Organic Chemistry

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630 CHAPTER 16 Reactions of Substituted Benzenes


Electron-donating substituents
increase the reactivity of the benzene
ring toward electrophilic aromatic
substitution.


Electron-withdrawing substituents
decrease the reactivity of the benzene
ring toward electrophilic aromatic
substitution.


depends on the substituent. Some substituents make the ring more reactive and some
make it less reactive than benzene toward electrophilic aromatic substitution.
The slow step of an electrophilic aromatic substitution reaction is the addition of an
electrophile to the nucleophilic aromatic ring to form a carbocation intermediate
(Section 15.9). Substituents that are capable of donating electrons into the benzene
ring will stabilize both the carbocation intermediate and the transition state leading to
its formation (Section 4.3),thereby increasing the rate of electrophilic aromatic
substitution. In contrast, substituents that withdraw electrons from the benzene ring
will destabilize the carbocation intermediate and the transition state leading to its
formation, thereby decreasing the rate of electrophilic aromatic substitution(Sec-
tion 16.4).Before we see how the carbocation intermediate is stabilized by electron
donation and destabilized by electron withdrawal, we will look at the ways in which a
substituent can donate or withdraw electrons.

There are two ways substituents can donate electrons into a benzene ring:inductive
electron donation and electron donation by resonance. There are also two ways sub-
stituents can withdraw electrons from a benzene ring:inductiveelectron withdrawal
and electron withdrawal by resonance.

Inductive Electron Donation and Withdrawal
If a substituent that is bonded to a benzene ring is less electron withdrawing than a
hydrogen, the electrons in the bond that attaches the substituent to the benzene ring
will move toward the ring more readily than will those in the bond that attaches the
hydrogen to the ring. Such a substituent donates electrons inductively compared with
a hydrogen. Donation of electrons through a bond is called inductive electron
donation(Section 1.18). Alkyl substituents (such as ) donate electrons inductive-
ly compared with a hydrogen.
Notice that the electron-donating ability of an alkyl group—not the electron-
donating ability of a carbon atom—is compared with that of hydrogen. Carbon is
actually slightly less electron donating than hydrogen (because C is more electronega-
tive than H; see Table 1.3), but an alkyl group is more electron donating than hydrogen
because of hyperconjugation (Section 4.2).

If a substituent is more electron withdrawing than a hydrogen, it will withdraw the
electrons away from the benzene ring more strongly than will a hydrogen. With-
drawal of electrons through a bond is called inductive electron withdrawal.The
group is a substituent that withdraws electrons inductively because it is more
electronegative than a hydrogen.

Resonance Electron Donation and Withdrawal
If a substituent has a lone pair on the atom that is directly attached to the benzene ring,
the lone pair can be delocalized into the ring; these substituents are said to donate
electrons by resonance. Substituents such as OH, OR, and Cl donate electrons
by resonance. These substituents also withdraw electrons inductively because the
atom attached to the benzene ring is more electronegative than a hydrogen.

NH 2 ,

+NH
3

s

s

CH 3 NH 3

+
substituent donates
electrons inductively
(compared with a
hydrogen)

substituent withdraws
electrons inductively
(compared with a
hydrogen)

CH 3

s

s

s

Z donates electrons
into the benzene ring

Y withdraws electrons
>>from the benzene ring

ZY

relative rates of electrophilic substitution
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