Resonance - Which contributes most?

Unveiling the Major Players: Resonance Structures and Their Contribution

Resonance structures, those seemingly interchangeable depictions in organic chemistry, hold a deeper truth. While they represent the same molecule, some structures contribute more significantly to its overall electronic structure than others. Let's delve into the factors that make a resonance structure a major contributor, and explore these concepts with some examples:

Stability is King

The most important contributors are the most stable resonance structures. This translates to structures with:

1. The one with the most atoms has a full octet.
2. The one with the fewest # of atoms with a formal charge
3. Negative charge on most electronegative elements (or positive charge on most electropositive atoms)
4. Like charges separated by max distance (or oppositely charged atoms closest together).

Example (Acetic Acid)

Let's consider acetic acid's resonance structures.  The major resonance contributor to acetic acid is structure A below since it has a full octet on all atoms and no formal charges.  This is how we would normally draw acetic acid.  The structure on the C is the second most important, since all atoms have a full octet but have a strike against it since it charges are separated more than structure B. 

most atoms with a full octet	✓	✗	✓
fewest formal charges	✓	✗	✗
Negative charge on most electronegative elements (or positive charge on most electropositive atoms)		✓	✗
Like charges separated by max distance (or oppositely charged atoms closest together).		✓	✗

•  

  • Minimized Formal Charge: Consider the nitrate ion (NO3-). Structure A (with one negative charge on an oxygen) is more stable than Structure B (with two negative charges on one oxygen) because spreading the charge makes the molecule more comfortable electronically.

    O-N=O   <-- Structure A (more stable, lower formal charge)
     vs.
    [O-]-N=O   <-- Structure B (less stable, higher formal charge)
    

  • Octet Rule Adherence: Look at the molecule ozone (O3). Structure A follows the octet rule for all atoms, while Structure B forces an oxygen atom to hold more than eight electrons. Structure A is the major contributor.

    O=O-O   <-- Structure A (more stable, octet rule followed)
    vs.
    O-O=O   <-- Structure B (less stable, octet rule violated)
    

  • Covalent Bond Bonanza: Benzene (C6H6) is a classic example. Each resonance structure depicts three double bonds and three single bonds. However, experiments show all six carbon-carbon bonds have the same length, somewhere between a single and double bond. This suggests the actual molecule is a resonance hybrid of all the structures, with the electron density delocalized across the ring, creating a stability not reflected in any single structure.

• Demystifying the Double-Headed Arrow: The double-headed arrow in resonance structures doesn't imply a constant flipping between forms. Instead, the actual molecule is a resonance hybrid, a blend of all the contributing structures, with the electron density delocalized across the molecule. Imagine the actual molecule as having a smeared-out electron cloud, not the discrete bonds shown in the individual resonance structures.

By understanding these factors and the reasoning behind them, you can identify the major contributors and gain a deeper understanding of the molecule's true electronic structure, which ultimately influences its reactivity and behavior.