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Correlating pka and pH with ionic forms (exercises)

  • Thread starter sd34
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Please post this type of questions in the homework section, filling the template.
I have the following molecule: benzotriazole. It has pka=12. At pH=6 which form prevails?

a) neutral
b) anionic
c) cationic
d) none of the above.

I thought the correct answer was c) cationic: pH<pka, benzotriazole has basic functional groups (amines).
 

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mjc123

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I don't know where your figure of 12 comes from. Wikipedia quotes a value of 8.2. Moreover, this is the pKa of the neutral molecule, not the conjugate acid. Benzotriazole behaves very differently from ordinary amines:
"Moreover, the proton does not tightly bind to any of the nitrogen atoms, but rather migrates rapidly between positions 1 and 3. Therefore, the BTA can lose a proton to act as a weak acid (pKa = 8.2)[3][4] or accept a proton using the lone pair electrons located on its nitrogen atoms as a very weak Bronsted base (pKa < 0)." (https://en.wikipedia.org/wiki/Benzotriazole)
People sometimes talk about the "pKa of an amine" when they mean the pKa of its conjugate acid, but this is sloppy, and in this case actually incorrect.
 
9
0
I don't know where your figure of 12 comes from. Wikipedia quotes a value of 8.2. Moreover, this is the pKa of the neutral molecule, not the conjugate acid. Benzotriazole behaves very differently from ordinary amines:
"Moreover, the proton does not tightly bind to any of the nitrogen atoms, but rather migrates rapidly between positions 1 and 3. Therefore, the BTA can lose a proton to act as a weak acid (pKa = 8.2)[3][4] or accept a proton using the lone pair electrons located on its nitrogen atoms as a very weak Bronsted base (pKa < 0)." (https://en.wikipedia.org/wiki/Benzotriazole)
People sometimes talk about the "pKa of an amine" when they mean the pKa of its conjugate acid, but this is sloppy, and in this case actually incorrect.
No.12 was given by our teacher ( this exercise was in the previous exam)...it’s more like hypothetically speaking. They just want to test if we know which form that molecule is in at pH x.

So...I guess in this case the correct answer is d).
 

mjc123

Science Advisor
813
368
In the circumstances, there is no correct answer. I don't know what your teacher was thinking. If it was a normal amine with pKa of conjugate acid 12, it would be cationic. If it was the pKa of the neutral amine (but with the wrong value), the answer would be neutral.
 

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