for Histidine,
why Rgroup Basic group(which is more
basic) has smaller pka , which in turn has bigger pkb value , than
alpha Basic group ?
I suspect you still won't understand this after you read my post.
In which case, go ask your private tutor or school teacher. Don't
waste my time.
The R group iminium is less acidic (ie. smaller Ka or larger
pKa) than the alpha COOH, but more acidic (ie. larger Ka or smaller
pKa) than the alpha NH3+.
The R group imine is more basic (ie. larger Kb or smaller pKb)
than the alpha COO-, but less basic (ie. smaller Kb or larger pKb)
than the alpha NH2.
Bonus : For a challenging A grade question, Cambridge can ask
you to *explain* (ie. not just assign values without explanation)
why is it the case that for Histidine (unusual among alpha amino
acids), the R group imine is more basic (ie. larger Kb or smaller
pKb) than both the alpha COO- and the R group amine, and why the R
group imine is less basic (ie. smaller Kb or larger pKb) than the
alpha amine. Or conversely, explain the relative acidities of the
conjugate acids of these (abovementioned) groups and their Ka or
pKa values.
My BedokFunland JC students can check their answers with me
during their next tuition session, while the rest of you can go ask
your school teacher or your own private tutor. I won't reveal the
answers here.
