Effet nivelant et solutions acido-basiques non aqueuses

The leveling effect occurs when a solvent neutralizes stronger acids or bases, preventing them from reacting as intended with other compounds. When a base stronger than the solvent's conjugate base is used, it deprotonates the solvent until consumed. Similarly, an acid stronger than the solvent's conjugate acid protonates the solvent until depleted. The solvent essentially limits the effective strength of these reactive species.

Amide ions are stronger and less stable than water's conjugate base, so they deprotonate water to form hydroxide ions. This consumes the amide ions, making them unavailable to deprotonate acetylene, which has a higher pKa than water. The leveling effect of water neutralizes the amide ions before they can react with the desired compound.

The solvent must facilitate the acid-base reaction without undergoing significant reaction itself. In aqueous solutions, the leveling effect limits strong acids and bases. However, using a more basic solvent like ammonia allows stronger bases such as amide ions to successfully deprotonate compounds like acetylene. Solvent selection determines whether desired reactions can proceed effectively.

A strong base deprotonates water to produce the conjugate base of water, which is the hydroxide ion. Over time, the base gets completely consumed through this reaction with the solvent. Once depleted, the base becomes unavailable to deprotonate any acid weaker than water, demonstrating the leveling effect.

A strong acid protonates water molecules, producing more conjugate acid of the solvent. Eventually, none of the original acid remains in solution. This prevents the acid from protonating any base weaker than water. The solvent's leveling effect neutralizes the acid's reactivity toward weaker bases.

For successful acid-base reactions, the solvent must not react significantly with the acid or base being studied. If the solvent reacts with the analyte through the leveling effect, it consumes the reactive species and prevents the desired reaction from occurring. Choosing an appropriate solvent ensures accurate solution composition during acid base titrations.

A compound can be deprotonated only if the base used is stronger than the solvent's conjugate base. If the base is weaker, the leveling effect causes it to react with the solvent instead, becoming unavailable for the desired deprotonation. The compound's pKa relative to the solvent's pKa determines whether deprotonation is possible.