Organic Chemistry

Section 21.9 Aromatic Five-Membered-Ring Heterocycles 897

A primary amine is obtained from the reduction of a nitroalkane, and an arylamine is
obtained from the reduction of nitrobenzene.

PROBLEM 13

Excess ammonia must be used when a primary amine is synthesized by reductive amina-

tion. What product will be obtained if the reaction is carried out with an excess of the car-

bonyl compound instead?

21.9 Aromatic Five-Membered-Ring Heterocycles

Pyrrole, Furan, and Thiophene
Pyrrole,furan, and thiopheneare five-membered-ring heterocycles. Each has three
pairs of delocalized electrons: Two of the pairs are shown as bonds, and one pair
is shown as a lone pair on the heteroatom. Furan and thiophene have a second lone pair
that is not part of the cloud. These electrons are in an orbital perpendicular to the
porbitals. Pyrrole, furan, and thiophene are aromatic because they are cyclic and pla-
nar, every carbon in the ring has a porbital, and the cloud contains threepairs of
electrons (Sections 15.1 and 15.3).

N

H

OS

pyrrole furan thiophene

p p

p sp^2

p p

NO 2 + H 2 NH 2

Pd/C

nitrobenzene aniline

CH 3 CH 2 NO 2 + H 2 CH 3 CH 2 NH 2

Pd/C

nitroethane ethylamine

O

CH OCH+ CH 3 CH 2 NH 2 NCH 2 CH 3 CH 2 NHCH 2 CH 3

NaBH(OCCH 3 ) 3

catalytic

H+

catalytic

H+

O

NaBH(OCCH 3 ) 3

+ CH 3 NH

CH 3

CH 3

CH 3

ON

CH 3

CH 3

N

an imine

an enamine

N H O

orbital structure of pyrrole

these

electrons

are part of

the cloud

these

electrons

are part of

the cloud

orbital structure of furan

these

electrons are

in an sp^2 orbital

perpendicular

to the p orbitals

3-D Molecules:

Pyrrole;

Furan;

Thiophene