Effects on reactivity Antiaromaticity

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antiaromaticity can have significant effect on pka. linear compound 1-propene has pka of 44, relatively acidic sp carbon center because resultant allyl anion can resonance stabilized. analogous cyclic system appears have more resonance stabilized, negative charge can delocalized across 3 carbons instead of two. however, cyclopropenyl anion has 4 π electrons in cyclic system , in fact has substantially higher pka 1-propene because antiaromatic , destabilized. because antiaromatic compounds short-lived , difficult work experimentally, antiaromatic destabilization energy modeled simulation rather experimentation.




some antiaromatic compounds stable, larger cyclic systems (in antiaromatic destabilization not substantial). example, aromatic species 1 can reduced 2 relatively small penalty forming antiaromatic system. antiaromatic 2 revert aromatic species 1 on time reacting oxygen in air because aromaticity preferred.




the loss of antiaromaticity can driving force of reaction. in following keto-enol tautomerization, product enol more stable original ketone though ketone contains aromatic benzene moiety (blue). however, there antiaromatic lactone moiety (green). relief of antiaromatic destabilization provides driving force outweighs loss of aromatic benzene.










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