I was looking for the $\mathrm{p}K_\mathrm{a}$ of imidazole and found this $\mathrm{p}K_\mathrm{a}$ table [1]:
\begin{array}{ll} \text{Compound} &\mathrm{p}K_\mathrm{a} \\ \hline \text{Imidazolium} &7.38 \\ \text{2-Aminoimidazolium} &8.46 \\ \text{1-Methyl-2-aminoimidazolium} &8.65 \\ \text{4,5-Dimethyl-2-aminoimidazolium} &9.21 \end{array}
Wouldn't you predict totally inverse order of $\mathrm{p}K_\mathrm{a}$ values?
For example, $\ce{-NH2}$ is a donor by resonance, it will make the cation more stable than imidazole, as far as I know. So, how to understand those values of $\mathrm{p}K_\mathrm{a}$?
I upload the first two compound structures:
IMIDAZOLE
AMINOIMIDAZOLE
- Storey, B. T.; Sullivan, W. W.; Moyer, C. L. The Journal of Organic Chemistry 1964, 29 (10), 3118–3120. DOI 10.1021/jo01033a537.
Edit
Only for completeness I should add that pka values are of the protonates species, I mean imidazolium, 2-aminoimidazolium, and so on..