Introduction to Benzene's Chemistry

Introduction to Benzene's Chemistry: Aromatic Substitution

Though benzene contains three double bonds, its chemical behavior is completely different from that of alkenes. Aromatic compounds show chemical properties that are only characteristic of aromatic compounds instead. The aromatic π system survives in most reactions of aromatic compounds, as the destruction of the aromatic system requires an ample amount of energy. Frequently, the aromatic system is first broken and then restored during a reaction. The electrophilic aromatic substitution is such a reaction.

Electrophilic aromatic substitution

Contrary to alkenes, reagents, such as bromine and HBr, are usually not completely added to aromatic compounds. Though, the electrophilic part of the reagent is added, the nucleophilic counterpart is not. Thus, the first step is similar to that of the addition to an alkene. However, in the first step the aromatic system is broken, which requires a great amount of energy. Therefore, the second step is not the addition of the nucleophilic part of the reagent to the intermediate cation, but rather the abstraction of a proton from the cationic species, as the aromatic system is then recovered. As a result, a hydrogen atom of the aromatic compound has been substituted by an electrophilic reagent. This reaction type is therefore known as the electrophilic aromatic substitution.

Fig.1
Conversion of benzene with $DBr$.

In the reaction of benzene with DBr, an exchange of a hydrogen (H) for a deuterium (D) takes place, while no 1,2- or 1,4-addition is obtained. In other words, a hydrogen atom of the aromatic ring is substituted by deuterium. However, the back reaction can happen, as well. Nevertheless, if a great excess of DBr is applied, virtually all hydrogens will be exchanged for deuterium. One examples of an additions to the aromatic systems is the Birch reduction.

Fig.2
Second step in electrophilic aromatic substitution.
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