Why Do Carboxylic Acids Deprotonate?

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Discussion Overview

The discussion revolves around the deprotonation of carboxylic acids in neutral water, particularly focusing on the relationship between pKa values and pH levels. Participants explore the implications of these values on the protonation state of carboxylic acids and amino groups, as well as the concept of zwitterions.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the charge states of carboxylic acids and amino groups in neutral water, questioning the interpretation of pKa values and their implications for protonation.
  • Another participant provides the relationship between pKa and pH, indicating that the majority of a species is protonated when pH is less than pKa, and deprotonated when pH is greater than pKa.
  • A question is raised regarding the zwitterionic form of amino acids at neutral pH, suggesting a connection to the discussion on carboxylic acids.
  • A participant acknowledges the complexity of the situation, noting that the presence of hydroxyl groups at pH 7 contributes to the deprotonation of carboxylic acids and relates this to the concept of zwitterions.

Areas of Agreement / Disagreement

Participants exhibit some agreement on the relationship between pKa and pH, but there remains confusion and differing interpretations regarding the implications of these values for the protonation states of carboxylic acids and amino groups.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the behavior of carboxylic acids in water, the definitions of protonation states, and the specific conditions under which these behaviors are observed.

Racer77
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Can someone explain to me why the carboxylic acid group and the amino group both have charges on them in neutral water? The pKa for carboxylic acid is around 2-5 depending on its environment. I am confused that a pKa of 2.3 would give a ratio of 80000 deprotonated to 1 protonated? If the acid has a pKa it means it is weak, a pKa of 2.3 gives a concentration lower than 1. I assume the majority would be protonated. But all info I read states that the majority is deprotonated at a pH of 7. Can someone explain why to me it is this way? Thank you!
 
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pK_{a}-pH=log{{[HA]}\over{[A^{-}]}}

The majority is protonated when pH<pK_{a}.

The majority is deprotonated when pH>pK_{a}.
 
Last edited:
Are you asking why an amino acid is a zwitterion at neutral pH?
 
Thanks for the replies! I was having problems with looking at everything relative, because of course if you put a carboxyl group in water, it will make a low pH. What i did not understand is that a pH of 7 has so many hydroxyl groups to completely deprotonate the acid. So yeah it had to do with zwitterions too. Thank you guys.
 

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