nucleophile
Summary
A nucleophile is an electron-rich species that donates a lone pair or pi-bond pair to an electron-poor atom, usually carbon, forming a new bond. The testable point: nucleophilicity is a rate (kinetic) property, whereas basicity is thermodynamic, and the two often but not always track together.
Detail
Meaning: A nucleophile ("nucleus-loving") is an electron-rich species that donates an electron pair to an electron-poor atom (an electrophile) to form a new covalent bond. It has a lone pair or a pi bond, as in OH⁻, CN⁻, RO⁻, NH₃, H₂O and I⁻. Nucleophilicity is kinetic: it describes how fast the species attacks. Negative charge helps (OH⁻ > H₂O). Across a period, less electronegative atoms are stronger (NH₃ > H₂O). Down a group in polar protic solvents, larger atoms are stronger (I⁻ > F⁻). Steric bulk weakens it.
On the MCAT: Nucleophiles drive SN2, SN1, carbonyl addition and nucleophilic acyl substitution. Expect to rank reagents or predict rate changes. In SN2, the rate rises with a stronger or more concentrated nucleophile; in SN1, the nucleophile does not affect the rate. In curved-arrow mechanisms, the arrow starts at the nucleophile's lone pair or pi bond.
Common confusion: Nucleophilicity is not basicity. Basicity is thermodynamic (taking H⁺, reflected in the pKa of the conjugate acid). Nucleophilicity is kinetic (attacking an atom, usually carbon). They often rise together, but I⁻ is a good nucleophile and a weak base, while bulky tert-butoxide is a strong base and a poor nucleophile.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Klein)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.