Failures of the Resonance Picture for Aromatics If having these identical resonance structures were the sole cause of this pronounced stability,then ALL structures with conjugated systems of alternating double and single bonds should show analogous enhanced stabilities. These cyclic hydrocarbons with alternating double and single carboncarbon bonds are called Annulenes. 回 [4]Annulene [6]Annulene [8]Annulene [10]Annulene Benzene is the 6 membered annulene,and is called [6]annulene. For the double bonds to be totally conjugated,the molecule must be planar so that the p orbitals of the bonds can overlap. However,molecules like cyclobutadiene and cyclo-octatetraene DO NOT exhibit this increased stability-in fact quite the opposite! Cyclobutadiene has never been isolated and purified because it is so unstable-it reacts with itself to form dimers even at low temperatures. Cyclo-octatetraene has been shown to not exist in a planar structure,but instead it adopts a'tub'like conformation. Ch16 Aromatic Compounds(landscape).docx page 7 Ch16 Aromatic Compounds (landscape).docx page 7 Failures of the Resonance Picture for Aromatics If having these identical resonance structures were the sole cause of this pronounced stability, then ALL structures with conjugated systems of alternating double and single bonds should show analogous enhanced stabilities. These cyclic hydrocarbons with alternating double and single carbon carbon bonds are called Annulenes. Benzene is the 6 membered annulene, and is called [6] annulene. For the double bonds to be totally conjugated, the molecule must be planar so that the p orbitals of the  bonds can overlap. However, molecules like cyclobutadiene and cyclo-octatetraene DO NOT exhibit this increased stability - in fact quite the opposite! Cyclobutadiene has never been isolated and purified because it is so unstable - it reacts with itself to form dimers even at low temperatures. Cyclo-octatetraene has been shown to not exist in a planar structure, but instead it adopts a 'tub' like conformation