Organic Chemistry

638 CHAPTER 16 Reactions of Substituted Benzenes

All substituents that donate electrons
into the ring either inductively or by
resonance are ortho–para directors.

carbon are the most stable (electron-donating substituents) or the least stable (electron-

withdrawing substituents).

The only deactivating substituents that are ortho–para directors are the halogens,

which are the weakest of the deactivators. We have seen that they are deactivators

because they inductively withdraw electrons from the ring more strongly than they do-

nate electrons by resonance. The halogens nevertheless are ortho–para directors

because of their ability to donate electrons by resonance; they can stabilize the transi-

tion states leading to reaction at the ortho and para positions by resonance electron

donation (Figure 16.2).

In summary, as shown in Table 16.1, the halogens and all the activating substituents

are ortho–para directors. All substituents more deactivating than the halogens are meta

directors. Put another way,all substituents that donate electrons into the ring induc-

tively or by resonance are ortho–para directors, and all substituents that cannot

donate electrons into the ring inductively or by resonance are meta directors.

You don’t need to resort to memorization to determine whether a substituent is an

ortho–para director or a meta director. It is easy to tell them apart: All ortho–para

directors, except for alkyl, aryl, and groups, have at least one lone pair on

the atom directly attached to the ring; all meta directors have a positive charge or a

partial positive charge on the atom attached to the ring. Take a few minutes to examine

the substituents listed in Table 16.1 to convince yourself that this is true.

PROBLEM 10

a. Draw the resonance contributors for nitrobenzene.

b. Draw the resonance contributors for chlorobenzene.

PROBLEM 11

What product(s) would result from nitration of each of the following compounds?

a. propylbenzene d. benzenesulfonic acid

b. bromobenzene e. cyclohexylbenzene

c. benzaldehyde f. benzonitrile

PROBLEM 12

Are the following substituents ortho–para directors or meta directors?

a. c. e.

b. d. COOH f.

16.5 The Effect of Substituents on

We have seen that electron-withdrawing substituents increase the acidity of a com-

pound (Sections 1.18 and 7.10). Therefore, when a substituent either withdraws elec-

trons from or donates electrons into a benzene ring, the values of substituted

phenols, benzoic acids, and protonated anilines will reflect this withdrawal or dona-

tion. For example, the of phenol in at 25°C is 9.95. The of para-nitro-

phenol is lower (7.14) because the nitro substituent withdraws electrons from the ring,

whereas the of para-methyl phenol is higher (10.19) because the methyl sub-

stituent donates electrons into the ring.

pKa

pKa H 2 O pKa

pKa

pKa

NO 2 N“O

CH 2 “CHC‚N CH 2 OH CF 3

All substituents that cannot donate CH“CHR
electrons into the ring either inductively
or by resonance are meta directors.