donor
Summary
A donor is the species that supplies something to a partner. Mainly it is an electron-pair donor (a Lewis base, such as a ligand) that forms a coordinate covalent bond with a Lewis acid. A Brønsted-Lowry acid is a proton donor, and in a hydrogen bond the donor supplies the H.
Detail
Meaning: A donor supplies something to its partner. The main general chemistry sense is an electron-pair donor: a Lewis base uses a lone pair to bond to a Lewis acid, the acceptor, forming a coordinate covalent bond. In complex ions, the ligands (H₂O, NH₃, Cl⁻, CN⁻) are donors and the metal cation is the acceptor. Related uses: a Brønsted-Lowry acid is a proton donor, a reducing agent is an electron donor, and in a hydrogen bond the donor is the molecule whose H is bonded to N, O or F.
On the MCAT: Passages ask you to label the Lewis acid and base in BF₃ + NH₃ → F₃B–NH₃, where NH₃ donates its lone pair. Others ask for a complex's coordination number or whether a ligand is monodentate or bidentate. H-bond donors matter in boiling-point and solubility comparisons: alcohols donate and accept, while ethers accept but cannot donate.
Common confusion: Students assume donor means proton donor. The framework decides. In Brønsted-Lowry terms NH₃ is a proton acceptor, yet in Lewis terms it is an electron-pair donor. To settle it, ask what is being given: a proton, an electron pair, an electron, or a hydrogen in an H-bond.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.