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).		✓	✗

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.