[OfficialAnu]

Isn't the third form (rightmost) essentially the first, and aren't you supposed to "return" back to the first form after you resonate your last one in these ring structure type problems?

If you resonate the 3rd form, you return back to form 2, not 1, hence why I think there's only 2 forms.


Can anyone please clarify?

[mjc123]

3 is not 1, it is 1 reflected (or rotated) about an axis containing the lower left and upper right carbons. It is a distinct resonance structure.

You only return to structure 1 when you have a continuous ring of conjugated double bonds, as in benzene. Here you don't - the upper right carbon is sp³ (CH₂). That prevents you "going round the ring".