Nucleophile is defined as

• A. Electron pair acceptors
• B. Electron pair donors
• C. A species that donates a proton
• D. Electron pair donor

Answer: (D)

Nucleophiles are species that donate an electron pair to an electron-poor center. In Lewis terms, a nucleophile is a Lewis base because it provides a pair of electrons to form a bond with an electrophile. This is why the defining description is an electron pair donor. Donating a proton describes acid behavior, not nucleophilicity, and an electron pair acceptor would be an electrophile, not a nucleophile. Saying it is an electron pair donor (the standard way to state the definition) captures the essential idea that nucleophiles supply electrons to form new bonds.