Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: A Tree on May 28, 2010, 03:50:23 AM
-
I wonder whether two pyridine molecules can be linked by a H+ ion so as to form a dimer in acidic aqueous solutions.
-
http://en.wikipedia.org/wiki/Hydrochloride
-
http://en.wikipedia.org/wiki/Hydrochloride
I knew that pyridine tends to be protonated at the presence of H+. The pKa value of pyridinium ion (the protonated form of pyridine) is 5.21 (see http://en.wikipedia.org/wiki/Pyridine), which means pyridinium and pyridine are both available in the solution and their concentration is equal at such pH value. The problem is I'm not sure if hydrogen bondings will form between pyridinium ions and a pyridine molecules.
-
pKa acetic acid 4.77 pKb amonia 4.75 ammonium acetate rather unstable
pKa PyrH+ 5,21 pKb pyridine 8,79
What about the stability of (Pyr)2H+ ?
-
I wonder whether two pyridine molecules can be linked by a H+ ion so as to form a dimer in acidic aqueous solutions.
With the followup, this is almost two questions. The answer to the first question is that it would be possible, but as specified, no. In an acidic aqueous medium, the pyridine molecules would be individually protonated with the excess acid.
Can one form (Pyr)2H+ ? I presume that if one took a pyridinium salt and added a mole of pyridine to it, then one could find some pyridine hydrogen bonded to the proton of a protonated pyridine.
If you added a great molar excess of water to it, would the water disrupt the hydrogen bonded dimer? I presume it would, but I wouldn't know to what extent. I am also uncertain how one would answer this question definitively. How do you measure hydrogen bonding and hydrogen bonding species in water?
-
pKa acetic acid 4.77 pKb amonia 4.75 ammonium acetate rather unstable
pKa PyrH+ 5,21 pKb pyridine 8,79
What about the stability of (Pyr)2H+ ?
I wonder whether two pyridine molecules can be linked by a H+ ion so as to form a dimer in acidic aqueous solutions.
With the followup, this is almost two questions. The answer to the first question is that it would be possible, but as specified, no. In an acidic aqueous medium, the pyridine molecules would be individually protonated with the excess acid.
Can one form (Pyr)2H+ ? I presume that if one took a pyridinium salt and added a mole of pyridine to it, then one could find some pyridine hydrogen bonded to the proton of a protonated pyridine.
If you added a great molar excess of water to it, would the water disrupt the hydrogen bonded dimer? I presume it would, but I wouldn't know to what extent. I am also uncertain how one would answer this question definitively. How do you measure hydrogen bonding and hydrogen bonding species in water?
Thank you, AWK and Orgopete. In fact, I'm doing an experiment based on pyridine aqueous solutions and I need to know whether hydrogen bondings play a role in the system. Through the discussions, it seems to me that hydrogen bondings between PyrH+ and Pyr could be ruled out. Thanks again. :D
-
Probably you will not be able to isolate such a specie from water solution. However 2,2'-bipyridyl hydrochloride can be isolated