Acidity of Amines

We normally think of amines as bases, but it must be remembered that 1º and 2º-amines (not 3º-amines which have no N-H protons) are also very weak acids (ammonia has a pK_a = 34). In this respect it should be noted that pK_a is being used as a measure of the acidity of the amine itself rather than its conjugate acid, as in the previous section. For ammonia this is expressed by the following hypothetical equation:

NH₃ + H₂O ^____> NH₂^(–) + H₂O-H⁽⁺⁾

The same factors that decreased the basicity of amines increase their acidity. This is illustrated by the following examples, which are shown in order of increasing acidity. It should be noted that the first four examples have the same order and degree of increased acidity as they exhibited decreased basicity in the previous table. The first compound is a typical 2º-amine, and the three next to it are characterized by varying degrees of nitrogen electron pair delocalization. The last two compounds (shaded blue) show the influence of adjacent sulfonyl and carbonyl groups on N-H acidity. From previous discussion it should be clear that the basicity of these nitrogens is correspondingly reduced.

Compound					C6H5SO2NH2
pKa	33	27	19	15	10	9.6

The acids shown here may be converted to their conjugate bases by reaction with bases derived from weaker acids (stronger bases). Three examples of such reactions are shown below, with the acidic hydrogen colored red in each case. For complete conversion to the conjugate base, as shown, a reagent base roughly a million times stronger is required.

C6H5SO2NH2 + KOH C6H5SO2NH(–) K(+) + H2O	a sulfonamide base
(CH3)3COH + NaH (CH3)3CO(–) Na(+) + H2	an alkoxide base
(C2H5)2NH + C4H9Li (C2H5)2N(–) Li(+) + C4H10	an amide base