1. IB Chemistry Power Points
Topic 14 (AHL)
Bonding
www.pedagogics.ca
LESSON
Delocalization

2. Delocalized Electrons – classic example Benzene
Benzene has the formula C6H6 and consists of a six carbon ring
structure
σ bonds
There are 3 double
bonds shown here by
the pi orbital overlap.

3. Problem : there were two possibilities for location of
the C=C double bonds
Neither structure is correct.
Studies show all bonds are of equivalent length (but shorter
than a single bond and longer than a double bond).
A Lewis structure cannot show the true nature of this
molecule.

4. pi bond
sigma bond
Figure A: bonding in benzene with static single and
double bonds (not the true model)

5. Delocalization accounts for all the C-C bonds in
benzene being the same length and same
strength. The pi bond electrons “delocalize”
over the entire structure instead of above and
below alternating bonds. It is said that the
original structure of Benzene (alternating single
and double bonds) came to Freiderich Kekule in
a dream!

6. delocalized
region in pi
bonds
sigma bond
figure B: delocalized bonding in benzene (the true model)

7. Electron delocation explains resonance structures
–
• the bond lengths of CHO2 predicted by the Lewis
structure are incorrect (draw it)
[ ]
O –
H C
O
• The double CO bond should be shorter, and possess
a greater bond energy (due to the higher
concentration of electrons in a double bond)
• Yet, experimentally, both bonds are the same
• Answer . . . Delocalization: the pi bond spreads out
over both C-O bonds

8. Sometimes textbooks refer to “resonance” theory--the idea that
the double bond flips back and forth. Diagram 1 is the type
used to show the old resonance theory.
[ ]
–
[ ]
1 O O –
H C H C
O O
[ ]
O –
2
H C
O
Delocalization is more widely accepted. Diagram 2
shows how delocalized bonds are often represented.

9. Practice: Draw the delocalized structure of the carbonate anion