Bond Strengths of Toluenes, Anilines, and Phenols: To Hammett or Not

Abstract

The Hammett equation correlates the effects of Y on many different chemical properties of YC₆H₄ZX families of compounds. One of the most surprising is that the Z−X bond dissociation enthalpy (BDE), a homolytic property, can be correlated for some 4-YC₆H₄ZX families with electrophilic substituent constants, (Y), which were largely derived from the rates of the heterolytic S_N1 solvolyses of para-substituted cumyl chlorides. Although there is no Hammett correlation of the C−X BDEs in 4-YC₆H₄CH₂X (X = H, halide, OPh) families, there are good correlations of N−X BDEs with (Y) in 4-YC₆H₄NHX (X = H, CH₃, OH, F) and excellent correlations of O−X BDEs with (Y) in 4-YC₆H₄OX (X = H, CH₃, CH₂Ph) families. The reasons for this varied behavior are discussed.