Chapter 27.1, Problem 3P

Explain why the $\text{ p}{K}_{\text{a}}$ of the $-{\text{NH}}_3^{+}$ group of an $\text{α}$-amino acid is lower than the $\text{ p}{K}_{\text{a}}$ of the ammonium ion derived from a $1°$ amine $\left({\text{RNH}}_3^{+}\right)$ is to be stated. For example the $\text{ p}{K}_{\text{a}}$ of the $-{\text{NH}}_3^{+}$ group of an alanine is $9.87$ but the $\text{ p}{K}_{\text{a}}$ of the ${\text{CH}}_3{\text{NH}}_3^{+}$ is $10.63$.