Hammett correlation in competition experiments in dissociation of ionised substituted benzophenones and dibenzylideneacetones.

A convenient method of applying competition experiments to devise a Hammett correlation in the dissociation by α-cleavage of 17 ionised 3- and 4-substituted benzophenones, YC₆H₄COC₆H₅ [Y=F, Cl, Br, CH₃, CH₃O, NH₂, CF₃, OH, NO₂, CN and N(CH₃)₂] is reported and discussed. The results given by this approach, which rely on the relative abundance of [M-C₆H₅]⁺ and [M-C₆H₄Y]⁺ ions in the electron ionisation spectra of the substituted benzophenones, are compared with those obtained by previous methods. Various refinements of the method are considered, including reducing the ionising electron energy, making allowance for the relative abundance of ions such as C₆H₅⁺ and C₆H₄Y⁺, which may be formed to some extent by secondary fragmentation, and using substituent constants other than the standard σ constants. The reaction constant, ρ, of 1.08, which is in good agreement with that deduced previously, is consistent with a considerable reduction in electron density (corresponding to an increase in positive charge) at the carbon of the carbonyl group during fragmentation. This method has been successfully extended to the corresponding cleavage of 12 ionised substituted dibenzylideneacetones, YC₆H₄CH=CHCOCH=CHC₆H₅ (Y=F, Cl, CH₃, OCH₃, CF₃, and NO₂), which may fragment to form either a substituted cinnamoyl cation, [YC₆H₄CH=CHCO]⁺, or the cinnamoyl cation, [C₆H₅CH=CHCO]⁺. The derived ρ value of 0.76 indicates that the substituent, Y, influences the stability of the cinnamoyl cation somewhat less strongly than it does the analogous benzoyl cation.