Dissociation constants, meta ions

The substituent constants (known as Hammett constants) are measured for the add-dissociation eqrrUibrirrm of benzoic add and its para- or mcM-substituted derivatives, for which the reaction constant p is set to unity. Positive values of the Hammett constant indicate that the substituent is a better electron acceptor than H and can thus enhance addity by helping to stabilize the carboxylate ion negative values indicate that the substituent is a poorer electron acceptor than H (i.e., an electron donor instead). When the substituent R is in the para position, the resulting Cp values are taken as a measure of the polar effea of the substituent based on its combined resonance and inductive properties. When the substituent R is in the meta position, the resulting values are taken as a measme of the polar effea of the substituent based largely on its inductive properties. When a substituent is in the ortho position, it is assumed that the substituent effects are influenced by steric properties also. 

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