Can you really assign something that doesn't have an acidic proton a pka? Like you know how you said Dipea without the H has a pka of 50+, wouldn't it be more correct to give something that doesn't have a proton a pkb instead?
Ok so pka of aniline ( neutral nh2) is 30, pka of its conjugate acid is 4.6ish so NH3(+)
pKa and pKb are related to each other. For water, pKa is the negative log of [H
+]. pKb is the neg log of [HO
-]. If the pH is low (very acidic) the hydroxide concentration will be low.
pKa is the general convention for measuring the acidity and because they are related, the basicity. The conjugate base of a weak acid can deprotonate a stronger acid. LDA can deprotonate an ester or ketone, but not an alkane. We know that from the pKa's of the acids, diisopropylamine 36, ester 26, and ethane 50.
Theoretically, any proton can have an acidity, that is, how strong is the proton-electron pair bond? The further the electron pair is pulled from the proton, the weaker will be the bond and the more acidic the acid. If you increase the nuclear charge, it will increase the nuclear field to pull electrons toward it and it will also increase the nuclear-proton force repelling the proton. In my opinion, this simple physics, the inverse square law. This will explain why methane has a long bond and is a weak acid and HI has a long bond and is a strong acid. It isn't the nucleus-proton distance that matter (as much) as the proton-electron pair distance.
When reading pKa tables, one must use caution as to how they maybe written. The structure of an amine may be given, ammonia, but the pKa may state or have a footnote explaining it is actually for the conjugate acid of ammonia, NH4
+. So a compound like aniline can have two pKa values, one for aniline (~30) and one for anilinium ion (~4). Typographically, it is easier and less confusing (for some) to list the pKa of the conjugate acid while showing the structure or formula of the base. Presumably, the electrons of the ammonium nitrogen are unaffected by the counter ion. Therefore, separate entries are not required for ammonium chloride, bromide, iodide, bisulfate, triflate, etc.