JibbyDee - 26-11-2011 at 21:40
Ammonias pKa is something like 38 whereas trimethylamines pKa is 10.62. I thought methyl groups are inductive donators and would therefore increase
the basicity of nitrogen.
ziqquratu - 26-11-2011 at 22:01
Short answer - you're not comparing the right species here - the pKa of 38 for ammonia refers to deprotonation of NH3 (neutral); the 10.6 pKa you
quote for trimethylamine is ACTUALLY for trimethylammonium cation (i.e. Me<sub>3</sub>NH<sup>+</sup>
For ammonia, primary and secondary amines, you can measure a pKa of the amine itself, because it can lose a proton to become an amine anion (an
"amide", not to be confused with the functional group of the same name!). You can ALSO measure the pKa of the protonated ammonium cation, which will
be a much stronger acid than the parent amine (and thus, obviously, have a lower pKa).
On the other hand, a tertiary amine has no protons to lose, and thus only the pKa of the relatively acidic ammonium species can be measured.
JibbyDee - 26-11-2011 at 22:40
Ah right, that explains it. Thanks.