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

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.

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.

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

To determine which pairs of structures are correct representations of resonance forms in aromatic compounds, you need to consider a few key principles.

1. Resonance forms differ only in the placement of π (pi) bonds and lone pairs of electrons. The overall connectivity of atoms remains the same.

2. The number of π bonds and the positions of lone pairs should be the same in all resonance forms. This means that every atom needs to maintain its original number of bonds.

In the example you provided, there are three pairs of structures. Let's analyze them one by one:

Pair 1:

Structure 1 has a single bond between carbon 1 and carbon 2, a double bond between carbon 2 and carbon 3, and a single bond between carbon 3 and carbon 4. This structure follows the single bond, double bond, single bond pattern.

Structure 2 has a single bond between carbon 1 and carbon 2, a double bond between carbon 3 and carbon 4, and no bond between carbon 2 and carbon 3. Therefore, it doesn't follow the single bond, double bond, single bond pattern.

Since Structure 2 does not maintain the correct pattern, it is not a correct representation of a resonance form when compared to Structure 1. Thus, Pair 1 is not a correct pair of resonance forms.

Pair 2:

Structure 1 has a single bond between carbon 1 and carbon 2, a double bond between carbon 2 and carbon 3, and a single bond between carbon 3 and carbon 4. This structure follows the single bond, double bond, single bond pattern.

Structure 2 has a single bond between carbon 1 and carbon 2, a double bond between carbon 2 and carbon 3, and a single bond between carbon 3 and carbon 4. This structure also follows the single bond, double bond, single bond pattern.

Both Structure 1 and Structure 2 have the same pattern and maintain the correct connectivity of atoms. Thus, Pair 2 is a correct pair of resonance forms.

Pair 3:

Structure 1 has a single bond between carbon 1 and carbon 2, a double bond between carbon 2 and carbon 3, and a single bond between carbon 3 and carbon 4. This structure follows the single bond, double bond, single bond pattern.

Structure 2 has a single bond between carbon 2 and carbon 3, a double bond between carbon 3 and carbon 4, and a single bond between carbon 1 and carbon 4. Therefore, it doesn't follow the single bond, double bond, single bond pattern.

Since Structure 2 does not maintain the correct pattern, it is not a correct representation of a resonance form when compared to Structure 1. Thus, Pair 3 is not a correct pair of resonance forms.

In summary, Pair 2 is the correct pair of resonance forms in the given structures, while Pair 1 and Pair 3 are not correct pairs as they do not maintain the correct pattern or connectivity of atoms.