Current theory for predicting regioselectivity in reactions classed as nucleophilic aromatic photosubstitutions by the SN2 Ar* mechanism uses frontier molecular orbital theory. It asserts that reaction is favored at the aromatic ring position (bearing a nucleofugic group) with the largest calculated electron density (square of the HOMO coefficient) in the ground electronic state. There are theoretical reasons to question this model, not least of which are that it predicts chemical reactivity based on calculated properties of the reactant alone, and that it ignores relative activation energies. We showed recently for 2-chloro-4-nitroanisole with hydroxide ion that the temperature dependence of the distribution of the three photoproducts can predicted exactly by use of the Arrhenius Equation. This shows that activation energies rule the distribution. Accordingly, we have attempted to determine the relative activation energies by computation. The relative activation energies cannot be computed with sufficient accuracy to predict regioselectivity. The computed enthalpy changes to the first sigma-complex intermediates, however, are related to the experimental activation energies. Plots of Eact vs. computed delta H for all four of the 2-halo-4-nitroanisoles (halogen = F, Cl, Br, and I) in their reactions with hydroxide ion are highly linear. This exemplifies the Bell-Evans-Polanyi Principle, which provides a theoretically sound basis for predicting regioselectivity.

Getting beyond frontier molecular orbital theory to predict regioselectivity of nucleophilic aromatic photosubstitution reactions

249th National Meeting (2015)

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