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

  • Thread starter Thread starter cncbmb
  • Start date Start date
  • Tags Tags
    Change
AI Thread Summary
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.
cncbmb
Messages
16
Reaction score
0
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)
 
Chemistry news on Phys.org
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.
 

Thread 'Solubility of MgSO47H2O in water'

It seems like a simple enough question: what is the solubility of epsom salt in water at 20°C? A graph or table showing how it varies with temperature would be a bonus. But upon searching the internet I have been unable to determine this with confidence. Wikipedia gives the value of 113g/100ml. But other sources disagree and I can't find a definitive source for the information. I even asked chatgpt but it couldn't be sure either. I thought, naively, that this would be easy to look up without...
View full post »

Thread 'Why does having 8 valence electrons make an element inert?'

I was introduced to the Octet Rule recently and make me wonder, why does 8 valence electrons or a full p orbital always make an element inert? What is so special with a full p orbital? Like take Calcium for an example, its outer orbital is filled but its only the s orbital thats filled so its still reactive not so much as the Alkaline metals but still pretty reactive. Can someone explain it to me? Thanks!!
View full post »

Hot Threads

Recent Insights

Back
Top