18.4
Substitution aromatique électrophile : aperçu
Dans une réaction de substitution aromatique électrophile, un électrophile remplace l'hydrogène d'un composé aromatique.
De nombreux groupes fonctionn…
Les substitutions aromatiques électrophiles sont des réactions dans lesquelles un électrophile remplace l’un des hydrogènes aromatiques.
Ces réactions permettent l’introduction de différents groupes fonctionnels sur les cycles aromatiques.
Dans la première étape du mécanisme réactionnel, le système π du cycle aromatique attaque l’électrophile pour former un ion arénium, qui est stabilisé par résonance.
L’ion arénium est également appelé complexe sigma car l’électrophile forme une liaison sigma avec le cycle aromatique.
Dans la deuxième étape, l’ion arénium est déprotoné, ce qui rétablit l’aromaticité et donne le produit substitué.
Comme le montre le diagramme de l’énergie libre, la première étape est endergonique car le cycle perd sa stabilité aromatique. Cette étape a une énergie libre d’activation plus élevée et est lente. Il s’agit donc de l’étape qui détermine le taux.
En revanche, la deuxième étape est exergonique car elle redonne de la stabilité aromatique au système. Il a une énergie libre d’activation plus faible et est rapide.
Dans l’ensemble, les substitutions aromatiques électrophiles sont des réactions exergoniques.
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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.