The main difference is that nucleophilicity is much more sensitive to steric hindrance.

Basicity is a measure of the equilibrium between a Lewis base and a proton (H+). These reactions are often reversible. So the position of the equilibrium tells you the relative energy differences between the starting material and reactants.

Nucleophilicity is a measure of the reaction between a Lewis base and any other electrophile besides H+ (most commonly carbon in our cases). These reactions are generally not reversible. So to measure nucleophilicity we generally have to measure the reaction *rates*.

Reactions between Lewis bases and carbon are much more sensitive to steric hindrance than are reactions of Lewis bases with H+ (because attacking carbon requires orbital overlap with a sigma* orbital that may be difficult to access on a tetrahedral carbon atom via backside attack).

The order of basicity F(-), Cl(-), Br(-), and I(-) is the same in both polar protic and polar aprotic solvents.

However the *magnitude* of basicity for F(-) is much higher in polar aprotic solvents than in polar protic solvents. Hydrogen bonding greatly attenuates the basicity of F(-). In cases where water can be (mostly) removed, anhydrous F(-) is an extremely strong base.

Because the other product is HO(-) which is a considerably stronger base than F(-) (by about a factor of 11 orders of magnitude, compare the pKa of H2O (14) to that of HF (3). ). That’s why it doesn’t proceed.

Cl(-) *is* a weaker base than F(-).