Why does nucleophilicity in a protic solvent decrease with increase with basicity and why does it increase with increase in basicity in an aprotic solvent?
1 Answer
All reactivity in solution is moderated by the identity of the solvent.
Explanation:
It is well-known that sodium or potassium hydroxide is a more powerful base in ethanol, and that hydroxide anion is a more powerful base and nucleophile in diethyl ether than in water.
Why? Well the given APROTIC solvents have the property that while they stabilize the cation, here sodium or potassium ion, they are less competent to stabilize the anion, here hydroxide ion. The solvent is clearly LESS capable of stabilizing a negative charge. And thus in ethereal or alcoholic solution hydroxide anion is both a more powerful base, and a more powerful nucleophile. And chemists exploit this phenomenon on the bench when they use an alcoholic base bath to clean their glassware of grease.
Of course, we have to get the solute up into solution, and this is something that is not so straightforward. But, once it is there, the hydroxide performs as a stronger base, and as a stronger nucleophile, as compared to its performance in aqueous solution.
