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

1. Apr 5, 2009

### cncbmb

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. Apr 6, 2009

### Staff: Mentor

Because they are idiots that list pKa values naming them pKb.

3. Apr 6, 2009

### cncbmb

pKb
CH3NH2 3.36
CH3CH2NH2 3.27
CH3CH2CH2NH2 3.16
CH3CH2CH2CH2NH2 3.39

Why does the pattern break for CH3CH2CH2CH2NH2?

4. Apr 6, 2009

### alxm

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.

5. Apr 6, 2009

Thanks.