3.14:
Leveling Effect
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 pKa value higher than that of water.
3.14:
Leveling Effect
In acid-base chemistry, the leveling effect refers to the limitation imposed by the solvent on the strength of acids and bases in solution. When a base stronger than the solvent's conjugate base is used, it deprotonates the solvent until the base is entirely consumed, making it ineffective against weaker acids. Conversely, an acid stronger than the solvent's conjugate acid protonates the solvent until the acid is depleted, rendering it ineffective against weaker bases. Essentially, the solvent neutralizes stronger acids and bases, preventing them from reacting as intended with other compounds.
For example, in water (an aqueous solution), a strong base like the amide ion deprotonates water, predominantly forming hydroxide ions and leaving few amide ions. This prevents the amide ions from effectively deprotonating compounds like acetylene, which have a higher pKa than water. However, if a more basic solvent like ammonia is used, the amide ions can successfully deprotonate acetylene, facilitating the desired reaction.
In summary, the solvent's acidity limits the effectiveness of strong bases, and its basicity limits the effectiveness of strong acids. The chosen solvent must, therefore facilitate the acid-base reaction without undergoing a significant reaction itself.