Question

The carboxylic acid group of an amino acid has a pKa of approximately 2. However, carboxylic acids like benzoic acid and acetic acid have pKas in the range of 4-5 pKa units. Explain why the carboxylic acid of an amino acid is more acidic.

Answer 1

The key to understanding this trend is to consider the hypothetical conjugate base in each case: the more stable (weaker) the conjugate base, the stronger the acid. Look at where the negative charge ends up in each conjugate base.Remember the periodic trend in electronegativity: it also increases as we move from left to right along a row, meaning that nitrogen is the more electronegative than carbon. The more electronegative an atom, the better it is able to bear a negative charge.Hence,more stable and more acidic.