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Why is pyridine so much more acidic than other similar imines (5.5 versus ~9, see picture)? Wouldn't delocalization of the positive charge into the aromatic system stabilize it and hence decrease acidity? Also the lone pair of pyridine is not delocalized into the perpendicular $\pi$-system and hence not any less available than the one of the projected imine.

Organic Chemistry by Clayden et al. p. 1149.

(Source: Organic Chemistry by Clayden et al. p. 1149. )

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  • $\begingroup$ For future visitors, the corresponding passage in the 2nd edition is at the top of p 726. $\endgroup$ – orthocresol Feb 27 '17 at 15:47
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This is not a definitive answer to the question but seems to at least give the insight that the premise might not be true (i.e. pKa of typical 6-membered cyclic imine is ~9). I decided to e-mailed the main author of the book that made the above claim, prof. Jonathan Clayden. He kindly replied (if you read, many thanks!) the following:

I have been trying to find the original data for the statement that the cyclic imine (or rather its conjugate acid) has a pKa of around 9. Typically, imines have a pKa between 5 and 7, so this seems unusually high for a start. If this number is wrong then it means that pyridine may have quite a typical pKa for a cyclic imine of this type. I've not had time for a detailed search but I suspect this number may not be fully accurate. Imine pKas are notoriously hard to determine because of their hydrolytic instability.

If any additional information comes up, I will edit this answer accordingly.

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  • $\begingroup$ +1 for going to source for more information! Hopefully that might cause errata and an update in a future edition. $\endgroup$ – MaxW Feb 27 '17 at 18:22
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Why is pyridine so much more acidic than other similar imines?

When we discuss the acidity of a compound, we are really discussing where the equilibrium between the acid and its conjugate base lies. Therefore, when we compare acidities we are really comparing equilibria, the relative stabilities of the reactant (acid) and product (conjugate base).

The following diagram illustrates the equilibrium between the iminium ion and the corresponding imine, it's conjugate base.

enter image description here

We can't draw any good resonance structures for the imine, but we can draw 2 resonance structures to describe the iminium ion, A and B in the figure. In the pyridinium ion - pyridine case we can draw 2 resonance structures for pyridine (just like in benzene), but we can't draw any resonance structures for the pyridinium ion that delocalize the positive charge without destroying the aromaticity of the ring.

So, in the iminium ion - imine case

  • we can stabilize the iminium ion through resonance - this stabilizes the iminium ion and shifts the equilibrium to the left making the iminium ion less acidic.

Whereas in the pyridinium ion - pyridine case

  • we can't draw good resonance structures that stabilize the positive charge in the pyridinium ion - this makes pyridinium ion more acidic.

Both factors operate in the same direction and tend to make the pyridinium ion ($\mathrm{p}K_\mathrm{a}=5.5$) more acidic than the iminium ion ($\mathrm{p}K_\mathrm{a}$ $\sim$ 9).

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    $\begingroup$ (Just a minor detail.) Regarding your opening paragraph, the authors make it very clear early on in the book that pKa refers to the pKa of the conjugate acid, so there is a bit of context missing in the picture above. $\endgroup$ – orthocresol Feb 27 '17 at 15:42
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One interpretation of acidity is the ability to stabilize a negative charge (since the electron is generally kept by the acid, and the proton harbors the positive charge, if we go by bronsted lowry)

Thus, the negative charge is delocalized and thus stabilized. Its impact is not felt as greatly on one atom, and therefore it is more stable, not seeking a proton as much.

Also, the negative charge is better for nitrogen, an electronegative atom which can better stabilize it (but this doesn't explain the difference. I think the main difference is resonance)

This website may help:http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/heterocyclen/arom_gessaet_heterocyclen.vlu/Page/vsc/en/ch/12/oc/heterocyclen/sechsringaromat/sechsring_aromat.vscml.html

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  • $\begingroup$ @Jori is talking about acidity of cation (which he should mention specifically). $\endgroup$ – Mithoron Apr 5 '15 at 22:01

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