IS ANTI-AROMATICITY ABSOLUTE

The concept of antiaromaticity as applied to 4n-π electron monocyclic conjugated systems is examined and two subtypes distinguished. Relative antiaromaticity is the term which describes cyclic systems which are less stable than acyclic conjugated analogues. Absolute antiaromaticity refers to cyclic systems which are less stable than even nonconjugated models (e.g., ethylene). The extensive experimental evidence for absolute antiaromaticity (particularly for cyclobutadiene and cyclopropenide) is critically examined and judged inconclusive in view of the existence of alternate, plausible rationalizations. Theoretical analysis also challenges the reality of absolute antiaromaticity, at least in the monocyclic series. Relative antiaromaticity is affirmed, but only for the three smallest monocyclic systems (the two mentioned above plus cyclopentadienylium). © 1978, American Chemical Society. All rights reserved.