Title: Cyclic and heterocyclic aromatic compounds Chapter 14
1Cyclic and heterocyclic aromatic compounds
Chapter 14
2 Outline 1. Polycyclic aromatic hydrocarbons 2.
Basicity and acidity of the nitrogen heterocycles
3. Chemistry of pyrrole, furan, and thiophene 4.
Chemistry of pyridine
3 Outline 1. Polycyclic aromatic hydrocarbons 2.
Basicity and acidity of the nitrogen heterocycles
3. Chemistry of pyrrole, furan, and thiophene 4.
Chemistry of pyridine
4Orientation of the electrophilic substitution on
the example of naphthalene
5When the reaction system is close to the
equilibrium, substitution is directed to the more
stable ?-position
Naphthalene is more reactive, than benzene
6 Outline 1. Polycyclic aromatic hydrocarbons 2.
Basicity and acidity of the nitrogen heterocycles
3. Chemistry of pyrrole, furan, and thiophene 4.
Chemistry of pyridine
72. Basicity and acidity of the nitrogen
heterocycles
83. Chemistry of pyrrole, furan, and thiophene
9Electrophilic substitution in pyrrole, furan and
thiophene
X O, NH, or S
10Examples
11Furan is able to act as a diene in the reactions
of cycloaddition
The Fisher synthesis of indoles
124. Chemistry of pyridine
Electrophilic substitution in pyridine
Pyridine is less active, than benzene toward
electrophilic agents, because nitrogen is more
electronegative, than carbon and acts like an
electron withdrawing substituent, including the
meta-directing effect. Example
13Nucleophilic substitution in pyridine
The presence of nitrogen enables pyridine to
react with nucleophilic agents, like an electron
withdrawing substituents enables benzene to
participate in such reactions, including the
ortho-directing effect.
14Another example
These reactions require very strong nucleophiles
and heat, because H- is a very weak leaving
group. In ortho- or para-substituted pyridines
nucleophilic substitution proceeds much easier.
15Tautomerism of 2-pyridones
Unlike phenol, 2-hydroxypyridine prefers the
carbonyl form (2-pyridone), because the
isomerisation does not break aromaticity.
However, 2-aminopyridine is the preferred isomer
due to the strong conjugation between electron
donating amino-group and electron withdrawing
pyridine ring and weaker conjugation
in 2-pyridoneimine
16Pyridinium salts