Organic Chemistry

Pyrrole is an extremely weak base because the electrons shown as a lone pair are

part of the cloud. Therefore, when pyrrole is protonated, its aromaticity is destroyed.

Consequently, the conjugate acid of pyrrole is a very strong acid that

is, it has a strong tendency to lose a proton.

The resonance contributors of pyrrole show that nitrogen donates the electrons de-

picted as a lone pair into the five-membered ring.

Pyrrolidine—a saturated five-membered-ring heterocyclic amine—has a dipole mo-

ment of 1.57 D because the nitrogen atom is electron withdrawing. Pyrrole—an unsat-

urated five-membered-ring heterocyclic amine—has a slightly larger dipole moment

(1.80 D), but as we see from the electrostatic potential maps, the two dipole moments

are in opposite directions. (The red areas are on opposite sides of the two molecules.)

Apparently, the ability of pyrrole’s nitrogen to donate electrons into the ring by reso-

nance more than makes up for its inductive electron withdrawal (Section 16.3).

In Section 7.6, we saw that the more stable and more nearly equivalent the res-

onance contributors, the greater is the resonance energy. The resonance energies of

pyrrole, furan, and thiophene are not as great as the resonance energies of benzene and

the cyclopentadienyl anion, compounds for which the resonance contributors are all

equivalent. Thiophene, with the least electronegative heteroatom, has the greatest

resonance energy of these five-membered heterocycles; and furan, with the most elec-

tronegative heteroatom, has the smallest resonance energy.

Because pyrrole, furan, and thiophene are aromatic, they undergo electrophilic aro-

matic substitution reactions.

slow fast

mechanism for electrophilic aromatic substitution

Y

H

+

+ Y+ + H+

NNN Y

H H H

relative resonance energies of some aromatic compounds

− SNH O

>>>>

H

N

H

N

= 1.57 D = 1.80 D

−+

N

HH H

N

H

N

H

N

N

H

+N + −

−

+

−

δ+

δ−

δ− δ−

δ−

resonance contributors of pyrrole

resonance hybrid

1 pKa=-3.8 2 ;

p

pyrrolidine pyrrole

898 CHAPTER 21 More About Amines • Heterocyclic Compounds

Pyrrole, furan, and thiophene undergo
electrophilic substitution preferentially
at C-2.