Relative strengths of forty aromatic carboxylic acids in benzene at 25-degrees-C

Abstract

The r ela tive stre ngths of ben zoic acid, 31 of its monosubstit ut ed derivatives, 7 disub-stit uted d erivatives, and one trisubstituted derivative have been measured in t erms of the equilibrium constants (I(") for a ssociation with t he reference base 1,3-diphenylguanidine in benzene at 25° C. The measurements were performed spectrophotometrically, using the indicator a cid bromophthalein m a gen ta E (tetrabromophenolphthalein ethyl ester) as the reference acid, t he equilibrium con stant [(' for association of the indicat or with diphenylguauidine being known from pre vious work. Equilibrium con st ants for p-orsellinic (2,6-dihydroxy-p-toluic) acid and 2,4,6-trinitroben zoic a cid could not be m ea sured , a s t hese a cids appear t o associat e completely with diphenylguanidine under the condit ions of t h e experiments. Plotting log [(" values against t he corresponding p[(v a lues for aqueous solutions of t he a cids gives a n essentially linear relationship in the case of acids with m ela-a nd par a-subst ituents only, but the line for pam-substituted acids has a slightly differen t slope from t he line for m-subst i tuted acids. Similar comparisons were made with published data on relative strengths in a lcohols or dioxane-wat er mixtures. orlho-Substit uted a cids show m arked solvent effects. Explanations of the solvent effeets h ave been suggested , a nd various t heoretical implications and possible applications of the r esults are discussed. The t h e-or·etical discuss ion includes e valuation of constants of the H ammett equation.