Organic Chemistry

654 CHAPTER 16 Reactions of Substituted Benzenes

Electron-withdrawing substituents
increase the reactivity of the benzene
ring toward nucleophilic substitution
and decrease the reactivity of the
benzene ring toward electrophilic
substitution.

Notice also that the strongly electron-withdrawing substituents that activatethe

benzene ring toward nucleophilic aromatic substitutionreactions are the same sub-

stituents that deactivatethe ring toward electrophilic aromatic substitution. In other

words, making the ring less electron rich makes it easier for a nucleophile—but more

difficult for an electrophile—to approach the ring. Thus, any substituent that deacti-

vates the benzene ring toward electrophilic substitution activates it toward nucleo-

philic substitution and vice versa.

Nucleophilic aromatic substitution takes place by a two-step reaction known as an

reaction(substitution nucleophilic aromatic). In the first step, the nucleophile

attacks the carbon bearing the leaving group from a trajectory that is nearly perpendic-

ular to the aromatic ring. (Recall from Section 10.8 that leaving groups cannot be

displaced from carbon atoms by back-side attack.) Nucleophilic attack forms a

resonance-stabilized carbanion intermediate called a Meisenheimer complex, after

Jakob Meisenheimer (1876–1934). In the second step of the reaction, the leaving

group departs, reestablishing the aromaticity of the ring.

In a nucleophilic aromatic substitution reaction, the incoming nucleophile must be a

stronger base than the substituent that is being replaced, because the weaker of the two

bases will be the one eliminated from the intermediate.

The electron-withdrawing substituent must be ortho or para to the site of nucleo-

philic attack because the electrons of the attacking nucleophile can be delocalized onto

the substituent only if the substituent is in one of those positions.

A variety of substituents can be placed on a benzene ring by means of nucleophilic

aromatic substitution reactions. The only requirement is that the incoming group be a

stronger base than the group that is being replaced.

Cl

N+

+HO−

Cl

+

−O

O O−

O

N+

HO−

electrons are delocalized

onto the NO 2 group

sp^2

SNAr

3-D Molecule:

para-Fluoronitrobenzene

X

NO 2 NO 2

Y

N NO 2

X Y

+ Y− + X−

−−

−

− N NO^2

−O + O OO+ −

slow

general mechanism for nucleophilic aromatic substitution

fast

X Y X Y X Y

F

NO 2

∆

+ CH 3 O−

∆ HO−

++CH 3 CH 2 NH 2 H 2 O

OCH 3

NO 2

+ F−

Br

NO 2

NH 2 CH 2 CH 3 Br−

NO 2

NHCH 2 CH 3

NO 2

NO 2

+

p-fluoronitrobenzene

N-ethyl-2,4-dinitro-

aniline

p-nitroanisole

1-bromo-2,4-dinitrobenzene

NO 2 NO 2