Organic Chemistry

The moderately deactivating substituentsall have carbonyl groups directly attached to

the benzene ring. Carbonyl groups withdraw electrons both inductively and by resonance.

The strongly deactivating substituentsare powerful electron withdrawers. Except

for the ammonium ions ( and ), these substituents with-

draw electrons both inductively and by resonance. The ammonium ions have no reso-

nance effect, but the positive charge on the nitrogen atom causes them to strongly

withdraw electrons inductively.

Take a minute to compare the electrostatic potential maps for anisole, benzene, and

nitrobenzene. Notice that an electron-donating substituent makes the ring

more red (more negative), whereas an electron-withdrawing substituent makes

the ring less red (less negative).

PROBLEM 8

List the members of the following sets of compounds in order of decreasing reactivity to-

ward electrophilic aromatic substitution:

a. benzene, phenol, toluene, nitrobenzene, bromobenzene

b. dichloromethylbenzene, difluoromethylbenzene, toluene, chloromethylbenzene

PROBLEM 9 SOLVED

Explain why the halo-substituted benzenes have the relative reactivities shown in Table 16.1.

SOLUTION Table 16.1 shows that fluorine is the least deactivating of the halogen sub-

stituents and iodine is the most deactivating. We know that fluorine is the most electroneg-

ative of the halogens, which means that it is best at withdrawing electrons inductively.

Fluorine is also best at donating electrons by resonance because its 2porbital—compared

with the 3porbital of chlorine, the 4porbital of bromine, or the 5porbital of iodine—can

better overlap with the 2porbital of carbon. So the fluorine substituent is best both at do-

nating electrons by resonance and at withdrawing electrons inductively. Because the table

shows that fluorine is the weakest deactivator of the halogens, we can conclude that elec-

tron donation by resonance is the more important factor in determining the relative reactiv-

ities of halo-substituted benzenes.

(NO 2 )

(OCH 3 )

O −O O

OOH NS CN NR 3

+ +

strongly deactivating substituents

+NR

3

+NHR

2 ,

+NH

2 R,

+NH

3 ,

moderately deactivating substituents

C

H

O

C

R

O

C

OR

O

C

OH

O

634 CHAPTER 16 Reactions of Substituted Benzenes

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