Chapter 8. Organic Acids and Bases: Acidity Constant and Partitioning Behavior

Summary

The exchange of protons with aqueous media converts many organic compounds into ionic molecules. The tendencies to undergo such transformations are quantified via acidity constants. These acid-base reactivities depend on the ability of the structure near the ionizable functional groups to take on charge via inductive and delocalization effects. The acidity constants within compound families related to benzoic acid can be correlated to one another via the Hammett equation. In this linear free energy relationship, substituents are characterized for their ability to withdraw or donate electron density to the reaction site, and this ability is quantified with Hammett sigma values. Additionally, differences between compound families are parameterized with susceptibility factors. Finally, the impact of acid-base reactions on compound partitioning is demonstrated for cases in which the ionized species is unable to move from the aqueous to other nonaqueous phases (e.g., gas, organic solvent).