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Thread: Imadizole ring question

  1. #1 Imadizole ring question 
    Forum Freshman
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    Hi,

    Does anyone know if the 2 nitrogens on an imadizole ring can both be ionised at once? I am guessing that protonating one of the nitrogens would effectively 'cancel' the other nitrogen. Is this correct? If only one can be ionised, is it always the same one?

    I'm asking because I need to know 'how many ionisble groups histamine has'?

    Any help would be greatly appreciated.
    Thank in advance.


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  3. #2  
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    My guess is that the answer to ionizing both groups at once is "probably not." The imidazole ring normally exists in several tautomeric forms stabilized by resonance. You can protonate either nitrogen and still maintain some resonance, but the other nitrogen atom becomes more positive and thus resists further protonation.

    But you can consider that histidine (or your histamine) has two ionizable groups even though only one nitrogen atom at a time can acquire a proton.

    Again, I'm only giving you an educated guess. Determine for yourself if this answer meets your needs.

    *


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  4. #3  
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    Hi Steve,

    The imidazole ring normally exists in several tautomeric forms stabilized by resonance. You can protonate either nitrogen and still maintain some resonance, but the other nitrogen atom becomes more positive and thus resists further protonation.
    Thanks, that perfect sense. If I have to explain it, I'll write this down. Markers won't know what hit them

    Again, I'm only giving you an educated guess. Determine for yourself if this answer meets your needs.
    The teacher was kind of hinting it would be on an exam I have tommorow, that's all. Since you said either nitrogen can be protonated I'm guessing I will be OK if I say it has 3 ionisble groups (there is also an amino group on histamine).

    Thanks again for your help. Much appreciated
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