Information about the mechanism of reaction of a series of substituted benzenes can be obtained by plotting the logarithm of the observed rate constant obtained at a particular pH against the Hammett substituent constant (σ) for the particular substituent. The σ value for hydrogen is 0. Electron-donating substituents have negative σ values; electron-withdrawing substituents have positive σ values. The more strongly electron donating the substituent, the more negative its σ value will be; the more strongly electron withdrawing the substituent, the more positive value its σ value will be. The slope of a plot of the logarithm of the rate constant versus σ is called the (rho) value. The p value for the hydroxide-ion-promoted hydrolysis of a series of meta- and para-substituted ethyl benzoates is +2.46; the p value for amide formation for the reaction of a series of meta- and para-substituted anilines with benzoyl chloride is -2.78.
a. Why does one set of experiments give a positive p value while the other set of experiments gives a negative p value?
b. Why do you think that ortho-substituted compounds were not included in the experiment?
c. What would you predict the sign of the p value to be for the ionization of a series of meta- and para-substituted benzoic acids?