[Woopy]

Hello,

I am having difficulty discerning which resonance structures are considerable enough to include and which are so unimportant that they should be discarded. There seems to be no general rule on this that I can find, so I was wondering if such a rule existed or if someone can lead me in the right direction.

[fledarmus]

You learn by thinking about what sort of structures are UNLIKELY to exist. For example:

• Carbon atoms don't form five bonds - any resonance structure you draw that has five bonds to a carbon isn't going to contribute much.
• Unfilled shells aren't as stable as filled shells - structures having atoms with only six electons don't usually contribute much (with the rare exception of boron).
• Charge separation isn't usually stable - neutral resonance structures usually contribute more than structures containing both a negative and a positive charge.
• If the negative charge is on the more electronegative atom and the positive charge is on the more electropositive atom, that is even worse and will contribute even less.

The resonance structures that contribute the most are ones that are lowest energy forms related either by rotation of the molecule or rotation of a bond. For example, the two resonance forms of cyclohexatriene that are frequently drawn to indicate benzene, or the two resonance forms of a carboxylate anion. In these cases, the two resonance forms contribute equally to the most stable form of the molecule, and the measured form is an average of the two resonance forms.

[orgopete]

In my book, I used different types of examples. If you had a structure with a negative charge, then I begin moving the electrons away from the atom containing the charge. If a structure has a positive charge, then I move electrons toward that charge. With neutral structures, then choose electron pairs that will give the most stable cation or anion.

I find it difficult to write simple rules for all situations because this may lead to exceptions. I prefer to describe how electrons may move in certain structures in an effort for us to try to grasp the nature of the atoms donating and accepting electrons.

[souro10]

You do not need any special rules to decide which structures will have a significant contribution to the hybrid, and which structures will not. All you need to do is analyze the relative stabilities of the resonance contributing structures. Draw the structures and look for favors that might make a structure relatively stable, and look for factors that might make the structure unstable. A few ' rules of thumb ' that may not be always true are: 1. Greater the number of multiple bonds in a structure, more stable it is. 2. If negative charge resides on the more electronegative atom and positive charge on the more electropositive atom, it is stable, but if positive charge resides on the more electronegative atom, it becomes less stable. 3. Like charges cannot reside side by side. It'll have negligible to zero contribution. 4. A structure that has octet on all atoms is exceptionally stable. This is important. It is an important driving force for many rearrangements. 5. Separation of charges ("moving electrons") requires energy, and hence decreases stability. That is, charge separated structures are less stable. 6. A neutral structure is more stable than charge-seperated/electron moved. Hope this helps.