Bond strengths of toluenes, anilines, and phenols: To Hammett or not

The Hammett equation correlates the effects of Y on many different chemical properties of YC(6)H(4)ZX 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-YC(6)H(4)ZX families with electrophilic substituent constants, sigma(p)(+)(Y), which were largely derived from the rates of the heterolytic S(N)l 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 sigma(p)(+)(Y) in 4-YC6H4NHX (X = H, CH3, OH, F) and excellent correlations of O-X BDEs with sigma(p)(+)(Y) in 4-YC6H4OX (X = H, CH3, CH2Ph) families. The reasons for this varied behavior are discussed.