What is the order of nucleophilicity increase in protic solvents?

In protic solvents, nucleophilicity increases down the periodic table. This trend can be explained by considering how nucleophiles interact with the solvent.

In protic solvents, nucleophiles are often stabilized by hydrogen bonding with solvent molecules. As you move down a group in the periodic table, atoms generally become larger, which means that the nucleophiles' ability to donate electrons and participate in nucleophilic attacks increases due to a decrease in the electronegativity of the atoms involved. A larger atom can better accommodate negative charge and is generally less solvated, which enhances its nucleophilicity.

For instance, consider the halide ions: fluoride is a small ion that is heavily solvated because it can form strong hydrogen bonds with protic solvents, making it a poor nucleophile in these conditions. On the contrary, iodide is larger, experiences less solvation, and is therefore a stronger nucleophile in protic solvents.

In summary, the trend of increasing nucleophilicity down the periodic table in protic solvents is due to the balance between atomic size, charge density, and solvation effects.