Chemistry - What's the difference between a nucleophile and a base?

Solution 1:

The two are related, in that most nucleophiles are (Lewis) bases and vice versa. Some good nucleophiles are also strong bases, e.g. $\ce{HO-}$. However, a species can be a good nucleophile and a weak base, e.g. $\ce{I-}$; or a species can be a weak nucleophile and a strong base, e.g. $\ce{t-BuO-}$. How can we separate this behavior?

Nucleophilicity is a kinetic phenomenon.

Nucleophilicity is most often defined based on the relative rate of the reactions of nucleophiles with a standard substrate in a standard solvent.

For example, a standard reaction might look like:

$$\ce{CH3I ->[Nu-][H2O] CH3Nu}$$

The nucleophilicity will be related to the relative rate constant of reaction with the nucleophile (relative to the rate constant of the reaction with water $\equiv 1$).

Basicity is a thermodynamic phenomenon.

Basicity is based on the position of equilibrium:

$$\ce{B + HSol <=> BH+ + Sol-}$$

Solution 2:

There is a difference indeed: basicity is a particular kind of nucleophilicity. A nucleophile is a chemical species that donates an electron pair to an electrophile. A nucleophile can also be called a base when this donation occurs towards a particular electrophile, which is an hydrogen ion (a proton).