Information about the mechanism of the reaction undergone by a series of substituted

Chapter 16, Problem 95

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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 will be. Electron-withdrawing substituents have positive s values; the more strongly electron withdrawing the substituent, the more positive its s value will be. 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 . Y OCH2CH3 Y Y ortho-substituted meta-substituted para-substituted C O OCH2CH3 C O OCH2CH3 C O 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 experiment? c. What would you predict the sign of the r value to be for the ionization of a series of meta- and para-substituted benzoic acids?