Question

Question: Explain why the${\mathbf{pk}}_{\mathbf{a}}$of the COOH group of glycine is much lower than the ${\mathbf{pk}}_{\mathbf{a}}$ of the COOH of acetic acid (2.35 compared to 4.8).

Short answer

Answer

The pk_a value of the COOH group of glycine is much lower than that of the COOH group in acetic acid as the conjugate base of glycine is more stable.

Step-by-step solution

Step-by-Step SolutionStep 1: Conjugate base

The species formed by the removal of ${\mathbf{H}}^{\mathbf{+}}$ ion from a molecule is referred to as its conjugate base.

The stability of the conjugate base directly influences the strength of an acid.

The conjugate base of a strong acid is weak, i.e., it is highly stable (less reactive).

Acidity and  pka

The ${\mathrm{pk}}_{\mathrm{a}}$ value of a substance indicates its acidity.

The species having low role="math" localid="1648729969594" ${\mathbf{pk}}_{\mathbf{a}}$ will be more acidic, and the ones having high will be less acidic.

Acidity of glycine

The conjugate base of glycine is more stable than that of acetic acid due to the presence of an electron-withdrawing group. It stabilizes the conjugate base by pulling the excess electron density towards itself.

This makes the acid (glycine) more acidic.

Hence, the ${\mathrm{pk}}_{\mathrm{a}}$ value of the COOH group of glycine is much lower than that of the COOH group in acetic acid.

Glycine and its conjugate base