# Why is the iodide anion a good nucleophile but a poor base?

The $\mathrm{pK_{aH}}$ of $\ce I^-$ is very low which indicated that it is not favourable for it to bond with a proton. However, why would it be likely to bond with any other atom (mostly carbon when considering it's nucleophilic properties).

You should clarify whether the iodide anion is a good nucleophile in a polar protic or polar aprotic solvent.

Also let's note that nucleophilicity is a kinetic property, while acidity/basicity are thermodynamic properties. Iodide ion's lack of basicity in water reflects its conjugate acid's ($\ce{HI}$) lack of stability; it's relatively easy to ionize its proton due to the weak hydrogen-iodide bond.

On the other hand, the high nucleophilicity of iodide ion in water reflects the high rate of reaction between iodide ion and some electrophilic atom (other than hydrogen).

The relative lack of basicity of the iodide anion in water is precisely what makes it a good nucleophile. Its lack of affinity toward protons - i.e. the protons of water - physically frees $\ce{I^-}$ to attack electrophilic atoms (not limited to carbon). There is little hydrogen bonding between iodide ion and water.

On the other hand, the relative basicity and thus strong hydrogen-bond forming capabilities of $\ce{F^-}$ makes it a poorer nucleophile in water. Rates of reaction between fluoride anion and some electrophilic atom are slowed because the fluoride anion is trapped (strongly solvated) by water molecules. And there will also be a sizable number of fluoride anions that have completely abstracted a proton from water to make $\ce{HF}$, which isn't nucleophilic at all.

In a polar aprotic solvent, however, where there are no positively polarized hydrogens for a nucleophile to hook onto; $\ce{F^-}$ becomes a much stronger nucleophile. Actually, $\ce{F^-}$ becomes a better nucleophile than $\ce{I^-}$, likely due to $\ce{F^-}$'s greater charge density.

The bond between I and H is weak so I- is a weak base. I- is a strong nucleophile because it is polarizable, making it faster for its orbitals to overlap with the electrophile. Remember that basicity is a thermodynamic concept and nucleophilicity is a kinetic concept.

I have been told that I- can be added to reactions as a catalyst because it is a good nuclephile and a good leaving group. Leaving group capability seems to be inversely related to basicity.

• Why doesn't it overlap fast with protons? – RobChem Dec 21 '14 at 12:20
• What if it does but the rate of the bond between H and I breaking is even faster than the rate of the H-I bond being formed? – Brinn Belyea Dec 21 '14 at 15:37
• It probably is faster. – Brinn Belyea Dec 22 '14 at 18:04
• Because the bond making is faster. Nucleophilicity refers to how fast the bond can be made, not how stable it is. – Brinn Belyea Dec 23 '14 at 0:39
• In both cases the rate constant for bond breaking should be higher than making for most reactions involving a bond with C and I or H and I. My point is that nucleophilicity refers to how fast a nucleophile/base can react and basicity refers to how stable the products of the reaction are. Look at fluoride. Fluoride makes a stronger bond with C than I does but I reacts faster than F. – Brinn Belyea Dec 23 '14 at 1:09