What Are the Acid-Base Properties of Amines

Like ammonia, all amines are weak bases, and aqueous solutions of amines are basic. The following acid-base reaction between an amine and water is written using curved arrows to emphasize that, in this proton-transfer reaction, the unshared pair of electrons on nitrogen forms a new covalent bond with hydrogen and displaces hydroxide ion  

The equilibrium constant for the reaction of an amine with water, K, has the following form, illustrated for the reaction of methylamine with water to give methylammonium hydroxide  

Because the concentration of water in dilute solutions of methylamine in water is essentially a constant ([H2O] = 55.5 mol/L), it is combined with in a new constant called a base ionization constant, K. The value of for methylamine is 4.37 X 10 (p-SJ, = 3.36)  

It is also common to discuss the basicity of amines by referring to the acid ionization constant of the corresponding conjugate acid, as illustrated for the ionization of the methylammonium ion  

Values of pAT and pATj, for any acid—conjugate hase pair are related hy the equation 

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