TAFT SUBSTITUENT CONSTANTS, SIGMA-STAR AND SIGMA-I, AND HUHEEY GROUP ELECTRONEGATIVITY

For 34 groups, Taft's substituent constants sigma* and sigma(I) are found to correlate well (r = 0.962 and 0.948) with Huheey's group electronegativity (chi(H)) when Taft's steric parameter E(s) for a group is mixed with sigma-* and sigma(I): chi(H) = 0.360(sigma-* - 0.107 E(s)) + 2.255 chi(H) = 3.048(sigma(I) - 0.014 E(s)) + 2.196 From these equations, it follows that for a particular chemical group G, sigma-* (G) almost-equal-to 8-sigma(I)(G). Since the sigma-* scale is essentially based on the sigma(I) scale, the small inherent steric component in sigma(I) is amplified in the case of the sigma-* by a factor of ca 8. An analysis shows that chi(H) represents the electronegativity of a group when the group is considered to be a mere collection of non-bonded, charged and suitably hybridized atoms. Thus sigma-* and sigma(I) are found to represent the electronegativity of a group in the spirit of the chi(H). It is concluded that a chemical group can be viewed as a collection of non-interacting atoms. Although there is evidence that a molecule can sometimes be treated similarly, for a group such a situation is novel.