Electronegativity Equilibration and the Methylene Increment in Organic Thermochemistry

Abstract

It is proposed that the magnitude of the ' methylene increment' ∆∆ fH°(CnH2n+1X, Cn+1H2n+3X) between the heats of formation of successive members of a homologous series can be rationalized by considering the variation of carbon-carbon bond enthalpies Eij. These are deemed to be related to the products qiqj of the fractional charges on adjacent carbon atoms by the relation: Eij=E°-aqiqj where E° and a are empirical constants, and the fractional charges are estimated by electronegativity equilibration. For the homologous series CnH2n+1X (X = H, Cl , Br, I, OH, OMe , NH2), a linear correlation is found between the mean increment [∆∆ fH °(CH3X, CnH2n+1)]/(n-1) and the mean charge product ∑qiqj/(n - 1). The empirically obtained value for E° is in close agreement with the experimental C-C bond enthalpy in diamond. For all series, the calculated methylene increment converges to a common value of 20.6 kJ mol-1. The treatment may provide an attractive alternative to existing methods for the estimation of the heats of formation of organic compounds, requiring fewer empirical parameters.