Electrophilic Aromatic Substitution

In this section, we will look into nature, mechanism and the reactivity of electrophilic aromatic substitution reaction in which the electrophile is a halogen.

Benzene is an aromatic hydrocarbon with pie bonds similar to an alkene. However, in case of an alkene, halogen adds instantaneously across the double bond by the anti-addition method and the reaction is exothermic. The halogen addition across the double bond is not observed in the case of Benzene under standard reaction conditions.

If an analogous comparison is done between halogen addition across alkene and Benzene, it is observed that for Benzene, the reaction is thermodynamically unfavorable and endothermic due to the decrease in entropy and loss of the ring aromaticity.

why does benzene undergo substitution reactions

Benzene is an aromatic hydrocarbon but behaves differently from alkane hydrocarbons. It does not undergo free radical halogenation reactions like alkanes in the presence of a halogen source and UV light.

Take the example of Methane; it undergoes free-radical substitution reactions that give a mixture of mono and polyhalogenated products.

 

Benzene under similar reaction condition gives a polychlorinated hydrocarbon; Benzene hexachloride (BHC) also known as 1,2,3,4,5,6- hexachlorocyclohexane as the product by losing its aromaticity entirely. Such kind of polyhalogenation is observed only for highly reactive halogen Chlorine and not for Bromine.

If an alkyl group is attached to Benzene, then the alkyl group behaves like an alkane to give mono and polyhalogenated product but Benzene ring is unaffected in this case.

To summarize, we can say reactions that favor aromaticity of the Benzene ring is preferred over-reactions that destroy it. Also, halogen addition to the ring does not occur by free-radical substitution reactions or by electrophilic addition reactions. It occurs by electrophilic substitution reaction on treatment with halogen and a Lewis acid catalyst in which a proton is substituted with the halogen. This suggests that only an electrophile can attack the ring and atoms can only the attack the alkyl substituent attached to the phenyl ring.

As the electrophiles, in this case, are the halogens, and we know that the electronegativity and electrophilicity decrease from F to I in the periodic table. Fluorine is most electrophilic, and Iodine is least. Therefore, fluorination is highly reactive, and iodination is highly unreactive for electrophilic aromatic substitution reactions.

The exothermic rates of aromatic halogenation also decrease from Fluorine to Iodine. That means Fluorination is highly exothermic and can be explosive. Such reaction is hard to control at the mono fluorinated stage and usually gives polyfluorinated products.

For Iodination, the reaction is endothermic and cannot be done using conventional method using Lewis acid catalysts and requires other oxidizing agents.

The other method to synthesize 1- fluorobenzene is by diazotization method starting from Aniline. The diazonium salt formed is treated with Fluoroboric acid giving the desired product.

For Chloro substituted Benzene, the reaction is done using Chlorine gas and Lewis acid catalysts such as AlCl3 or FeCl3. Similarly, for a Bromo substituted product, Bromine and Lewis acid catalyst such as AlBr3 or FeBr3 is used.

Let us look at the general reaction mechanism and discuss the changes happening using an energy profile diagram for more understanding;

 

electrophilic aromatic substitution mechanism

In the first step, the pie electrons in the ring accept electrophile and forming a carbocation intermediate. This intermediate formed has sp3 carbon and is not aromatic.

As the bond formed with the electrophile is a sigma bond, the complex is called a sigma complex. As aromaticity of the ring is lost in this step, it is a slow step and, therefore, the rate-determining step of the reaction. It is also endothermic in nature with almost 36 kcal of energy consumed. The intermediate carbocation is then stabilized by resonance lowering the energy; with the positive charges delocalized at the ortho and the para positions.

In the next step, the base tries to abstract the proton resulting in an energy rise and finally, due to proton abstraction, the aromaticity is regained with significant loss in energy. In the end, a stable product with less energy than the starting material is formed.

Why is the substitution of a proton with a halogen favorable reaction? This is because, as compared to proton, halogen with its electron-donating resonance effect and electron-withdrawing inductive effect makes the product more stable than the starting material. The halogens, tries to stabilize the carbocation arenium ion at the transition state by its stronger –I effect than weaker +R effect. So, any group that stabilizes the carbocation will favor the transition state. Carbocation can be stabilized by making the ring more nucleophilic that is by increasing the electron density of the ring by introducing electron donating groups. For e.g. groups such as –CH3, -OCH3, -NH2, -NR2

A look at Bromination reaction mechanism will help us understand more. As the halogens are homonuclear diatomic molecules and electrophilicity decreases down the group, its reactivity towards Benzene ring is low. The reactivity of the halogens is increased by making it more positive using Lewis acid catalysts.

Lewis acid catalysts make it more electrophilic than diatomic Br2 but less than when it exists as bromonium ion. Lewis acid such as Fe or Al has an empty orbital that attracts the lone pair of electrons from one of the bromine in the bond making the terminal Bromine quite electrophilic. This terminal halogen is picked by the Benzene ring for the reaction, and the intermediate formed is stabilized by resonance. The central electrophilic Bromine cannot be picked as it is already trivalent and cannot expand its valency to four.

The Lewis acid complex FeBr4 then acts as a base by picking up the proton. In the end, the Lewis acid catalyst is regenerated for taking part in the reaction again along with Bromobenzene and HBr as the final products. The overall reaction is exothermic releasing 45kJ/mol of energy.

In short for faster reactions, groups that increase nucleophilicity and electrophilicity favor the reaction by stabilizing the transition state.

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