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Aromatic electrophilic substitution: mechanism and examples

The electrophile, many times, is generated during the same molecular mechanism, product of the mixture of reagents and a catalyst, which consists of a Lewis acid, for example AlCl 3 or FeCl 3 . These catalysts increase the avidity of the electrophile for the electrons of the aromatic ring, thereby accelerating the reaction.

In electrophilic aromatic substitution, it is the benzene ring that attacks the electrophile. Source: Gabriel Bolívar via MolView.

In the image above we have a simple representation of the electrophilic attack of benzene towards the E + electrophile . Note that the attack comes from the electrons of one of its double bonds; that is, it is the electrons of the conjugated π system that initiate the SEAr.

This reaction makes it possible for benzene and other aromatic compounds to acquire substituents such as OH, NO 2 , SO 3 H, Cl, Br, R, COR, COCH 3 groups , among others. For example, phenol is synthesized via SEAr starting from benzene and other derivatives.

Aromatic electrophilic substitution mechanism

Step 1: Electrophilic attack

General mechanism for SEAr. Source: SoonLorpai via Wikipedia.

The upper image depicts the mechanism of aromatic electrophilic substitution in more detail. Any of the three double bonds of benzene attack the electrophile E + , to form an intermediate species known as arenium ion (delocalized cyclohexadienyl), enclosed in red brackets.

Note that the positive charge on E + now moves into the ring. But not only that: it delocalizes between three carbon atoms in positions ortho (contiguous) and para (opposite) to the carbon linked to E (CE). This intermediary exists precisely thanks to the stability conferred by its resonance structures.

Step 2: Loss of the proton or hydrogen ion

However, the arenium ion must soon neutralize its positive charge by losing a proton or hydrogen ion. This is where the substitution culminates. The electrophile E + therefore replaces one of the hydrogens of benzene, leaving this as an H + ion outside the ring, so that the positive charges are conserved.

If you look closely, all the steps are in equilibrium, so the substitution is reversible. That is, if the concentrations of H + are increased , then a hydrogen will replace E and we will obtain the reactants again.

Examples of Aromatic Electrophilic Substitution

Benzene

The SEAr of benzene is the simplest of all, already represented above. Any of the hydrogens can be substituted for E + , since they are all chemically equivalent.

Phenol

Ortho, para and goal attacks

Resonance structures for phenol in its electrophilic aromatic substitution reaction. Source: Pete Davis, Public domain, via Wikimedia Commons

Now consider the SEAr for phenol. This time, the electrophile is the nitronium cation, NO + , which, when linked to the benzene ring, becomes the nitro group, -NO 2 .

Now that an OH group is present, the other hydrogens are no longer chemically equivalent; some are more susceptible to being substituted than others. And furthermore, the OH exerts a direct influence at this point.

Above we have three substitutions in different positions relative to the OH: ortho, para and meta attacks. Note that in all three we have the arenium cation and its resonance structures. In ortho and para attacks, the positive charge inside the benzene ring is located right on the carbon linked to the OH; whereas in the meta attack, this does not occur.

OH as ortho group and for director

OH has the ability to give up electrons to the ring via resonance and induction. In both, it can help to “dissipate” the positive charge on the carbon to which it is bound, thereby stabilizing the structure. On the contrary, when the meta attack occurs, the OH cannot stabilize the positive charge in the same way, the structure being more unstable.

Therefore, the ortho and para attacks are more energetically favored. OH is then said to be an ortho-para directing group, which can also activate the benzene ring towards SEAr. That is, phenol reacts much faster than benzene, which is demonstrated by measuring reaction rates.

Arylamines

Resonance structures for aniline in its electrophilic aromatic substitution reaction. Source: V8rik at English Wikipedia, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

The case of arylamines, like that of aniline (upper image), is similar to that of phenol. Note that in its mechanism this time the stabilization of the positive charge on the nitrogen atom (H 2 N + =) is shown, helping the substitution to be oriented towards the ortho and para positions, as happens with OH.

However, aniline is more reactive than phenol against electrophilic substitution. Why? Because the nitrogen atom is less electronegative than oxygen, which is why it gives up its pair of free electrons more easily to the aromatic ring. Oxygen, being more electronegative, gives up one of its pairs of electrons with less “greed”.

Also, the atomic radius of nitrogen is closer in size to that of carbon. This has an impact on the fact that resonance takes place especially between atoms with similar radii or sizes. Therefore, the resonance between carbon and nitrogen is slightly more stable and efficient than that between carbon and oxygen.

Chlorobenzene

In chlorobenzene, on the other hand, the Cl atom slows down substitution due to its electronegativity. And although it is also capable of donating electrons by resonance towards the ring, its atomic radius is considerably larger than that of carbon, thus reducing this electronic contribution.

In response, chlorobenzene reacts 50 times slower than benzene, as its ring is deactivated by chlorine. And furthermore, by attracting electrons to itself, chlorine is a directing target group, so substitutions will predominantly occur at that position.

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