Information about the mechanism of the reaction undergone by a series of substituted benzenes can be obtained by plotting the logarithm of the observed rate constant determined at a particular pH against the Hammett substituent constant (s) for the particular substituent. The s value for hydrogen is 0. Electron- donating substituents have negative s values; the more strongly electron donating the substituent, the more negative its s value. Electron-withdrawing substituents have positive s values; the more strongly electron withdrawing the substituent, the more positive its s value. The slope of a plot of the logarithm of the rate constant versus s is called the r (rho) value. The r value for the hydroxide-ion-promoted hydrolysis of a series of meta- and para-substituted ethyl benzoates is +2.46; the r 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 r value, whereas the other set of experiments gives a negative r value?
b. Why were ortho-substituted compounds not included in the xperiment?
c. What do you predict the sign of the r value to be for the ionization of a series of meta- and para-substituted benzoic acids?
Trending now
This is a popular solution!
Step by step
Solved in 5 steps with 9 images
