Molecules that are fully conjugated, but each of whose Kekulé structures contains
at least two atoms that are not π-bonded. Examples of non-Kekule
hydrocarbons are trimethylenemethane (2-methylidenepropane-1,3-diyl)
1,
m-benzodiquinomethane (benzene-1,2-diyldimethyl)
2, and 1,2,4,5 - tetramethylenebenzene (benzene-1,2,4,5-tetrayltetramethyl)
3.
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