Resonance Tutorial
7/31/09 9:29 PM
Chemistry Tutorials
Resonance in Motion
What is resonance?
Resonance can be considered as the condition ocurring when more than one
valid Lewis structure can be written for a particular molecule.
This can happen any time there are two or more adjacent p-tpye orbitals in
the same plane.
Important!
Resonance structures are imaginary. They represent extremes of electron
location. Resonance can be considered as a valence theory solution to a
molecular orbital problem.
In general chemistry the concept of resonance was introduced through
inorganic anions such as NO3- , NO2- , ClO4- , SO 4-2, CO3-2.
Note the three Lewis structures below for the nitrate anion (ignore the
curved arrows in structures 1 and 2 for the moment). The position of the
colored atoms is the same in each structure. The only difference is the
location of the electrons. Structure 1 has a double bond to the red oxygen,
structure 2 to the blue oxygen, and structure 3 to the green oxygen.
Structures 1-3 are not identical, but they are equivalent.
Rule #1: Resonance structures must have the same number of electrons.
(you should prove it to yourself by counting them).
Rule #2: All resonance structures must be valid Lewis dot structures.
Rule #3: The hybridization does not change between resonance
structures.
Rule #4: The positions of atoms are the same in all structures.
http://www.nku.edu/~russellk/tutorial/reson/resonance.html
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Rule #5: Resonance structures do not have to be equivalent.
Important!
The actual structure of a molecule is a weighted average of all
resonance contributors. The result is that the electrons are
distributed over the entire molecule, and not isolated, as
suggested by individual resonance structures. For the
Resonance in Motion examples below you can think of is as if
the strucures were changing infinitely fast, so that electrons
and charges were everywhere at once.
See the resonance in motion
How does one show the interconversion between resonance structures?
The arrows in the figure tell the story. Each double headed arrow
represents the movement of two electrons. To convert between structure 1
and structure 2, one of the lone pairs of electrons on the blue oxygen moves
to form a new double bond. It would not be possible to stop at this point
because it would result in an invalid Lewis dot structure, nitrogen would
have 10 electrons around it. To maintain a valid Lewis dot structure one
pair of electrons from the double bond between nitrogen and the red oxygen
moves to become a lone pair on the red oxygen. The same process can be
repeated to get from structure 2 to structure 3. Except in this case the
electrons start moving from from the green oxygen and end at the blue
oxygen.
Problems
1. Draw the arrows necessary to go from structure 3 to structure 1.
2. Draw the arrows necessary to go from structure 2 back to structure 1.
3. Instead of starting to move electrons from the blue oxygen in structure 1,
could you start drawing new resonance structures by moving a lone pair on
the green oxygen first? The red oxygen?
http://www.nku.edu/~russellk/tutorial/reson/resonance.html
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Other examples of resonance in motion:
Inorganic
CO 3 -2
SO 3 -2
SO 4 -2
O3
Organic
1. Aromatic
Benzene
2. Cationic
Napthalene
Cyclopropenyl cation Tropylium ion
3. Anionic
Acetate ion
Cyclopentadienyl anion
4. Neutral Dioplar
http://www.nku.edu/~russellk/tutorial/reson/resonance.html
An enolate
Acetoacetate anion
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Diazomethane
An ylide
5. Donor - Acceptor Pairs
Nitromethane
4-(N,N-dimethylamino)pyridine
N,N-dimethyl 4-nitroaniline
DMAP
Resonance in motion was designed and created by KC Russell and Miodrag Micic. If you have any
questions or would like to make comments or suggestions, please send e-mail to
[email protected] May 18, 2000
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