Resonance and delocalization

A wide array of organic molecules exhibit delocalized bonding.  Benzene is the most famous example, but modern understanding has revealed that electron delocalization is a very important feature of many systems.

Molecular orbital theory offers some obvious advantages for explaining delocalized bonding.  However, if we properly understand the background of VB theory, we can use the simple method of using bond-line structures to convey concepts that otherwise require numerical analysis of MO calculations.

Consider, for example, the allyl cation.  Ignoring the sigma framework for the moment, we find that three p orbitals can interact to form 3 molecular orbitals, shown here in descending order of their energy:

Load the frontier molecular orbitals to visualize the π bonding:
Show the antibonding MO (empty)
Show the nonbonding MO (empty in the cation)
Show the bonding π MO (filled--occupied by 2 electrons)

(There are, of course, additional MOs arising from the sigma framework.)

The lowest energy orbital will have 2 electrons, and the other two are empty "virtual" orbitals.  We can extract two important features from this picture:

  1. The 2 electrons are delocalized across the three carbon atoms.
  2. The lowest energy empty orbital, which reflects how a nucleophile will react with the molecule, is centered on atoms 1 and 3; there is no densty at all on carbon 2.  We therefore expect that any nucleophile will react equally at the two terminal carbons. A similar way of saying this is that the positive charge is half on each terminal carbon.

The same conclusions can be drawn from the proper use of resonance forms:

res_1.gif (1433 bytes)

The proper interpretation of this diagram is that:

  1. The multiple bond is equally shared between C1 and C2  and between C2 and C3.
  2. The positive charge is equally distributed at C1 and C3.  Therefore, nucleophiles will attack both positions.

The picture we draw is effectively identical to that arising from MO theory.  Use of the resonance forms provides a concise way to convey the same thing as the MO drawings, but is quicker to use and easier (in most cases) to interpret.  There are situations where use of resonance structures is inadequate; in these cases a more quantitative picture may be developed using molecular orbital computations.

Rules for Writing and Interpreting Resonance Structures

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Last updated:  09/21/2000