Question

WHY did the COOH pKa on glycine increase 82X in acidity (not 82%, but 82 times), or What increases the pKa of a carboxyl?

Answer 1

An increase in pKa shows a decrease in acidity as pKa is the negative log of acid dissociation constant for a given acid. Thus to determine the reason for change in pKa, the factors that influence the acidity should be determined. In case of glycine, the alpha-amino group is electron-withdrawing inductively. Since mesomeric effects cannot play here due to lack of conjugation, the conjugate base glycinate anion is made more electron-deficient resulting the carboxyl oxygen atom to withdraw more electron-density from the carboxylic acid hydrogen thus making it more acidic or increases its proton-like character. This also stablizes the conjugate base of glycinate anion. Therefore it can be summarized that the captodative effect in alpha-amino acids like glycine increases their acidity compared to that of unsunstituted carboxylic acids. This results in a decrease in pKa of glycine or an 82% increase in its acidity compared to acetic acid or such conventional carboxylic acids.

Similarly, substituents that are electron-donating increase the electron-density on the hydroxyl group of carboxylic acids, decreasing the stability of conjugate bases by increasing hydrogenic character of the hydroxyl hydrogen which results in a decrease in acidity of the species and an increase in pKa.