18.4
Substituição Eletrofílica Aromática: Visão Geral
Em uma reação de substituição eletrofílica aromática, um eletrófilo substitui um hidrogênio de um composto aromático.
Muitos grupos funcionais podem s…
Substituições eletrofílicas aromáticas são reações nas quais um eletrófilo substitui um dos hidrogênios aromáticos.
Essas reações permitem a introdução de diferentes grupos funcionais em anéis aromáticos.
Na primeira etapa do mecanismo de reação, o sistema π do anel aromático ataca o eletrófilo para formar um íon arenium, que é estabilizado por ressonância.
O íon arenium também é chamado de complexo sigma porque o eletrófilo forma uma ligação sigma com o anel aromático.
Na segunda etapa, o íon arenium é desprotonado, restaurando a aromaticidade e dando o produto substituído.
Como fica evidente no diagrama de energia livre, o primeiro passo é endergônico porque o anel perde sua estabilidade aromática. Esta etapa tem uma energia livre de ativação mais alta e é lenta. É, portanto, a etapa de determinação da taxa.
Em contraste, a segunda etapa é exergônica porque restaura a estabilidade aromática do sistema. Tem menor energia livre de ativação e é rápido.
No geral, as substituições eletrofílicas aromáticas são reações exergônicas.
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Q1: What happens in the first step of an electrophilic aromatic substitution reaction?
In the first step, the π system of the aromatic ring attacks the electrophile, forming an arenium ion, also called a sigma complex. This intermediate is resonance-stabilized because the electrophile forms a sigma bond with the aromatic ring. This step is endergonic and rate-determining because the ring loses its aromatic stability.
Q2: Why is the first step of electrophilic aromatic substitution the rate-determining step?
The first step is rate-determining because it has a higher free energy of activation and is slow. During this step, the aromatic ring loses its aromatic stability, making the reaction endergonic. The second step, which restores aromaticity, is fast and exergonic with lower activation energy.
Q3: What is an arenium ion in aromatic substitution?
An arenium ion is the resonance-stabilized intermediate formed when the π system of an aromatic ring attacks an electrophile. It is also called a sigma complex because the electrophile forms a sigma bond with the aromatic ring. This intermediate is then deprotonated in the second step to restore aromaticity.
Q4: How does the second step of electrophilic aromatic substitution restore aromaticity?
In the second step, the arenium ion is deprotonated, which restores the aromatic character of the ring and yields the substituted product. This step is exergonic because it restores aromatic stability to the system. It has a lower free energy of activation and proceeds rapidly compared to the first step.
Q5: What functional groups can be introduced through electrophilic aromatic substitution?
Many functional groups can be added to aromatic compounds through electrophilic aromatic substitution reactions. These include halogens, nitro groups, sulfonic acid groups, and alkyl or acyl groups. The specific functional group introduced depends on the electrophile used in the reaction.
Q6: Is electrophilic aromatic substitution an overall exergonic or endergonic reaction?
Overall, electrophilic aromatic substitution reactions are exergonic. Although the first step is endergonic and rate-determining, the second step is exergonic and restores aromatic stability. The favorable energy release in the second step outweighs the energy cost of the first step, making the overall process thermodynamically favorable.
Q7: How does a free energy diagram illustrate the two-step mechanism of electrophilic aromatic substitution?
A free energy diagram shows that the first step has a higher activation energy and is endergonic, representing the rate-determining step where aromaticity is lost. The second step has lower activation energy and is exergonic, restoring aromaticity. The overall reaction is exergonic, with the final product at lower free energy than the starting material.