5.6 : Leveling Effect and Non-Aqueous Acid-Base Solutions

This lesson defines the leveling effect in acidic and basic solutions and its role in aqueous and non-aqueous solutions. It is essential to understand the competing nature of various species in a chemical system.

The Leveling Effect of a Solvent

A generic acid (HA) reacts with the generic base (B^-) to yield the corresponding conjugate base (A^-) and conjugate acid (HB):

Figure 1: A generic acid-base reaction

However, if the reaction takes place in a solvent (HX), the solvent can also participate in the reaction, depending on the strength of its corresponding conjugate acid or base. This leads to two situations.

For the first kind, assume that the generic acid (HA) in a reaction is a weaker acid than the solvent (HX). In such a case, B− will deprotonate the solvent to produce the solvent's conjugate base (X-), leading to B^- being entirely consumed and unavailable to interact with the reactant (HA):

Figure 2: A reaction depicting the leveling effect of a solvent on a generic base

This phenomenon is referred to as the leveling effect of the base by a solvent.

Alternatively, assume that the generic base (B-) in a reaction is a weaker base than the solvent (HX). In such a case, HA will protonate the solvent to produce the solvent's conjugate base (H2X), leading to HA being entirely consumed and unavailable to interact with the reactant (B-):

Figure 3: A reaction depicting the leveling effect of a solvent on a generic acid

This phenomenon is referred to as the leveling effect of the acid by a solvent.

The Leveling Effect of Water on a Strong Base

To visualize the leveling effect of solvent on strong bases, consider an aqueous solution of acetylene reacting with sodium amide. In this example, acetylene (pK_a=25) is a weaker acid than the solvent, water (pK_a=15.7), as evident from the inverse relationship between acidity and pK_a value. Therefore, as provided in Figure 4, the amide ion deprotonates the water instead of acetylene, demonstrating the leveling effect of water on strong bases.

Figure 4: Example of the leveling effect in a reaction between acetylene, sodium amide, and water

Since the hydroxide ions are more stable in this reaction, the equilibrium favors the hydroxide ions' formation that replace the amide ions in the solution. However, the hydroxide ions are not basic enough to deprotonate the acetylene, leaving it in the solvent intact. Therefore, to deprotonate acetylene using amide, the choice of solvent plays a key role. It is necessary to use a solvent like ammonia with a pK_a of 38 that is greater than the pK_a of acetylene (25). This makes acetylene the stronger acid to ensure the solvent is not deprotonated.

The Leveling Effect of Water on a Strong Acid

Similarly, to understand the leveling effect of solvent on strong acids, consider an aqueous solution of perchloric acid interacting with morpholine. In this example, morpholine (pK_a=8.36) is a weaker base than the solvent that is water (pK_a=15.7), as evident from the direct relationship between basicity and pK_a value. Therefore, as provided in Figure 5, the perchloric acid protonates the water instead of morpholine, demonstrating the leveling effect of water on strong acids.

Figure 5: Example of the leveling effect in a reaction between perchloric acid, morpholine, and water

Since the hydronium ions are more stable in this reaction, the equilibrium favors the formation of hydronium ions that replace the solution's perchlorate ions. However, the hydronium ions are not acidic enough to protonate the morpholine, leaving it in the solvent intact. Therefore, to protonate morpholine using perchloric acid, the choice of solvent plays a key role. It is necessary to use a solvent like benzoic acid with a pK_a of 4.2 that is lower than the pK_a of morpholine (8.36). This makes morpholine the stronger base to ensure the solvent is not protonated.

In summation, the choice of solvent must satisfy key conditions -it should not be deprotonated by the stronger base or protonated by the stronger acid before interacting with the other reactant. Typically, water is the solvent used in most reactions, enforcing a leveling effect on strong acids and bases. Hence, reactions employing acids stronger than H₃O⁺ and bases stronger than OH^- cannot be used in water.