Hammett Parameter and Molecular-Modeling Correlations of Substituent Effects on Esterification Kinetics

The effect of substituents on the rate of reactions is taught extensively throughout organic chemistry. The Hammett equation is useful in quantifying this dependence; however, there are relatively few experiments that investigate this relationship. We describe an advanced undergraduate experiment suitable for exploring the substituent effect on esterification kinetics of trifluoroacetic acid with a variety of alcohols. We find a negative correlation between the Hammett substituent constant and the rate of the esterification, indicating a slower rate of reaction for the substituted alcohols. The decrease in measured rates is also correlated with the calculated charge on the oxygen atom for the substituted alcohols. The combination of the experimental data with the theoretical calculations should increase the student's understanding of substituent effects on reaction rates. © 2010 The American Chemical Society and Division of Chemical Education, Inc.