In terms of aromatic compounds, how do I determine which pairs of structures are correct representations of resonance forms?

Question

For example in these structures:

img33.imageshack.us/img33/2756/70725074.jpg

I know there has to be a single bond, double bond, single bond pattern. But I'm not too sure about this one.

Answer 1

First, I am not an organic chemist; however, it appears to me that in 1, 2, and 3, all you have done is to move the double bonds from the carbon in the first picture to an adjacent carbon in the other picture (and the same for the other double bonds). Therefore, I think 1, 2, and 3 are equivalent resonance structures between the first one shown and the second one shown. However, 4 is not the same. Note in 4 that the double bonds have not shifted BUT the substituted groups have changed. The first one is an ortho dimethyl and the second one is a meta dimethyl substituted. Check my thinking. Try pushing the electrons for 1, 2, and 3 to see what I am talking about.

Answer 2

Yes I believe you are correct Dr. Bob. the fourth one is not a resonance, as they are different structures. Thanks again!