Question

Why are the carboxylic acid groups of the amino acids so much more acidic (pka~ 2) than a carboxylic acid such as acetic acid (pka=4.76)?

Answer 1

Amino acids are more acidic than simple carboxylic acids due to the inductive, resonance, and electrostatic stabilization offered by the amino group, making their carboxylate ion more stable and thus lowering the pKa values.

Step-by-step explanation:

The carboxylic acid groups in amino acids have lower pKa values, indicating they are more acidic, compared to standard carboxylic acids like acetic acid. This increased acidity is due to several effects:

The inductive effect from the attached amino group, which stabilizes the negative charge on the carboxylate after deprotonation.

Resonance stabilization of the carboxylate ion where the negative charge can be delocalized over two oxygen atoms.

The proximity of the amino group in amino acids, which, when protonated to an ammonium ion, can further stabilize the negative charge through an electrostatic effect.

These effects result in a carboxylate ion that is more stable than in other carboxylic acids, thereby making the amino acid more acidic (i.e., having a lower pKa value). For example, the amino acid glycine has a pKa around 2, which is significantly more acidic than acetic acid with a pKa of 4.76.

Answer 2

The groups you mention are withdrawing electrons with inductive effect. Because there is no conjugation between the acids (pk-2 and pka 4.76), nitrogen makes it impossible to bind to the carboxyl group. Acids are affected in two ways, the first since the electron subtraction of charged oxygen molecules, and the second way by polarizing the O-H bond with carboxylic acid, this helps the protons to split easily.