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.