18.4
Sustitución aromática electrofílica: descripción general
En una reacción de sustitución aromática electrófila, un electrófilo sustituye el hidrógeno de un compuesto aromático.
Mediante estas reacciones se pu…
Las sustituciones aromáticas electrofílicas son reacciones en las que un electrófilo sustituye a uno de los hidrógenos aromáticos.
Estas reacciones permiten la introducción de diferentes grupos funcionales en los anillos aromáticos.
En el primer paso del mecanismo de reacción, el sistema de π del anillo aromático ataca al electrófilo para formar un ion arenio, que se estabiliza por resonancia.
El ion arenio también se denomina complejo sigma porque el electrófilo forma un enlace sigma con el anillo aromático.
En el segundo paso, el ion arenio se desprotona, restaurando la aromaticidad y dando el producto sustituido.
Como se desprende del diagrama de energía libre, el primer paso es endergónico porque el anillo pierde su estabilidad aromática. Este paso tiene una mayor energía libre de activación y es lento. Es, por lo tanto, el paso determinante de la tasa.
Por el contrario, el segundo paso es exergónico porque restaura la estabilidad aromática del sistema. Tiene menor energía libre de activación y es rápido.
En general, las sustituciones aromáticas electrofílicas son reacciones exergónicas.
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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.