# How can I- be both a nucleophile and a weak base?

I am just confused as to how $\ce{I-}$ cannot attract $\ce{H+}$, but it can attack a carbocation. It is sharing electrons in both cases, so what is the difference?

• Nucleophilicity is a kinetic property, basicity is a thermodynamic property. They are friends some of the time, but they don't always hold hands as your example shows. – Jori Aug 25 '15 at 15:56
• Also, iodide is a base. Whether it reacts as a base depends on where the proton is coming from. – jerepierre Aug 25 '15 at 21:25

In your case, $\ce{I-}$ is a good nucleophile in both protic and non-protic solvents. It's a rather big ion - the biggest of the halide anions. That means its negative charge is relatively dilute. This in turn means it's not as strongly solvated by the solvent molecules; it doesn't have as many tenacious solvent molecules hindering its trek toward a carbocation. So $\ce{I-}$ can reach carbocations relatively quickly. Note that terms dealing with time usually indicate that the writer is speaking about a chemical's kinetic properties.
For the same reason that $\ce{I-}$ is a good nucleophile it's a poor base. It's dilute negative charge (due to its size) means that $\ce{H+}$ has better things to do than be attracted to $\ce{I-}$. $\ce{I-}$ has a dilute amount of negative charge so it cannot effectively stabilize the hydrogen proton as well as many solvents might - especially water, which extensively associates with $\ce{H+}$; $\ce{H3O+}$ only begins to describe how well the proton is associated with water molecules; the proton has been found to be associated with 2, 3, 4, or more water molecules!