How To Figure Out Resonance Structures Using “The Rule of 3”

Organic chemistry and resonance structures

This concept comes up more and more as you move farther into your class. Below, I will show you how to figure out all your resonance structures WITHOUT needing to draw them.

Start by numbering your structure

Here is a quick way to figure out all the resonance structures you have without needing to go through the hassle of drawing all the structures. Start by numbering 1,2 and 3 on your alkene.

In the above example (3-carbon Alkene), place the number 1 on the carbon that has the charge. Then label the other carbons 2 and 3.

You can see that the double bond is on the numbers 2 and 3. With resonance structures, the charge will always be on 1 or 3.

How to apply this to a more complicated example

Ok – let’s look at an example that has more double bonds.

Start by numbering your charged carbon 1…and then continue to number 2 and 3. Now here is the cool part. Your 3 now becomes your “new 1”.

We know this because of the first example (charge on 1 or 3).

Label 1-3 again. Repeat until you go through all your Alkenes. Do you see the problem?

The last 1-3 (in red) does not have the alkene on 2 and 3. Therefore, that one does not count. So, the resonance structures are on green 1, blue 1 and red 1. This is a total of 3 resonance structures.

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