Download Chem 410 Chapter 11: Polyprotic Acids and Bases Part 1 How

Chem 410 Chapter 11: Polyprotic Acids and Bases Part 1
How common are they?
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In addition to common polyprotic acids such as carbonic acid, phosphoric acid, phthalic acid, oxalic
acid, lactic acid, citric acid, maleic acid, and many more, amino acids and therefore proteins are
polyprotic acids.
19 of the 20 amino acids have the following structure, where the R group varies with the different
amino acid:
Although we can draw the amino acid structure as on the left, it doesn’t exist in pure water of pH 7.
At a neutral pH, the amino acid undergoes an intramolecular acid/base reaction between the amine
group (NH2) and the carboxylic acid group.
So the carboxylic acid proton is deprotonated while the amine group is protonated as NH3+. This is
shown in the figure on the right.
The result is a dipolar compound ( one + and one – charge) which is overall a neutral compound.
This type of compound is called a zwitterion.
But these amino acid zwitterions are salts, actually internal salts. So they will have many physical
properties in common with salts. So they are crystals with high melting points, are highly water
soluble (but low solubilities in organic compounds), and they have large dipole moments.
If in the amino acid above is polyprotic, then it is also a polyprotic base.
So to determine what form the amino acid will be in at low, neutral, and high pH, we have to
compare the acidity or basicity of the two “acidic” or “basic” groups. Of course, the R group could
also contain an acidic or basic group to make a triprotic acid. For now, let’s just look at the two
groups above.
The two acidic groups to compare are the –CO2H group and the NH3+ group. These are the acid
groups as they are protonated. So which is stronger or which is weaker?
To answer this, look at the corresponding base groups: -CO2- and NH2. Now the question is which
base is stronger?
In general, the amine group is a stronger base than the carboxylate anion, partly due to the resonance
stabilization of the carboxylate group. So if the amine group is a stronger base, then the NH2 group
will protonate before the carboxylate group protonates.
Of course, the above also means that the carboxylic acid is a stronger acid than the NH3+ group, so
the carboxy acid will deprotonate before the NH3+ group deprotonates.
This explains why at neutral pH the zwitterion form exists.
What will exist at very low pH, when there is an abundance of hydronium ions?
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What will exist at very high pH, when there is an abundance of hydroxide ions?
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What does this say about the zwittterion form? It is both an acid and a base so it is amphiprotic.
The carboxy group is the basic group while the NH3+ group is the acidic group on the zwitterion.
Do you also see that for the typical diprotic amino acid (or any other diprotic acid), we then have a
diprotic acid or diprotic base?
For the typical amino acid, we have the fully protonated acid form H2A+, the zwitterion HA, and the
fully deprotonated basic form A-.
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The Basics of Polyprotic Acids
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Remember the relationship between pKa and pKb for monoprotic acids and their conjugate bases:
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This is also true for polyprotic acids, but you need to be careful to match the correct acid with the
correct conjugate base:
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Your book mentions that for the acid, the a subscript may be dropped, but the b subscript for the
base is never dropped.
Finding pH of Diprotic Systems
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We will focus on calculations for diprotic systems, as triprotic systems and beyond are more
complicated. You DO need to be able to do these calcs!!
So we have a generic diprotic acid H2A with its 2 pKa and 2 pKb values.
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Let’s now take a specific acid, glutamine (You budding docs or pharmacists, what is glutamine good
for, and what is it used for medically?):
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Let’s calculate the pH of a 0.0500M solution of the fully protonated (or fully acidic) glutamine,
H2G+, then the pH of a 0.0500M sln of the zwitterion glutamine (the amphiprotic form), HG, then
finally the pH of a 0.0500M sln of the fully deprotonated (or fully basic) glutamine, G-.
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Calculate the pH of a 0.0500M sln of glutamine hydrochloride (H2G+):
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Summary: A fully protonated diprotic acid behaves as if it were a monoprotic acid, where Ka1
= Ka .
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Calculate the pH of a 0.0500M sln of glutamine (HG): this is the hardest one!
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Calculate the pH of a 0.0500M sln of the fully deprotonated sodium glutaminate (G-):
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Problem: 1) What is the pH of a 0.075M sln of tryptophan hydrochloride? 2) What is the pH of a
0.075 M sln of tryptophan? 3) What is the pH of a 0.075 M sln of sodium tryptophanate?
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Summary of fully basic form: The fully basic form of a diprotic acid acts as though it were a
monoprotic base, B, where Kb1 = Kb.
•Overall Summary for Diprotic Systems:
Treat H2A as a monoprotic acid
Treat A2- as a monoprotic base
Treat HA or HB+ as a mixture with pH 0.5(pKa1 + pKa2)