pK of reaction Imidazol -> Imidazol- + H+ (?)
Fred Buchholz
(Andrey Bliznyuk) wrote:
> Dear Chemists,
>
> I am looking for the constant of the reaction of dissociation of Imisazol in
> water ( Imidazol -> Imidazol anion + H+ ). Any ideas where to look
> are appreciated. Can't find any data in usual textbooks - only protonation
> of Imidazol.
>
> Andrey
The 75th Ed. of the CRC Handbook of Chemistry and Physics lists the pKa of
1H-imidazole as pKa=6.953 at 25 C.
Also many other amines, etc. under the index heading Dissociation Constants.
--
Fred Buchholz
Midland, Michigan
John Bartmess
Andrey Bliznyuk's <Andrey....@anu.edu.au> question:
AB:
> I am looking for the constant of the reaction of dissociation of
> Imisazol in water ( Imidazol -> Imidazol anion + H+ ). Any ideas
> where to look are appreciated. Can't find any data in usual
> textbooks - only protonation of Imidazol.
>The 75th Ed. of the CRC Handbook of Chemistry and Physics lists the
> pKa of 1H-imidazole as pKa=6.953 at 25 C.
Wrong reaction. That's for imidizolium (M+H+) ion going to H+ and
neutral imidazole.
In DMSO solvent: pKa = 18.6,
Bordwell, F.G. _Acc. Chem. Res._ *1988* _21_, 456.
This is *not* directly extrapolatable to water. Some pKas increase
by up to ca. 5 pK units on going from DMSO to water, some decrease
by a like amount, and some don't change. I suspect that imidazole is
too weak and acid to be appreciably deprotonated by hydroxide in
water.
John
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