Name: solvent effects on strong acids and bases
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 11 s
Channel: JoVE

In an acid&#8211;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&#8211;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

leveling effect and non-aqueous acid-base solutions

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.
<h4>The Leveling Effect of a Solvent </h4>
A generic acid (HA) reacts with the generic base (B-) to yield the corresponding conjugate base (A-) and conjugate acid (HB):
<img alt="Acid-base reaction mechanism diagram: H-A + B&#8315; &#8594; A&#8315; + H-B, showing proton transfer steps." src="/files/ftp_upload/17738/17738_Reaction%20equation%20-%20Generic%20acid-base%20reaction.svg" style="text-align: center; height: 40px;" data-altupdatedat="1754904545000" data-alt="Acid-base reaction mechanism diagram; proton transfer from HA to B forming A- and 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&#8722; 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):
<img alt="Acid-base reaction equation diagram, H-A + B showing bond dissociation, ion interaction." src="/files/ftp_upload/17738/17738_Reaction%20equation%20-%20Generic%20base%20leveling%20reaction%20in%20a%20solvent.svg" style="text-align: center; height: 40px;" data-altupdatedat="1754904545000" data-alt="Acid-base reaction mechanism diagram, H-A + B + H-X &#8594; H-A + H-B + X-, illustrating proton transfer." /> 
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-):
<img alt="Acid-base reaction mechanism, equation diagram, showing proton transfer and ion formation." src="/files/ftp_upload/17738/17738_Reaction%20equation%20-%20Generic%20acid%20leveling%20reaction%20in%20a%20solvent.svg" style="text-align: center; height: 45px;" data-altupdatedat="1754904545000" data-alt="Acid-base reaction mechanism; equation diagram; proton transfer between acids and bases." /> 
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.
<h4>The Leveling Effect of Water on a Strong Base</h4>
To visualize the leveling effect of solvent on strong bases, consider an aqueous solution of acetylene reacting with sodium amide. In this example, acetylene (pKa=25) is a weaker acid than the solvent, water (pKa=15.7), as evident from the inverse relationship between acidity and pKa 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.
<img alt="Chemical reaction diagram showing hydrolysis of nitrile forming ammonia and alcohol." src="/files/ftp_upload/17738/17738_Reaction%20equation%20-%20Leveling%20effect%20of%20amide.svg" style="text-align: center; height: 45px;" data-altupdatedat="1754904545000" data-alt="Alkyne reaction mechanism with ammonia, CO2, arrow showing product formation; organic chemistry." /> 
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 pKa of 38 that is greater than the pKa of acetylene (25). This makes acetylene the stronger acid to ensure the solvent is not deprotonated.
<h4>The Leveling Effect of Water on a Strong Acid</h4>
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 (pKa=8.36) is a weaker base than the solvent that is water (pKa=15.7), as evident from the direct relationship between basicity and pKa 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.
<img alt="Acid-catalyzed amide hydrolysis reaction mechanism diagram with curved arrow notation." src="/files/ftp_upload/17738/17738_Reaction%20equation%20-%20Leveling%20effect%20of%20perchloric%20acid.svg" style="text-align: center; height: 100px;" data-altupdatedat="1754904545000" data-alt="Acid-catalyzed mechanism, equation, cyclic amide hydrolysis, chemical reaction diagram." /> 
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 pKa of 4.2 that is lower than the pKa 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.

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 ...

leveling-effect-and-non-aqueous-acid-base-solutions

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Organic Chemistry

Unit 1

Acids and Bases

KEY TERMS AND CONCEPTS

solvent effects on strong acids and bases

Solvent effects can change how strong acids and bases react in a chemical system. The leveling effect is one important example. It shows that a solvent can take part in the reaction before the acid and base react with each other.

For a generic acid-base reaction, a generic acid (HA) reacts with a generic base (B−) to form the conjugate base (A−) and the conjugate acid (HB). In a solvent called HX, the solvent may also react if it is stronger than one of the reactants. If HA is weaker than the solvent, B− deprotonates the solvent instead of HA. This is called the leveling effect of the base by a solvent.

The same idea works for acids. If B− is weaker than the solvent HX, then HA protonates the solvent instead of B−. This consumes HA before it can react with the base. This is called the leveling effect of the acid by a solvent.

Water shows this effect clearly with strong bases. In an aqueous solution of acetylene and sodium amide, acetylene has a pKa of 25, while water has a pKa of 15.7. Because water is the stronger acid, the amide ion deprotonates water first and forms hydroxide ions. The hydroxide ions are stable, but they are not basic enough to deprotonate acetylene.

To deprotonate acetylene with amide, a different solvent is needed. Ammonia is one example because its pKa is 38, which is greater than the pKa of acetylene. That choice keeps the solvent from being deprotonated and allows the reaction to proceed toward acetylene.

Water also levels strong acids. In an aqueous solution of perchloric acid and morpholine, morpholine has a pKa of 8.36, while water has a pKa of 15.7. Perchloric acid protonates water first, which forms hydronium ions. The hydronium ions are more stable, but they are not acidic enough to protonate morpholine.

To protonate morpholine with perchloric acid, a different solvent is needed. Benzoic acid is one example, with a pKa of 4.2, which is lower than the pKa of morpholine. That makes morpholine the stronger base and prevents the solvent from being protonated first. In many reactions, solvent choice must stop the solvent from being deprotonated by a strong base or protonated by a strong acid.

Water is used in most reactions, so it often limits how strong the acid or base can be in solution. In water, acids stronger than H3O+ and bases stronger than OH− cannot be used effectively. Understanding this limit is key when choosing aqueous or non-aqueous conditions for acid-base chemistry.

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Solvent effects can change how strong acids and bases react in a chemical system. The leveling effect is one important example. It shows that a solvent can take part in the reaction before the acid and base react with each other. For a generic acid-base reaction, a generic acid (HA) reacts with a generic base (B−) to form the conjugate base (A−) and the conjugate acid (HB). In a solvent called HX, the solvent may also react if it is stronger than one of the reactants. If HA is weaker than the...

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Learn how solvents like water can level strong acids and bases. See why pKa and solvent choice control acid-base reactions in aqueous and non-aqueous solutions.

Solvent Effects on Strong Acids and Bases | JoVE K12

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Solvent Effects on Strong Acids and Bases

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