November 28, 2021, 06:01:29 PM
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Topic: Question on pKa change with addition of a carboxyl group  (Read 420 times)

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Offline peachy-peachy

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Question on pKa change with addition of a carboxyl group
« on: September 16, 2021, 03:51:48 PM »
The amino acid glycine has an carboxyl group, amino group (NH3), and 2 hydrogens attached to the alpha carbon. If an additional adjacent carboxyl group were added, would the pKa of the other carboxyl group increase or decrease?

I was thinking it would decrease since I know that dicarboxylic acids tend to generally be more acidic than just 1 carboxylic acid by itself. However, I'm not sure how the presence of the NH3 group affects this. I know it is also electron withdrawing, but oxygen beats out nitrogen in terms of electronegativity, so I don't know if that makes a difference here.

Offline rolnor

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Re: Question on pKa change with addition of a carboxyl group
« Reply #1 on: September 16, 2021, 04:29:13 PM »
This is aminomalonic acid, you can probably find the pKa in the literature.
Here is a link:
https://wap.guidechem.com/encyclopedia/propanedioic-acid-2-amino--dic373029.html

I dont find the pKa, you need Scifinder etc.

Offline peachy-peachy

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Re: Question on pKa change with addition of a carboxyl group
« Reply #2 on: September 16, 2021, 04:37:22 PM »
This is aminomalonic acid, you can probably find the pKa in the literature.
Here is a link:
https://wap.guidechem.com/encyclopedia/propanedioic-acid-2-amino--dic373029.html

I dont find the pKa, you need Scifinder etc.

The pKa of the carboxyl group (without the additional carboxyl group) and just the amino acid by itself is 2.34. But would the 2.34 increase or decrease in the presence of an adjacent carboxyl group?

Offline Babcock_Hall

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Re: Question on pKa change with addition of a carboxyl group
« Reply #3 on: September 16, 2021, 08:59:32 PM »
How does the presence of the ammonium group affect the pKa in glycine itself?

Offline rolnor

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Re: Question on pKa change with addition of a carboxyl group
« Reply #4 on: September 17, 2021, 01:04:08 AM »
Here is malonic acid, the second proton is not easy to pull because you get two charges so close together:

https://en.m.wikipedia.org/wiki/Malonic_acid

Here is glycine

https://en.m.wikipedia.org/wiki/Glycine

And acetic acid:
https://en.m.wikipedia.org/wiki/Acetic_acid

As you can se, malonic acid is much more acidic than acetic acid


Offline Babcock_Hall

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Re: Question on pKa change with addition of a carboxyl group
« Reply #5 on: September 17, 2021, 10:12:11 AM »
@OP, Now compare acetic acid and glycine.  I would think about electrostatics.

Offline VP_JQ

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Re: Question on pKa change with addition of a carboxyl group
« Reply #6 on: September 17, 2021, 06:15:03 PM »
Hi!

Isn't it possible an intramolecular Hydrogen bond between both carboxyl groups? I think, in that way, one charge will stabilize much more and pKa will decrease...

Offline rolnor

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Re: Question on pKa change with addition of a carboxyl group
« Reply #7 on: September 18, 2021, 08:42:36 AM »
Yes, thats possible, maybe this can be observed in a proton-NMR spectrum? The hydrogen bond would be strong and the 1H-shift would probably be downfield a lot.

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