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

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The discussion centers on the pKb values of various organic bases, specifically amines, and the observed trend of decreasing pKb with longer alkyl chains. However, a notable anomaly occurs with n-Butylamine (CH3CH2CH2CH2NH2), where the pKb value increases to 3.39, which contradicts the expected trend. This discrepancy is further complicated by conflicting pKb values found on different websites, with one source listing n-Butylamine's pKb as 10.78 at 20 degrees Celsius. The conversation suggests that the differences in pKb values may stem from errors in labeling pKa values as pKb and highlights the complexities of accurately predicting pKa/pKb values due to factors like solvation effects and entropy, which influence the measurements beyond simple protonation/deprotonation energies.
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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 \Delta H 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.
 

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