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

  1. Apr 5, 2009 #1
    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)
     
  2. jcsd
  3. Apr 6, 2009 #2

    Borek

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    Staff: Mentor

    Because they are idiots that list pKa values naming them pKb.
     
  4. Apr 6, 2009 #3
    pKb
    CH3NH2 3.36
    CH3CH2NH2 3.27
    CH3CH2CH2NH2 3.16
    CH3CH2CH2CH2NH2 3.39

    Why does the pattern break for CH3CH2CH2CH2NH2?
     
  5. Apr 6, 2009 #4

    alxm

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    Science Advisor

    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.
     
  6. Apr 6, 2009 #5
    Thanks.
     
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