THE BASICITY OF P-SUBSTITUTED PHENOLATES AND THE ELIMINATION-SUBSTITUTION RATIO IN P-NITROPHENETHYL BROMIDE - A HSAB THEORETICAL-STUDY

The influence of basicity in a set of para-substituted phenolates on t he elimination-substitution ratio obtained upon reaction with p-nitrop henethyl bromide has been studied. A correlation between experimental equilibrium data (pK(a)) and reactivity indices obtained from density functional theory (DFT) was looked for in order to show that the hard and soft acids and bases principle (HSAB) is well suited to describe t he basicity properties of the para-substituted phenolates. When the ba sicity of para-substituted phenolates increases, their global hardness and their condensed softness at the oxygen atom increase; the negativ e net charge, condensed nucleophilic fukui function, and condensed nuc leophilic softness of the oxygen atom increase. The proposal that the alkyl halide substrate possesses a hard beta-hydrogen atom and a soft alpha-carbon was studied for p-nitrophenethyl bromide; it turns out th at the beta-hydrogen atom is soft, even softer than the carbon atom. T he experimental results for the elimination-substitution ratio for p-n itrophenethyl bromide can be explained from a local-local HSAB viewpoi nt for the para-substituted phenolates and p-nitrophenethyl bromide an d not from a global-local level viewpoint as suggested before. The res ults suggest that para-substituted phenolates with higher basicity (ha rder), less delocalized negative charge into the fragment R-C6H4, and a more polarizable oxygen atom (softer) do have a lower (relative) att raction toward an alkyl carbon atom (soft) than toward a hydrogen atom (softer) from p-nitrophenethyl bromide. The beta-hydrogen and alpha-c arbon atoms can be considered to act as a Lewis acid toward the para-s ubstituted phenolate nucleophile considered as a base.