NOTE ON THE Y-RULE IN CLAR THEORY

Abstract

The recently proposed Y-rule for constructing Clar-type aromatic sextet formulas for benzenoid molecules is examined, and the limits of its applicability are established. According to the Y-rule, the π-electron configuration of a benzenoid hydrocarbon is best represented by the Clar-type formula(s) in which all internal carbon atoms (the so-called Y-carbon atoms) are covered by aromatic sextets. The Y-rule is, by definition, applicable only to perifusenes (because catafusenes do not possess internal carbon atoms). We show that the rule is not applicable to all perifusenes, and that in some case it leads to numerous “best” Clar-type formulas.

Keywords:

• Benzenoid hydrocarbons
• Clar aromatic sextet theory
• Clar structure
• Y-rule

Acknowledgments

Y. R-M acknowledges the support under the project D.00406 of the Instituto Mexicano del Petróleo (IMP).