Organic Chemistry

Section 21.9 Aromatic Five-Membered-Ring Heterocycles 901

delocalization. (Recall that the more stable the base, the stronger is its conjugate acid;
Section 1.17).

PROBLEM 14

When pyrrole is added to a dilute solution of in 2-deuteriopyrrole is formed.

Propose a mechanism to account for the formation of this compound.

PROBLEM 15

Use resonance contributors to explain why pyrrole is protonated on C-2 rather than on

nitrogen.

PROBLEM 16

Explain why pyrrole is less acidic than cyclopentadiene even

though nitrogen is considerably more electronegative than carbon.

Indole, Benzofuran, and Benzothiophene
Indole, benzofuran, and benzothiophene contain a five-membered aromatic ring fused
to a benzene ring. The rings are numbered in a way that gives the heteroatom the low-
est possible number. Indole, benzofuran, and benzothiophene are aromatic because
they are cyclic and planar, every carbon in the ring has a porbital, and the cloud of
each compound contains fivepairs of electrons (Section 15.1). Notice that the elec-
trons shown as a lone pair on the indole nitrogen are part of the cloud; therefore, the
conjugate acid of indole, like the conjugate acid of pyrrole, is a strong acid
In other words, indole is an extremely weak base.

N

H

(^1) OS
2
(^43)
5
6
7
indole benzofuran benzothiophene
1 pKa=-2.4 2.
p
p
p
1 pKa' 172 1 pKa= 152 ,
D 2 SO 4 D 2 O,
N
H
N
H

• H+
  N
  −


• H+
  N
  −
  pKa = ~17 pKa = ~36
  N
  H
  N
  H NH
  N
  H NH NH
  N
  H
  N
  H


• 
  
  
  
  • HN
    N
  
  
  
  


• N
  N


• 
  
  
  
  • H H
    N+
  
  
  
  


• 
  

  H
  H
  N
  HHHH

  
  


• 
  

  NNH

  
  


• 
  

  HN NH
  pKa = −2.4 pKa = 1.0 pKa = 2.5 pKa = 4.85 pKa = 5.16
  pKa = 11.1 pKa = ∼ 17
  pKa = −3.8
  pKa = 6.8 pKa = 8.0 pKa = 14.4 pKa = ∼ 36
  TABLE 21.1 The pKa Values of Several Nitrogen Heterocycles
  3-D Molecules:
  Indole;
  Benzofuran;
  Benzothiophene
  Tutorial:
  Basic sites of nitrogen
  heterocycles