3

Resonance energy is responsible for the properties of aromatic compounds.
undergo substitution rather than addition:

These

1.3.

Orbital Picture of Benzene

sp2hybrids i.e. benzene is flat (and symmetrical)

1.3.1.Frost-Musulin Diagrams

A method for determining molecular orbitals (MOs)

1.4.

Aromatic Character

•
•

Highly unsaturated but undergo substitution rather than addition
Unusual stability:Low heats of hydrogenation
Low heats of combustion
Cyclic andflat / all C-C bonds of equal length
Protons show diagnostic chemical shifts in their 1H-NMR spectra. Around 7 ppm
rather than 4-5 ppm for vinylic protons, due to diamagnetic ring currents.

•
•

1.5.

Hückel Rule

How do we predict aromaticity?

“planar monocyclic completely conjugated hydrocarbons
aromatic when the ring contains (4n+2) π electrons.”

will

be

also

planar monocyclic completely conjugated hydrocarbons will be
antiaromatic (and therefore unstable) when the ring contains 4n π
electrons.

H
E

E

H
E

NuH

H+

Nu

Br

Br

Br

Br2

Br2

+HBr

H

H

H

H

H

H

H

H

H