Why is there a change in trend for pKb for n-butylamine?

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Discussion Overview

The discussion centers on the observed trend in pKb values for n-butylamine and other organic bases, specifically addressing why the trend appears to break for n-butylamine compared to shorter-chain amines. Participants explore discrepancies in reported pKb values from different sources and the complexities involved in determining these values.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants note that pKb values for organic bases generally decrease with longer alkyl chains, but question why n-butylamine (CH3CH2CH2CH2NH2) shows an increase in pKb to 3.39.
  • There is mention of a significant discrepancy in pKb values for n-butylamine, with one source listing it as 10.78, leading to confusion about the accuracy of these values.
  • One participant suggests that the differences in pKb values may stem from the complexities of solvation effects, entropy, and the relative auto-dissociation of water, indicating that pKa/pKb values are more complicated than they seem.
  • Another participant expresses frustration with the inconsistency in reported values, attributing it to potential errors in naming conventions between pKa and pKb.

Areas of Agreement / Disagreement

Participants express differing views on the reasons behind the break in trend for n-butylamine, with no consensus reached regarding the accuracy of the reported pKb values or the underlying reasons for the observed discrepancies.

Contextual Notes

Participants acknowledge that exact pKa/pKb values can be difficult to predict theoretically and may depend on various factors, including solvation effects and experimental conditions.

cncbmb
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I read that pKb for the organic bases below generally decreases as the alkyl chain gets longer:

http://www.chemguide.co.uk/basicorg/acidbase/bases.html#top
pKb
CH3NH2 3.36
CH3CH2NH2 3.27
CH3CH2CH2NH2 3.16
CH3CH2CH2CH2NH2 3.39

Why does the pattern break for CH3CH2CH2CH2NH2?

Why is that we find such a different pKb number when we search "n-Butylamine" on this website? http://www.sanderkok.com/techniques/laboratory/pka_pkb.html (it says pKb=10.78 for 20 degrees C which is much different than 3.39 on chemguide.co.uk)
 
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cncbmb said:
Why is that we find such a different pKb number when we search "n-Butylamine" on this website? http://www.sanderkok.com/techniques/laboratory/pka_pkb.html (it says pKb=10.78 for 20 degrees C which is much different than 3.39 on chemguide.co.uk)

Because they are idiots that list pKa values naming them pKb.
 
pKb
CH3NH2 3.36
CH3CH2NH2 3.27
CH3CH2CH2NH2 3.16
CH3CH2CH2CH2NH2 3.39

Why does the pattern break for CH3CH2CH2CH2NH2?
 
cncbmb said:
Why does the pattern break for CH3CH2CH2CH2NH2?

No simple reason I'd suspect. The differences are fairly small, and exact pKa/b values are notoriously difficult to predict theoretically.

Put it this way: the enthalpy [tex]\Delta H[/tex] of protonation/deprotonation for a single molecule in vacuum probably does follow a fairly regular trend there. But the pKa/pKb values include the solvation effects, entropy and all that, and does so relative the auto-dissociation of water. So pK values are actually a bit more complicated and unpredictable than they might appear from simply thinking in terms of the protonation/deprotonation energy.
 
Thanks.