Organic Chemistry

If a substituent is attached to the benzene ring by an atom that is doubly or triply
bonded to a more electronegative atom, the electrons of the ring can be delocalized
onto the substituent; these substituents are said to withdraw electrons by resonance.
Substituents such as and withdraw electrons by resonance. These
substituents also withdraw electrons inductively because the atom attached to the
benzene ring has a full or partial positive change and, therefore, is more electronega-
tive than a hydrogen.

PROBLEM 7

For each of the following substituents, indicate whether it donates electrons inductively,

withdraws electrons inductively, donates electrons by resonance, or withdraws electrons

by resonance (inductive effects should be compared with a hydrogen; remember that many

substituents can be characterized in more than one way):

a. Br c. e.

b. d. f.

Relative Reactivity of Substituted Benzenes
The substituents shown in Table 16.1 are listed according to how they affect the reac-
tivity of the benzene ring toward electrophilic aromatic substitution compared with
benzene—in which the substituent is a hydrogen. Theactivating substituentsmake
the benzene ring more reactive toward electrophilic substitution; thedeactivating
substituentsmake the benzene ring less reactive toward electrophilic substitution.
Remember that activating substituents donate electrons into the ring and deactivating
substituents withdraw electrons from the ring.
All the strongly activating substituentsdonate electrons into the ring by resonance
and withdraw electrons from the ring inductively. The fact that they have been found
experimentally to be strong activators indicates that electron donation into the ring by
resonance is more significant than inductive electron withdrawal from the ring.

strongly activating substituents

NH 2 OH OR

N

+

CH 2 CH 3 NHCH 3 (CH 3 ) 3

OCH 3

O

CCH 3

N

O

withdrawal of electrons from a benzene ring by resonance

+

+

+

O−

N

OO−

N

−OO−

N

+

−OO−

+ +

N

−OO−

+

+

C“O,C‚N, NO 2

p

OCH 3 +OCH 3 +OCH 3 +OCH 3 OCH 3

donation of electrons into a benzene ring by resonance

−

−

−

Section 16.3 The Effect of Substituents on Reactivity 631

Tutorial:

Donation of electrons

into a benzene ring

Tutorial:

Withdrawal of electrons

from a benzene ring

anisole

nitrobenzene