nucleophilicity
Summary
Nucleophilicity is how fast a species donates an electron pair to an electrophilic atom, usually carbon; it is kinetic, unlike basicity. It often tracks basicity, but not always: in polar protic solvents I⁻ > F⁻, while in polar aprotic solvents the order follows basicity (F⁻ > I⁻).
Detail
Meaning: Nucleophilicity is how fast a nucleophile (an electron-pair donor) attacks an electrophilic atom, usually carbon. It is kinetic, while basicity is thermodynamic (the equilibrium for accepting H⁺). Nucleophilicity generally rises with negative charge (HO⁻ > H₂O), falls with electronegativity across a row (NH₂⁻ > HO⁻ > F⁻), and falls with steric bulk (CH₃O⁻ > (CH₃)₃CO⁻). In polar protic solvents, hydrogen bonding cages small anions, so I⁻ > Br⁻ > Cl⁻ > F⁻. In polar aprotic solvents (DMSO, acetone) anions are less solvated and the order follows basicity: F⁻ > Cl⁻ > Br⁻ > I⁻.
Equation: rate = k[Nu][R–X] for SN2, where rate is in mol L⁻¹ s⁻¹, k is the rate constant (L mol⁻¹ s⁻¹), and [Nu] and [R–X] are concentrations (mol/L).
On the MCAT: A passage varies the nucleophile, solvent, or substrate, and you rank SN2 rates or predict the effect of switching solvents. SN2 rate depends on nucleophile strength; SN1 rate does not.
Common confusion: A strong base is not always a strong nucleophile. tert-Butoxide is a strong but bulky base and a poor nucleophile, and I⁻ is a weak base but a good nucleophile in a protic solvent. Sterics and solvent affect rate, not basicity.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.