Leveling effect

The term leveling effect refers to a solvent's ability to level the effect of a strong acid or base dissolved in it.

Process
When a strong acid is dissolved in water, it reacts with it to form H3O+ in the following reaction (where "HA" is a generic strong acid such as "HCl"): "HA + H2O -> A- + H3O+"

Any acid that is stronger than H3O+ reacts with H2O to form H3O+; therefore, no acid can be stronger than H3O+ in H2O.

This is true with any solvent, although usually to a lesser extent than with water. Bases may be also leveled in solvents.

Example
Solomons & Fryhle's Organic Chemistry (8th Ed.) describes the leveling effect with the following example:

If you were to add NaNH2 to water in an attempt to carry out a reaction using the very powerful base, the amide ion (NH2-, the following reaction would take place immediately: H-O-H + NH2- --> H-O- + NH3

(Stronger Acid) (Stronger Base)   --> (Weaker Base)  (Weaker Acid)

The Amide ion would react with water to produce a solution containing hydroxide ions (a much weaker base) and ammonia.

Solution
To use bases stronger than the hydroxide ion, use a solvent that is a weaker acid then water.

水平化効果