Organic Chemistry

benzene

pyridine

Section 21.10 Aromatic Six-Membered-Ring Heterocycles 903

SOLUTION An amide will notbe formed because the positively charged nitrogen caus-

es pyridine to be an excellent leaving group. Therefore, the final product of the reaction

will be a carboxylic acid. (If the final pH of the solution is greater than the of the car-

boxylic acid, the carboxylic acid will be predominantly in its basic form.)

Pyridine is aromatic. Like benzene, it has two uncharged resonance contributors.
Because of the electron-withdrawing nitrogen, it also has three charged resonance
contributors that benzene does not have.

The dipole moment of pyridine is 1.57 D. As the resonance contributors and the elec-
trostatic potential map indicate, the electron-withdrawing nitrogen is the negative end
of the dipole.

Because it is aromatic, pyridine (like benzene) undergoes electrophilic aromatic
substitution reactions ( is any base in the solution).

Electrophilic aromatic substitution of pyridine takes place at C-3 because the most sta-
ble intermediate is obtained by placing an electrophilic substituent at that position
(Figure 21.2). When the substituent is placed at C-2 or C-4, one of the resulting reso-
nance contributors is particularly unstable because its nitrogen atom has an incomplete
octet anda positive charge. The electron-withdrawing nitrogen atom makes the inter-
mediate obtained from electrophilic aromatic substitution of pyridine less stable than
the carbocation intermediate obtained from electrophilic aromatic substitution of ben-
zene. Pyridine, therefore, is less reactive than benzene. Indeed, it is even less reactive
than nitrobenzene. (Recall from Section 16.3 that an electron-withdrawing nitro group
strongly deactivates a benzene ring toward electrophilic aromatic substitution.)

NO 2

N

relative reactivity toward electrophilic aromatic substitution

NO 2

NO 2

>>>

NNN

slow fast

mechanism for electrophilic aromatic substitution

Y

H

B

+

+ Y+ + +BH

Y

≠B

N

= 1.57 D

N N N N N

−−−

+

+

+

resonance contributors of pyridine

H 2 O

N

RCCl

O

RC N

O

RCO−

O

+

N

+

+

pKa

Pyridine undergoes electrophilic

aromatic substitution at C-3.