Question

Why are the carboxylic acid groups of the amino acids more acidic

(pKa ~ 2) than a carboxylic acid such as acetic acid (pKa = 4.76)?

Short answer

The carboxylic acid groups of the amino acids are more acidic (pKa ~ 2) than a

carboxylic acid such as acetic acid (pKa = 4.76), because the electron-withdrawing

⁺NH₃

substituent on the alpha carbon atom increases the acidity of the carboxyl group.

Step-by-step solution

Acid-Base properties of Amino acids

Every amino acid contains an acidic carboxyl group and a basic amino group,

and each group can exist in an acidic form or a basic form, depending on the pH

of the solution in which the amino acid is dissolved.

• The carboxyl groups of the amino acids have pK a values of approximately 2, and the protonated amino groups have pK a values near 9. Therefore, both the groups are exists in the acidic form in a very acidic solution at pH = 0.
• At pH = 7, the pH of the solution is greater than the pK a of the carboxyl group, but less than the pK a of the protonated amino group; therefore, the carboxyl group is in the basic form and the amino group is in the acidic form.
• In a strongly basic solution (pH = 12), both groups are in the basic form.

Identify the acidic strength by the effect of substituents in amino acids

The carboxylic acid groups of the amino acids are more acidic (pKa ~ 2) than a

carboxylic acid such as acetic acid (pKa = 4.76), because the electron-withdrawing + NH 3

substituent on the alpha carbon atom increases the acidity of the carboxyl group.

The inductive effect also plays a very important role in increasing the acidity of the

carboxylic acid groups of the amino acid, because the negative inductive effect of + NH 3

substituent leads to the stabilization of carboxylate ion resulting in the delocalization of

electrons on the conjugate base, therefore proton concentration increases with higher

ionization constant results in increased acidity.