Organic Chemistry

benzene

pyridine

pyrrole

598 CHAPTER 15 Aromaticity • Reactions of Benzene

PROBLEM 4

The [10]- and [12]-annulenes have been synthesized, and neither has been found to be aro-

matic. Explain.

15.3 Aromatic Heterocyclic Compounds

A compound does not have to be a hydrocarbon to be aromatic. Many heterocyclic

compoundsare aromatic. A heterocyclic compoundis a cyclic compound in which

one or more of the ring atoms is an atom other than carbon. A ring atom that is not car-

bon is called a heteroatom. The name comes from the Greek word heteros, which

means “different.”The most common heteroatoms found in heterocyclic compounds

are N, O, and S.

Pyridine is an aromatic heterocyclic compound. Each of the six ring atoms of pyridine

is hybridized, which means that each has a porbital; and the molecule contains three

pairs of electrons. Don’t be confused by the lone-pair electrons on the nitrogen; they

are not electrons. Because nitrogen is hybridized, it has three orbitals and a p

orbital. The porbital is used to form the bond. Two of nitrogen’s orbitals overlap

the orbitals of adjacent carbon atoms, and nitrogen’s third orbital contains the

lone pair.

It is not immediately apparent that the electrons represented as lone-pair electrons

on the nitrogen atom of pyrrole are electrons. The resonance contributors, however,

show that the nitrogen atom is hybridized and uses its three orbitals to bond to

two carbons and one hydrogen. The lone-pair electrons are in a porbital that overlaps

the porbitals on adjacent carbons, forming a bond—thus, they are electrons. Pyr-

role, therefore, has three pairs of electrons and is aromatic.

+

−−

−−

N

H

N

H

+

N

H

+

N

H

+

N

H

resonance contributors of pyrrole

p

p p

sp^2 sp^2

p

this is a

p orbital

N

orbital structure of pyridine

these

electrons are

in an sp^2 orbital

perpendicular

to the p orbitals

sp^2 sp^2

p sp^2

p sp^2 sp^2

p

sp^2

N

heterocyclic compounds

pyrrole

H

pyridine

N

furan

O

thiophene

S