Question

Which is the strongest nucleophile in polar protic solvents?

All are the same strength in this type of solvent.

I-

Cl-

Br-

F-

Answer 1

A nucleophile is a chemical species that donates an electron pair to form a chemical bond in relation to a reaction.

The nucleophilicity of halogen anions in solvent is determined by the availablility of their lone pair electrons. Therefore, the nucleophilicity order among four halogens (F, Cl, Br, I) is dictated by the nature of solvent they are in.

In polar protic solvents (water, methanol, acetic acid, etc.) the nucleophilicity order is like:

I- > Br- > Cl- > F-

In such solvents, the presence of hydrogen bonds is important. F- can easily involved in the network of solvent hydrogen bonds system due to its strong electronegativity. Thus F- can’t spend its lone pair electrons for other electrophiles. This makes F- the weakest nucleophile in the protic solvents. On the other hand, I- has a low electronegativity, and seldom participates in such hydrogen bonding and it's lone pair of electrons is readily available for an electrophile.