A good nucleophile is characterized by several key factors that influence its ability to donate a pair of electrons to form a new covalent bond. These factors include:
- Charge: Nucleophilicity increases with increasing negative charge because a more electron-rich species is more willing to donate electrons. For example, conjugate bases (negatively charged) are generally better nucleophiles than their neutral acids
- Electronegativity: Lower electronegativity means the atom holds its electrons less tightly, making it more willing to donate them. Thus, nucleophilicity increases as electronegativity decreases across the periodic table
- Steric Hindrance (Bulkiness): Less steric hindrance means the nucleophile can more easily approach the electrophilic center. Bulky nucleophiles are less nucleophilic because their electron pairs are less accessible. For example, methoxide (small) is a better nucleophile than t-butoxide (bulky)
- Solvent Effects: The type of solvent affects nucleophilicity. In polar protic solvents, nucleophiles can be "caged" by hydrogen bonding, reducing their reactivity. Polar aprotic solvents do not strongly solvate nucleophiles, allowing them to be more reactive. For instance, iodide is more nucleophilic than fluoride in protic solvents because it is less solvated
- Polarizability: More polarizable atoms (larger atoms with more diffuse electron clouds) tend to be better nucleophiles because they can better stabilize the transition state during bond formation. For example, iodide is more nucleophilic than fluoride due to higher polarizability
In summary, a good nucleophile is typically a negatively charged, less electronegative, small (less hindered), and more polarizable species, and its nucleophilicity is enhanced in polar aprotic solvents where it is less solvated
