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Effects of strain and conjugation on the structure of the benzene-ring in a free benzene molecule, in benzofuran, 1-indanone, and phthalide

B. Vidal, J. Vardin, and A. Darryhenaut

Laboratoire de Chimie Organique, Faculté des Sciences, Universitě de La Reunion, В.P. 5 97490-Sainte-Clotilde, La Reunion (France DOM)

 

Abstract: MNDO calculations show that the alternation of bond lengths which is observed in a benzene ring, when there is a five-membered aliphatic fused ring (Mills—Nixon effect) as well as a heterocyclic one, is induced by the only lengthening of the bridgehead bond under the strain of the fused ring, and no other specific effect is to be involved. Such an alternation is a basic feature of the benzene ring when one of its bonds is lengthened. When alternation occurs, since the hexatriene structure is the model structure which should be reached for a bond infinitely lengthened, it is rather an open chain hexatriene-like structure which tends to be reached instead of a Kekulé-like one. We show that a weak conjugation does not prevent the alternation of bond lengths to occur under the strain of the five-membered fused ring, since it is observed in 1-indanone. When conjugation is more effective, involving the whole five-membered ring, as in benzofuran, conjugation tends to im­pose an alternation opposite to that induced by the strain of the same fused ring, and, as concerns the bond lengths, blurs the structural incidence of the Mills—Nixon effect. The opposition is the strongest in the region of the bridgehead bond where the strain is the most efficient. Conjugation is efficient through extended system, and it is stronger to impose its own alternation pattern inside the benzene moiety (apart the bridgehead bond) where the effect of the strain is rapidly decreasing. When using the π bond orders, the alternation pattern imposed by the conjugation with the five-membered ring is more clearly set up, since the bond orders are more typical of the conjugative process, and the new alternation pattern is no more a Mills—Nixon effect. Our work has been extended to phthalide to ascertain the generality of the alternation pattern under strain.

Full paper in Portable Document Format: 445a603.pdf

 

Chemical Papers 44 (5) 603–620 (1990)

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