Group electronegativities from electronegativity equilibration Applications to organic thermochemistry

Abstract

A simple scheme for electronegativity equilibration (as opposed to equalisation) allows the electronegativity of an atom in a molecule to be expressed as a weighted harmonic mean of the prebonded electronegativities of all the atoms. Fractional atomic charges and group electronegativities can be easily calculated. It is proposed that the heats of formation of gaseous organic substances can be estimated using constant bond-energy terms E(C–X) (X = H, Cl, etc.) and E(CC), except that E(C–C) varies according to the difference in electronegativity between the groups linked by the carbon–carbon bond, and E(C–H) is dependent on the hybridisation of the carbon atom. The enthalpies of formation of alkanes, alkenes, alkynes, alkyl radicals and halogenoalkanes can be satisfactorily reproduced using traditional Pauling electronegativities, although the results can be improved by making some adjustments. The charge distributions obtained by electronegativity equilibration help to rationalise the relative stabilities of primary, secondary and tertiary carbocations, as well as the apparent electron-donating function of alkyl groups relative to hydrogen atoms.