The effect of solvent polarity on the rate of the Mitsunobu esterification reaction
MetadataShow full item record
The rate of the Mitsunobu esterification reaction of ethanol or isopropanol with benzoic acid was found to be much faster in non-polar solvents. The logarithm of the rate constant was inversely proportional to the solvent polarity, as defined by ET values. Typically, the rate constant for ethyl benzoate formation in THF was 100 times greater than that in MeCN. The presence of either sodium benzoate or excess benzoic acid resulted in a decrease in rate. Each of the main species involved in the Mitsunobu esterification reaction, the alcohol starting material, dialkoxyphosphorane, alkoxyphosphonium salt and ester product, was detected by proton NMR analysis. The possible role of ion pair aggregates or clusters, prior to rate-determining SN2 attack of carboxylate on the alkoxyphosphonium ion, is discussed. An explanation is provided as to why the yield in the Mitsunobu reaction is often higher in non-polar solvents.
© 2015 Elsevier. Licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (http://creativecommons.org/licenses/by-nc-nd/4.0/) which permits unrestricted, non-commercial use, distribution and reproduction in any medium, providing that the work is properly cited.
Physical Organic Chemistry