how many resonance structures?

[Claude Chung 2F]

when given larger compounds such as ch3coch3, or larger compounds with many more atoms... how can we determine the number of resonance structures that it has and how to structure them? i know a common pattern is a triple bond between N and C (i think). do we just need to rely on patterns/memorization?

[Brian Bognuda 2I]

I know it's not an effective strategy and I'm sure there's better out there, but for resonance structures I like to plug in different ways and check if it is an example of resonance. For example, using a triple bond between N and C as you mentioned could also be turned into a double bond with another double bond occurring with a neighboring atom, and further with a single bond depending on the neighboring atom. In a way I am able to check the possible combinations with the bonding patterns like that, but I think it's also important to memorize certain common resonance structures for the test due to a time crunch. Hope this helps.

[Brandon Li]

Unfortunately, finding resonance structures, especially with larger compounds, is a matter of brute force. There is no given "formula" to find it. Just have to draw a lot out and see which ones fit and which ones do not based on their formal charges. Though, I doubt that any test would ask us to find all of the resonance structures of an incredibly complex compound without assistance (hopefully...)