Cyclic and heterocyclic aromatic compounds

Outline

1. Polycyclic aromatic hydrocarbons2. Basicity and acidity of the nitrogen heterocycles 3. Chemistry of pyrrole, furan, and thiophene4. Chemistry of pyridine

Outline

1. Polycyclic aromatic hydrocarbons2. Basicity and acidity of the nitrogen heterocycles 3. Chemistry of pyrrole, furan, and thiophene4. Chemistry of pyridine

E +

CH+

H E

CH+

H E

CH+ H

E

preferred reaction path

only resonance structures without breaking aromaticity are considered

-p o s it io n-p o s it io n

NO2

95%

H N O 3

H 2 S O 4

Orientation of the electrophilic substitution on the example of naphthalene

naphthalene anthracene phenanthrene

pyrene

When the reaction system is close to the equilibrium, substitution is directed to the more stable -position SO3H

SO3H

H 2 S O 4

4 0 oC

1 6 0 oC

85%

85%

Br

75%

B r 2 , C C l4 ( so lv e n t )

n o c a ta lys t

Naphthalene is more reactive, than benzene

Outline

1. Polycyclic aromatic hydrocarbons2. Basicity and acidity of the nitrogen heterocycles 3. Chemistry of pyrrole, furan, and thiophene4. Chemistry of pyridine

N N+

H

H+

Pyridine is a base due to the presence of a lone electron pair

pKa = 5.2

Imidazole is a stronger base due to stabilization of the conjugate acid

NN

HN

N+

H

HN

NCH

+

H

H

N+

N

H

H

H+

pKa = 6.95

Acidification of pyrrole breaks aromaticity of the system and produces a messy mixture

N

H

N+

H H

H+

mess

2. Basicity and acidity of the nitrogen heterocycles

N

H

N-

RMgBr

pKa = 23

Pyrrole is a stronger acid, than secondary amines, due to the aromatic stabilization of the conjugate base

N

H

pKa = 44

3. Chemistry of pyrrole, furan, and thiophene

>

= 1.7 D

All three heterocycles have an atom with at least one lone electron pair, involved to the aromatic conjugation. It is evidenced both by physical and chemical properties.

N

H

O S

O O

pyrrole furan thiophene

>

Reactivity in electrophilic substitution:

= 0.7 D

b.p. = 31.4 oC b.p. = 67 oC

>