Benzene cyclobutadiene

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

Benzene cyclobutadiene and cyclooctatetraene provide clear examples of Huckel s rule Benzene with six tt electrons is a An + 2) system and is predicted to be aromatic by the rule Square cyclobutadiene and planar cyclooctatetraene are An systems with four and eight tt electrons respectively and are antiaromatic... 

Hiberty, P. C. The Distortive Tendencies of Delocalized n Electronic Systems. Benzene, Cyclobutadiene and Related Heteroannulenes. 153, 27-40 (1990). 

Distortion Energies and QMREs (in kcal/mol) for Benzene, Cyclobutadiene, and Their Heteroanalogs According to Calculations (76JA2750 87JA363 88IC2219) ... 

Hiberty PC (1990) The Distortive Tendencies of Delocalized tt Electronic Systems. Benzene, Cyclobutadiene and Related Heteroannulenes. 153 27-40 Hintz S, Heidbreder A, Mattay J (1996) Radical Ion Cyclizations. 177 77-124 Hirao T (1996) Selective Transformations of Small Ring Compounds in Redox Reactions. 178 99-148... 

Like benzene, cyclobutadiene ([4]annulene) has a continuous ring of overlapping p orbitals. But it has four pi electrons (two double bonds in the classical structure), which is a (41V) system with N = 1. Huckel s rule predicts cyclobutadiene to be antiaromatic. 

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