Leveling Effect and Non-Aqueous Acid-Base Solutions

In an acid–base reaction, when a base stronger than the conjugate base of the solvent is used, it deprotonates the solvent to produce the conjugate base. Over time, the base gets completely consumed, making it unavailable to deprotonate any acid that is weaker than the solvent.

Similarly, if an acid stronger than the conjugate acid of the solvent is used, it protonates the solvent to produce more of the conjugate acid. Eventually, none of the acid is present to protonate any base that is weaker than the solvent.

In both cases, the solvent prevents the stronger base or the stronger acid from reacting with the desired compound. This is the leveling effect of the solvent.

For a successful acid–base reaction, the chosen solvent must facilitate the reaction without reacting.

To illustrate, consider an aqueous solution of amide ions. Since an amide ion is stronger and less stable than the conjugate base of water, it deprotonates water, favoring the formation of more hydroxide ions.

Consequently, the solution contains mostly hydroxide ions and few amide ions. Due to the leveling effect of water, the amide ions get consumed, and they are unavailable for the deprotonation of a compound like acetylene that has a pK_a value higher than that of water.

Suppose a more basic solvent like ammonia with a pK_a value higher than acetylene is used. In this case, amide ions will deprotonate acetylene instead of the solvent ammonia, and the reaction will proceed as desired, producing more of the conjugate base of acetylene.

To summarize, the acidity of the solvent levels the strength of strong bases; that is, the base used cannot be stronger than the conjugate base of the solvent.

Similarly, the basicity of the solvent levels the strength of strong acids, which means that the acid used cannot be stronger than the conjugate acid of the solvent.