Taft's substituent constants, σ* and σI, and Huheey's group electronegativity

Abstract

AbstractFor 34 groups, Taft's substituent constants σ* and σI are found to correlate well (r = 0.962 and 0.948) with Huheey's group electronegativity (χH) when Taft's steric parameter Es for a group is mixed with σ* and σI: equation image From these equations, it follows that for a particular chemical group G, σ*(G) ≈︁ 8σI(G). Since the σ* scale is essentially based on the σI scale, the small inherent steric component in σI is amplified in the case of the σ* by a factor of ca 8. An analysis shows that χH 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 σ* and σI are found to represent the electronegativity of a group in the spirit of the χH. 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.