Amphoteric Compounds

AMPHOTERIC COMPOUNDS

A glance at the pK_a values in Table 3.1 reveals that many classes of compounds can act either as acids or as bases, depending on the reaction environment. Such materials are termed amphoteric. They must have an acidic proton (i.e., a proton attached to an electronegative element or group) and unshared pairs of electrons that can be donated to a proton. For example, water, alcohols, and other hydroxylic compounds as well as amines and amides are all amphoteric materials. Comparing the pK_a’s of these materials permits an assessment of the predominant behavior in a given environment. For example, if an amine is dissolved in water, it could function as an acid or a base. To determine which behavior will predominate, the position of the equilibrium can be determined for each process. Comparison of these values will indicate which will be the principal behavior. Thus, as an acid, the amine would donate a proton to water to give an amide anion and the hydronium ion.

As a base the amine would accept a proton from water to give an ammonium ion and hydroxide.

Since the equilibrium constant for the amine acting as an acid is 10⁻³⁵ and that for the amine acting as a base is much larger at 10⁻⁶, reaction as a base will be the main behavior in aqueous solution. The magnitude of the equilibrium constant (10⁻⁶) indicates that it is only a weak base.

On the other hand, if you treat an amine such as diisopropyl amine with n-butyl lithium in tetrahydrofuran (THF), then a different behavior is indicated. In this case the equilibrium lies far to the right so as to be virtually irreversible. The amine therefore functions as an acid and donates a proton to the much stronger base butyl lithium. This is the standard method for the preparation of the versatile base LDA.