Question

3. Which reactant in each of the following pairs is a stronger nucleophile? Explain your reasoning. (3)

a. NaOH or H2O​​​

b. H2O or CH3COO-  ​​​

c. I- or Cl-

Answer 1

3.

(a) HO^-  is stronger nucleophile

NaOH is a strong nucleophile.  NaOH is a strong base.

NaOH    Na⁺ + OH^-

Negatively charged species are more nucleophilic than neutrally charged ones. so H₂O (Water) is less nucleophilic than HO^-.

(b) CH₃COO^-  is stronger nucleophile

Negatively charged species are more nucleophilic than neutrally charged ones, so H₂O (Water) is less nucleophilic than CH₃COO^-. As for the others, consider electronegativity, polarizability, and size.

(c) I^-  is stronger nucleophile.

Nucleophilicity is a kinetic parameter and I^- is usually a better nucleophile than Cl^-. The I^- reacts faster than Cl- because it gives up its 2e to the +ve center more easily than Cl^-.

Iodide is more polarizable than chloride, improving its ability to distribute electron density unevenly around the nucleus.

However, the nucleophilicty of the halides is highly dependent on the solvent in which they are used. Flouride is a more potent nucleophile than iodide in a polar aprotic solvent because it is loosely bound to its solvent shell, whereas the opposite is true in polar protic solvents.