2

1.

Introduction to Aromatic Chemistry

1.1.

Historical Aspects

Benzene parent molecule from coal tar.

Ladenburg

Dewar

Kekulé

1.2.

Resonance Energy and Reactivity

49.8

85.8

57.2 (55.4 observed)

28.6

36 kcal/mol resonance energy i.e. more stable

All carbon-carbon bonds in benzene are the same and are intermediate in length
between single and double bonds.

C=C typically 1.34 Å

C-Ctypically 1.54 Å

If Kekulé we would expect to find three short bonds and three long bonds.
bonds are of equal length:1.39Å.

Actually all

1.2.1

Valence Bond Representation

Structure that differ only in the arrangements of electrons (but not atoms!)

mule ~ donkey/horse

•

structures differ only in the arrangement of electrons - the “real” structure is a
resonance hybrid of all these structures.
when contributing forms have similar energy contents resonance becomes
important.
resonance hybrid is more stable than any of the contributing structures.

•

•