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Why amino acids as Zwitterons in solid state and ph neutral?

  1. Oct 6, 2013 #1
    Hi forum goers

    I'm reviewing some coursework for my General Chemistry class and cannot seem to find a reasonable explanation of why amino acids exist as zwitterons in solid state and ph7 solutions. I'm fairly certain the explanation has to do with its solubility in water, but I am not sure.

    Here is my thought process.
    Amino acids tend to exist as zwitterons because when in the solid state, there is no reaction for the positive charge(i think positive charge) to be pulled from the amino acid to the water.

    And in neutral ph's amino acids exist because of the balance of concentration of OH- and H2O, or similar idea of balance of concentration. It has something to do with equilibrium constant. I don't really know.

    The next (and last topic) on my objectives sheet says to know how to write Lewis structures for amino acids as they exist in low pH and high pH.
    I believe this is a further extension of why amino acids exist as zwitterons in neutral pH. I will tackle this one after getting clarification.

    Thanks for looking, have a good one!
    Any clarification is great, thank you!
  2. jcsd
  3. Oct 7, 2013 #2
    The carboxyl group on an amino acid generally has a pKa of around 2, while the amino group has a pKa around 9. Remember, when pH=pKa, the ionizable group exists half in its protonated form, and half in its deprotonated form. The deprotonated form of the carboxyl group is COO-; the deprotonated form of the amino group is NH2. At a pH above the pKa, the group exists primarily in its deprotonated from.
    That being said, at pH 7, which is above the pKa of the carboxyl group, but below the pKa of the amino group, the amino acid will exist in a zwitterionic state containing both a COO- and an NH3+ group.
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