COMPILATION AND CRITICAL-EVALUATION OF STRUCTURE-REACTIVITY PARAMETERS AND EQUATIONS .1. VALUES OF SIGMA(M) AND SIGMA(P) BASED ON THE IONIZATION OF SUBSTITUTED BENZOIC-ACIDS IN WATER AT 25-DEGREES-C

Data have been compiled for the pK(a) values in water at 25 degrees C of about 60 meta- or para-substituted benzoic acids, with a view to pr oposing recommended values of Hammett substituent constants, sigma. Th e number of substituents covered is about one-half of this number, but for certain substituents data are not available for both the meta and para position. The amount and the reliability of the data vary greatl y from one acid to another. Critical appraisal leads to the classifica tion of the substituents and their sigma values as belonging to a Firs t Division (with more reliable data) or a Second Division (with less r eliable data). The substituents in the former category are: Me, NO2, O Me, F, Cl, and Br (each in both the meta and the para position). Recom mended values of sigma are tabulated for substituents in each Division , but for certain members of the Second Division there are caveats bec ause the data show discrepancies.