[jd3005]

Hi!   I guess this is my first post here, however I am having some trouble on an assignment that was preceded by confusion that my professor may have caused.  I don't think asking for exact answers means that I learnt anything, so I just want to ask about concepts.

Firstly!   I guess this would be very intro level biochemistry that was likely covered in Gen Chem, BUT seeing as I have been away from school for a short while I need a little help.

FIRSTLY.  We were discussing titrations of amino acids and different forms existing at each plateau.  My professor made me understand that each intermediate between plateaus would be when there is a change in predominant species.  However, upon reading my text, I had always believed that the two species existed in an equilibrium that shifted as the pH of the solution changed.   I also thought that at the point where pKa values can be determined was exactly when the species were present at the same concentration.    This makes sense to me because if you were to move pH left or right even just a little bit, then one of the species would be more prevalent in solution, is that correct?   Is my thinking off-base?  If my thinking is not off-base, would there be two species at each plateau? and points between two (steep parts of curve) would be points where there is a single species present?

Secondly.   I have a compound and I was just told to find functional groups, which I was able to.    It has a very cyclic appearance, but would it not be cyclic if it contained an ether connecting two carbons?  Also one of the carbons it connects to has a CH2OH group, which would likely be the start of the chain if it isn't connected.  Or is CH2OH a functional group?

[Borek]

My professor made me understand that each intermediate between plateaus would be when there is a change in predominant species.

I am not entirely sure what he means, which probably means his description is ambiguous and open to interpretation. But in a way he is right - say we have a system

H₂A⁺ HA A^-

Before the first inflection point system is dominated by the H₂A⁺/HA, after it is dominated by the HA/A^-. So the pair that dominates the behavior of the plateau changes (not the single form).

However, upon reading my text, I had always believed that the two species existed in an equilibrium that shifted as the pH of the solution changed.

All species, not just two of them.

I also thought that at the point where pKa values can be determined was exactly when the species were present at the same concentration.

The two species involved in a single protonation reaction.

This makes sense to me because if you were to move pH left or right even just a little bit, then one of the species would be more prevalent in solution, is that correct?   Is my thinking off-base?  If my thinking is not off-base, would there be two species at each plateau? and points between two (steep parts of curve) would be points where there is a single species present?

There are no places where there is a single form present alone, but yes - at inflection point concentration of one form is typically higher than concentrations of all other forms. By how much depends on the pKa differences between forms.

Secondly.   I have a compound and I was just told to find functional groups, which I was able to.    It has a very cyclic appearance, but would it not be cyclic if it contained an ether connecting two carbons?  Also one of the carbons it connects to has a CH2OH group, which would likely be the start of the chain if it isn't connected.  Or is CH2OH a functional group?

Seeing the compound would help immensely. You can use SMILES to enter the structural formula (http://www.chemicalforums.com/index.php?topic=59314.msg230001#msg230001).