ANGULAR GROUP INDUCED BOND ALTERNATION (AGIBA) - A NEW-TYPE OF THE SUBSTITUENT EFFECT

Nonsymmetrical - angular substituents -XY induce a bond alternation le ading to the imbalance of the Kekule structures in the substituted der ivatives of benzene. An empirical rule is proposed - that cis CC bonds in the ring show increased double character (compared to the trans bo nds) when the substituent is of -X-Y kind, and less double character i f the substituent is of the -X=Y type. The effect is strongly conforma tion dependent. Analysis of the structural effects observed in the rin g for different conformations lead to the conclusion that the imbalanc e of Kekule structures for a ring is due to the combination of the str ain (rehybridization) effect and the through space pi electron interac tions between the XY substituent and the CC bond cis to XY group.