An AM1 study of the effect of substituents on the bond dissociation energies of anilines, phenols, and α-substituted toluenes

Two isodesmic equations (see Graphical Abstract) were used to separate the ground state and radical effect of para-substituents, Y, on the bond dissociation energies (BDEs) of YC6H4Z–H (Z=CHX, NX, or O). In all cases, the ground state is destabilized by electron-donating groups (p-EDGs), while stabilized by electron-withdrawing groups (p-EWGs). The radicals are stabilized by p-EDGs, the ‘captodative effect’, but the effect of p-EWGs depends upon the electronegativity of Z, which can be modified by an adjacent substituent, X. When Z is polarized by a strong EWG (e.g. NO2) or when Z is NH or O, the radicals are destabilized by p-EWGs, an ‘anti-captodative effect’. Relative to C6H5Z–H, the p-EWGs may either increase or decrease the BDE depending upon the ‘apparent electronegativity’ of Z.