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I am supposed to say what is more basic and I ran into a problem. I would like to ask if anyone has any idea what is the correct $\mathrm{p}K_\mathrm{a}\ce{H}$ value of pyrrole. I know it is easy to just google it or open a book to find the answer. I have done that, but got pretty interesting (confusing?) results:

Wikipedia: "Pyrrole is weakly basic, with a conjugate acid $\mathrm{p}K_\mathrm{a}$ of (−3.8)"

Human metabolome database (HMDB): "Pyrrole is a very weak base with a $\mathrm{p}K_\mathrm{a}\ce{H}$ of about (4)"

PubChem: "Pyrrole is a very weak base with a $\mathrm{p}K_\mathrm{a}\ce{H}$ of about (4)"

McMurry 9th edition: "pyrrole - $\mathrm{p}K_\mathrm{a}$ of ammonium ion: (0,4)"

University of Calgary: "pyrrole conjugated acid $\mathrm{p}K_\mathrm{a}$: (-3,8)"

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  • $\begingroup$ related chemistry.stackexchange.com/questions/48499/… $\endgroup$ – Mithoron Nov 16 at 22:29
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    $\begingroup$ Higher data are probably in DMSO. $\endgroup$ – Mithoron Nov 16 at 22:35
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    $\begingroup$ BTW while Pubchem repeats what metabolome says it gives -3.8 value in separate section. I wager pKa you seek is about -3.8 to -4; which are like exact same thing for stuff with negative pKa. Quite possibly in DMSO value may be about +4. $\endgroup$ – Mithoron Nov 16 at 22:41
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Op's question is what is the correct $\mathrm{p}K_\mathrm{a}\ce{H}$ value of pyrrole. Thus, I'm not going to elaborate OP's findings, but would try to give a reasonable answer to the question. The most reasonable answer I found for $\mathrm{p}K_\mathrm{a}\ce{H}$ value for the pyrrole is $0.4$ (for the novices, this is the $\mathrm{p}K_\mathrm{a}$ of the conjugate acid of pyrrole).

It is well known that the best way to quantify the basicity of an amine is to examine the $\mathrm{p}K_\mathrm{a}$ of its conjugate acid (Master Organic Chemistry). The concept is the higher the $\mathrm{p}K_\mathrm{a}$ of the conjugate acid, the stronger the base.

Let's compare two cyclic amine bases under this concept: Pyridine and Pyrrole. The following diagram illustrate the concept:

Pyrrole and Pyridine

Accordingly, pyrrolium ion is about $10^5$ times stronger acid than pyridinium ion. Thus, pyrrole is about $10^5$ times weaker base than pyridine. Why? Read the note on the bottom of the box in the diagram. This is also well explained in chemistry.msu.edu with the diagram attached:

In each case the heteroatom has at least one pair of non-bonding electrons that may combine with the four π-electrons of the double bonds to produce an annulene having an aromatic sextet of electrons. This is illustrated by the resonance description at the top of the following diagram. The heteroatom Y becomes $\mathrm{sp^2}$-hybridized and acquires a positive charge as its electron pair is delocalized around the ring. An easily observed consequence of this delocalization is a change in dipole moment compared with the analogous saturated heterocycles, which all have strong dipoles with the heteroatom at the negative end. As expected, the aromatic heterocycles have much smaller dipole moments, or in the case of pyrrole a large dipole in the opposite direction. An important characteristic of aromaticity is enhanced thermodynamic stability, and this is usually demonstrated by relative heats of hydrogenation or heats of combustion measurements. By this standard, the three aromatic heterocycles under examination are stabilized, but to a lesser degree than benzene. Additional evidence for the aromatic character of pyrrole is found in its exceptionally weak basicity ($\mathrm{p}K_\mathrm{a}$ ca. $0$) and strong acidity ($\mathrm{p}K_\mathrm{a} = 15$) for a $2^\circ$-amine. The corresponding values for the saturated amine pyrrolidine are: basicity $11.2$ and acidity $32$.

Pyrrole & Furans Dipoles

Furthermore, for everybody's convenience, I attached a orbital representation of pyrrole and furan to show their resistance to protonation.

Comment on different $\mathrm{p}K_\mathrm{a}$ for pyrrole in literature: The $\mathrm{p}K_\mathrm{a}$ value of $3.8$ (and $\approx 4$) is probably for the protonation of pyrrole at $\ce{C2}$ position. For protonation at $\ce{N}$, $\mathrm{p}K_\mathrm{a}$ is most probably $0.4$, as described above. We can trust the value given in McMurray's Organic Chemistry textbook, since it is widely used in sophomore organic chemistry university course in USA.

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  • $\begingroup$ That's a lot of writing for an answer that's not proving anything at all. 1) Did you see that there was "-" 3.8? 2) Any value in literature wouldn't be for protonation on nitrogen, because it's unfavourable 3) McMurry is a nice textbook for elementary org. chem. but hardly a proof for anything. Even a paper may have a wrong value. 4) For such well reasearched question I'd expect at least something like that: chemistry.stackexchange.com/a/85198/9961 to actually answer anything. $\endgroup$ – Mithoron Nov 17 at 18:10
  • $\begingroup$ @ Mithoron: I have given the answer in my first paragraph. Also, I said I don't want to elaborate wrongful information given in internet as usual. This answer is for a elementary organic chemistry student so do not need any literature proof; commonly used textbook is good enough. Finally, if you have a good answer, why don't you share with community? $\endgroup$ – Mathew Mahindaratne Nov 17 at 20:23
  • $\begingroup$ I already said what I think in comments. What makes you think it's for "elementary organic chemistry student" and one like that should just trust his textbook? If this site has any penchant, it's debunking textbook mistakes, often with experimental data. Also you seem to not take my 2) which is about factual error... $\endgroup$ – Mithoron Nov 17 at 21:18

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