EFFECTS OF SUBSTITUENTS ON THE ACIDITY OF ACETIC-ACIDS

The acidities of a wide range of substituted acetic acids have been determined in several solvents from potentiometric titrations. From multi-linear-regression (MLR) correlation equations in which substituent steric, polarizability, and field/inductive effects are considered, analysis of structural effects on the relative acidities was accomplished. The effects that alkyl substituents have on acidity variations come primarily from a substituent steric effect and not a substituent polarizability effect. The effect of polar substituents on acidity variations is caused mainly by a substituent field/inductive effect, and this effect is manifested most in polar basic solvents. The effect of the N,N-dimethylamino substituent differs from the other polar substituents studied. N,N-Dimethylglycine is less acidic than predicted by the MLR correlation equations in all solvents studied. The tautomeric equilibrium that governs the neutral/zwitterion formation is medium dependent, and the magnitude of that equilibrium constant dictates the observed acidity of N,N-dimethylglycine. The solubility of N,N-dimethylglycine gives an indication of the extent of zwitterion formation. N,N-Dimethylglycine is very soluble in solvents in which the zwitterion is favored, but the solubility decreases in solvents in which the neutral amino acid is the favored species.