In simple terms, electrophilic aromatic substitution proceeds in two steps. Initially, the electrophile E' adds to a carbon atom of the benzene ring in the same manner in which it would react with an alkene, but here the
n-electron cloud is disrupted in the process. However, in the second step the resultant carbocation eliminates a proton to regenerate the aromatic system (Scheme 2.1). The combined processes of addition and elimination result in overall substitution.
In the second step, a proton is abstracted by a basic species present in the reaction mixture. The attacked carbon atom reverts to sp2 hybridization and planarity and aromaticity are restored. This fast step is energetically favourable and is regarded as the driving force for the overall process. The product is a substituted benzene derivative. The energy changes that occur during the course of the reaction are related to the structural changes in the reaction profile shown in Figure 2.1. It should be noted that each step proceeds through a high-energy transition state in which partial bonds attach the electrophile and the proton to the ring and the n-cloud is incomplete.
Most examples of electrophilic aromatic substitution proceed by this sequence of events:
- Generation of an electrophile
- The electrophile attacks the n-cloud of electrons of the aromatic ring
- The resulting carbocation is stabilized by resonance
- A proton is abstracted from the carbocation, regenerating the
- A substituted aromatic compound is formed phi-cloud
In the following sections, various examples are reviewed, highlighting the source of the electrophile and any variations in mechanistic detail.
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