Electronegativity Equilibration and the Methylene Increment in Organic Thermochemistry

Abstract

It is proposed that the magnitude of the ' methylene increment' ∆∆ _fH°(C_nH_2n+1X, C_n+1H_2n+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 E_ij. These are deemed to be related to the products q_iq_j of the fractional charges on adjacent carbon atoms by the relation:

where E° and a are empirical constants, and the fractional charges are estimated by electronegativity equilibration. For the homologous series C_nH_2n+1X (X = H, Cl , Br, I, OH, OMe , NH₂), a linear correlation is found between the mean increment [∆∆ _fH °(CH₃X, C_nH_2n+1)]/(n-1) and the mean charge product ∑q_iq_j/(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.