Dewar structure, anthracene

The Dewar-type structure for the central ring of the anthracene system in 19 is possible only because the 9,10 substituents prevent the system from being planar. From 19 is the actual structure of the molecule and is not in resonance with forms like 20, although in anthracene itself, Dewar structures and structures like 20 both contribute. This is a consequence of rule 2 (p. 41). In order for a 20-like stracture to contribute to resonance in 19, the nuclei would have to be in the same positions in both forms. 

The Dewar-type structure for the central ring of the anthracene system in 16 is possible only because the 9,10 substituents prevent the system from being planar.39 16 is the actual structure of the molecule and is not in resonance with forms like 17, although in anthracene... 

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