Measures of Amine Basicity

Two conventions are used to measure the basicity of amines. One of them defines a basicity constant Ky, for the amine acting as a proton acceptor from water  

The other convention relates the basicity of an amine (R3N) to the acid dissociation constant of its conjugate acid (RsNH )  

The conjugate acid of ammonia is ammonium ion (NH4 ), which has = 5.6 X 10 ° (p/fa = 9.3). The conjugate acid of methylamine is methylammoifium ion (CH3NH3 ), which has = 2 X 10 (pATa = 10.7). The more basic the amine, the weaker is its conjugate acid. Methylamine is a stronger base than ammonia methylammonium ion is a weaker acid than ammonium ion. 

The relationship between the equilibrium constant Ky, for an amine (R3N) and for its conjugate acid (R3NH ) is  

Citing amine basicity according to the acidity of the conjugate acid permits acid-base reactions involving amines to be analyzed according to the usual Brpnsted relationships. By comparing the acidity of an acid with the conjugate acid of an amine, for example, we see that amines are converted to ammonium ions by acids even as weak as acetic acid  

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