distribution in a σ complex.

The semiempirical-quantum-mechanically calculated atom's electric charges of several σ complexes show a similar result:

Fig.5

Atom's charges (after Mulliken) of several σ complexes and benzene. The values shown are the sum of the charges of the respective carbon atom and the hydrogen bound to it.

The calculated values of the positive charge distribution in σ complexes indicate that the ring's total charge rises accordingly with an increase in the substituent's electronegativity. This is anything but surprising, as a substituent with a higher electronegativity ought to be able to withdraw more electron density from the ring, while the fundamental charge distribution in the ring remains unaltered.

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