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

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In summary, the pKb values for the organic bases listed on chemguide.co.uk decrease as the alkyl chain gets longer. However, the pattern breaks for n-Butylamine. When searching for pKb values on a different website, a significantly different value is found. This could be due to the complexities involved in predicting pK values, which include solvation effects and entropy.
  • #1
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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  • #2
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.
 
  • #3
pKb
CH3NH2 3.36
CH3CH2NH2 3.27
CH3CH2CH2NH2 3.16
CH3CH2CH2CH2NH2 3.39

Why does the pattern break for CH3CH2CH2CH2NH2?
 
  • #4
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.
 
  • #5
Thanks.
 

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

Why does the pKb value for n-butylamine change?

The pKb value for n-butylamine changes because it is affected by various factors such as temperature, solvent, and concentration. These can alter the equilibrium between the conjugate acid and base forms of n-butylamine, leading to a change in the pKb value.

What factors influence the change in pKb for n-butylamine?

The change in pKb for n-butylamine can be influenced by temperature, solvent, and concentration. Additionally, the presence of other molecules or ions in the solution can also affect the equilibrium and lead to a change in the pKb value.

How does temperature affect the pKb value for n-butylamine?

An increase in temperature can cause the pKb value for n-butylamine to decrease. This is because higher temperatures can increase the rate of dissociation of the conjugate acid, resulting in more base molecules and a lower pKb value.

What is the relationship between solvent and pKb for n-butylamine?

The pKb value for n-butylamine can vary depending on the solvent used. This is because different solvents have different levels of polarity, which can affect the strength of the interactions between the molecules and alter the equilibrium between the conjugate acid and base forms.

Can the concentration of n-butylamine affect its pKb value?

Yes, the concentration of n-butylamine can affect its pKb value. A higher concentration of n-butylamine will result in more base molecules and a lower pKb value. Conversely, a lower concentration will lead to fewer base molecules and a higher pKb value.

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