Determining correct pKas in organic chemistry

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Discussion Overview

The discussion revolves around determining the correct pKa values for various organic compounds, specifically in the context of a homework problem from an exam. Participants explore the reasoning behind pKa values and their implications in organic chemistry.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Joanna presents a homework problem asking which set of pKa values is correct, expressing confidence in answer #4 based on lecture materials but seeking a logical reasoning approach to arrive at the answer.
  • One participant asserts that HCl must have a negative pKa, prompting a request for justification.
  • Another participant suggests that since HCl is a strong acid, it should indeed have a negative pKa, questioning the relationship between pKa and acidity.
  • A different participant encourages others to manipulate the pKa equation to verify their understanding.
  • One participant shares a general rule learned in organic chemistry regarding the relationship between pKa values, stating that strong acids have low pKa values and strong bases have high pKa values, with weak acids being close to the pKa of water (approximately 16).

Areas of Agreement / Disagreement

Participants express varying levels of agreement regarding the nature of pKa values, particularly in relation to strong acids like HCl. There is no consensus on the specific pKa values presented in the homework problem, and the discussion remains exploratory.

Contextual Notes

Participants reference pKa values and their implications without providing definitive calculations or sources, leaving some assumptions and dependencies on definitions unresolved.

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Homework Statement


This is a problem taken from an old exam:
C. Which set of pKa’s is correct?
1. CH3COCH3 -7.0 2. CH3COCH3 -19.0
CH3COOH 4.8 CH3COOH -4.8
HCl 19.0 HCl 7.0

3. CH3COCH3 4.8 4. CH3COCH3 19.0
CH3COOH 7.0 CH3COOH 4.8
HCl 16.0 HCl -7.0


Homework Equations


None that I know of.


The Attempt at a Solution


I know from looking at the pKa values we were given in the lecture than the right answer is #4. Nevertheless, I was wondering if there was a way to get to this answer without learning the pKas by heart (a reasoning that would make the question logical).

Thank you,

Joanna.
 
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HCl has to have a pKa that is negative, tell me why.
 
The higher the pKa, the more basic a substance is. HCl is a strong acid, therefore it should be negative... correct?
 
Yep, tweak the pKa equation and see for yourself.
 
A general rule that I learned in Ochem...know water (about 16)...strong bases will have a high pKa and a strong acids will have a low one. Weak acids will be close to water.
 

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