Repaso Ex #2

Repaso #2
Marimar de la Cruz Bonilla
Capitulo 16: Benceno y sus reacciones.
•
Sustitución aromática electrofilica (SAE)
Electrófilo: centros deficientes de electrones que buscan cargas negativas
Las 5 reacciones siguen el mismo mecanismo. La única diferencia es la naturaleza del
electrófilo y como se forma en el medio de reacción.
Mecanismo General
o Primer Paso:
o
o
•


Se llama carbocatión doble alilico porque en la segunda forma resonante el C+
es alilico con respecto a ambos enlaces dobles.
Segundo Paso (A): Ataque de Nucleofilo al
carbocation para dar adición.
 Seria la reacción que ocurriría a un
alqueno aislado pero no ocurre en SAE
ya que resultaría en adición.
 Además en este paso se pierde
aromaticidad y por lo tanto estabilidad.
Segundo Paso (B): base se lleve H+ para dar
sustitución
 El producto mantiene aromaticidad y la
estabilidad asociada a esta.

o
o
En el primer paso benceno pierde aromaticidad y estabilidad.
El ion arenio no es aromático pero es un hibrido de resonancia donde la carga
positiva esta des localizada.
•
Halogenacion:
o Cloracion:
 FeCl3: cloruro ferrico o cloruro de hierro (III)
 Electrofilo:
• Cl-Cl: no actúa como electrofilo a solas
porque no es polar.
• FeCl3 actúa como agente
catalítico. Su función es polarizar
el enlace en la molécula de Cl.
Al polarizarlo uno de los Cl se va
a comportar en una molécula
positiva la cual será el electrofilo.
o
Bromación:
o
Yodación:
 En la yodación CuCl2 o H2O2 actuan como reactivos, agentes oxidantes y
reaccionan para formar I+.
•
•
Nitración:
o Ion nitronio actúa como electrofilo.
 El acido sulfúrico dona un protón y el acido nítrico lo acepta.
o
SAE:
o
El grupo nitro es un precursor del grupo amino.
Sulfonación:
o Trióxido de azufre es un gas y al mezclarse con acido sulfúrico se ve que sale un humo.
o El electrofilo es el Ion Sulfonio.
o
SAE:
o
o
La sulfonación es reversible por lo que pude ocurrir como sulfonación o desulfonacion.
La sulfonacion y nitración se utilizan
en la industria farmacéutica y en la
industria de tintes.
Sulfanilamida: miembro de la familia de antibióticos de las “drogas
sulfa”.
Alquilación Friedel-Crafts:
o Reacción de un anillo aromático con un haloalcano catalizada por
un acido de Lewis para producir un areno o alquilbenceno.
o
•
o
En la alquilación friedel-crafts el electrofilo es el carbocation que
surge de la reacción del aloalcano y el AlCl3.
o
SAE:
o
Limitaciones de la Alquilacion Friedel- Crafts:
 La reacción no ocurre si un halógeno esta
enlazado a un carbono sp2 porque los
carbocationes con la carga positiva en un
carbono sp2 son muy inestables y no se
froman a las condiciones de alquilación FC.
 La reacción no ocurre si el anillo ya tiene
como sustituyente un grupo NH2 o un
grupo que retire densidad electrónica del
anillo y lo desactive porque en condiciones
Friedel-Crafts el N de anilina froma un
complejo con el AlCl3 (acido de lewis), lo cual hace que anilina que es un grupo
altamente activante a condiciones Friedel-Crafts se convierte en un gruop
altamente desactivante.

Con frecuencia ocurre polialquilación ya que la presencia de un grupo alquilo
como sustituyente activa el anillo a reacciones de SAE. Para disminuir la
polialquilación se utiliza un exceso de compuesto aromático.

Con frecuencia se observan rearreglos estructurales en el Electrofilo
principalmente si se utiliza un haloalcano primario.
Pueden ocurrir rearreglos de carbocationes debido a migración de
hidruro o migración de grupo alquilo con el proposito de que exista el
carbocation mas estable.
Acilacion Friedel-Crafts:
o
•
o
o
Reacción de un anillo aromático con un cloruro acido catalizada por un acido de
Lewis (AlCl3), el producto es una arilcetona.
El electrofilo es el ion acilo, el cual es un hibrido de resonancia.
•
o
SAE:
o
En la acilacion se eliminan dos de las limitaciones de la alquilación.
 No se observa poliacilacion ya que la precencia de un grupo acilo como
sustituyente en el anillo desactiva el anillo a reacciones SAE.
 No habrá rearreglos estructurales en el electrófilo ya que el electrófilo se
encuentra estabilizado por resonancia.
SAE cuando benceno tiene sustituyente:
o Grupos Activantes y o,p-orientadores:
Activantes: cuando la se compara la velocidad de la reacción de algún anillo
con cualquiera de los sustituyentes ilustrados y la velocidad de la misma
reacción con benceno se nota que la velocidad de la reacción del anillo
sustituido es mas rápida.
 o,p-orientadores: Los sustituyentes ilustrados arriba orientan el electrófilo para
que se una al anillo en posición orto o para con respecto a ellos.
Grupos desactivantes y m-orientadores:

o
o
¿Por qué unos grupos activan y otros desactivan?
 La presencia de un sustituyente puede alterar la densidad electrónica del anillo
aromático por efectos inductivos o de resonancia.
 Efectos Inductivos: desplazamiento de electrones sigma debido a diferencias en
electronegatividad o polaridad de enlaces en grupos funcionales.
o Si el elemento directamente enlazado al anillo o enlazado al carbono
enlazado al anillo es electronegativo hala densidad electrónica.
 Resonancia: es la deslocalización de electrones pi o no enlazantes
generalmente por la interacción entre orbitales p puros.
o Al haber carbocationes en las formas resonantes del anillo (en el anillo),
el anillo esta deficiente en electrones ya que el grupo funcional ha
halado electrones del anillo.
o El sustituyente aumenta la densidad electrónica en el anillo por
resonancia cuando se ven carbonos negativos en los anillos de las formas
resonantes.
o Los efectos de resonancia se ven marcados en las posiciones orto y para
por lo que los carbocationes y carbonos negativos que dibuje en las

Al z donar densidad electrónica el ion se estabiliza esto se refleja en una
estabilización de su estado de trancision por lo que disminuye la energía de
activación y acelera el paso lento.
 Postulado de Hammond: una de sus consecuencias es que en un proceso
endergonico si el producto se estabiliza el estado de trancision se estabiliza.
 Si z retira densidad electrónica el ion se destabiliza y el estado de trancision
tambien por lo que aumenta la energía de activación.
¿Por qué unos grupos son orto, para orientadores mientras que otros son meta
orientadores?
 Meta Orientadores:

o
fromas resonantes deben estar localizados en esas posiciones con
respecto al sustituyente.
Hay grupos que por inducción halan o donan electrones y por resonancia
hacen lo opuesto, para saber que efecto predomina se debe probar
experimentalmente.
o Anilina:
 Hala densidad electrónica por inducción
 Dona densidad electrónica por resonancia. (Efecto dominante).
Se va a preferir sustitución meta porque el intermediario es de mayor
estabilidad.
o Los meta-orientadores tendrán carga positiva completa o parcial en el
átomo enlazado al anillo lo cual provocara que cuando el ataque es
orto o para hay una forma resonante inestable y entonces se enlazara en
posición meta.
Orto Para Orientadores:
o

o
Un o,p-orientador debe tener un par libre en el átomo que enlaza al
anillo provocando que cuando se enlace en posición orto y para haya
una cuarta forma resonante en la cual existe una gran estabilidad ya que
todos los átomos han completado el octeto.
o


•
–CH3, no tiene par libre pero es orto para orientador porque habrá una
forma resonante en la que existirá un carbocation terciario cuando el
electrófilo ataque en posición o,p por lo que el intermediario es mas
estable.
Los halógenos son o,p-orientadores y desactivantes.
o Desactivantes:
 Inducción: halan densidad electrónica. (efecto predominante)
 Resonancia: donan densidad electrónica.
o Son orto para orientadores porque enlazan al anillo con un átomo con
par libre.
**Al mover electrones deslocalizados para determinar figuras resonantes la
carga positiva debe estar en posición orto o para con respecto al carbono sp3.
SAE en anillos disustituidos:
o Para SAE en anillos disustituidos lo que ocurre se puede resumir en 3 casos.
 Ambos sustituyentes orientan a las mismas posiciones.

Los sustituyentes orientan a posiciones distintas y predomina la orientación del
grupo más activante.

Si los dos sustituyentes están en posición meta generalmente no se observa
sustitución en el carbono entre ellos debido a impedimento esterico.
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