Crown ether

A crown ether is a cyclic molecule with several oxygen atoms arranged around a central cavity. It selectively binds metal cations whose size and coordination fit that cavity.

Cation coordination: The formation of chemical bonds between a positively charged ion and surrounding molecules or ions. Crown ether oxygen atoms coordinate cations by donating electron pairs.
Macrocycle: A large ring-shaped molecule containing a substantial number of atoms in its cycle. Crown ethers belong to the macrocycles, whose ring cavities can bind guests.
Charles J. Pedersen: An American chemist who discovered crown ethers and shared the 1987 Nobel Prize in Chemistry. Pedersen first reported the compounds and their selective binding of metal ions.
Phase-transfer catalysis: A catalytic method that transfers a reactant between immiscible phases by carrying an ion across their interface. Crown ethers can carry inorganic cations into organic phases, enabling reactions with anions.
Acyclic polyether: A molecule with multiple ether linkages arranged in an open chain rather than a closed ring. It lacks the preorganized cavity that gives crown ethers characteristic size selectivity.
Host–guest chemistry: The study of complexes formed when a host molecule binds a guest within or around its structure. Crown ethers are molecular hosts that bind cation guests.
Polyether: A polymer or molecule containing multiple ether linkages in its structure. Repeated ether oxygen atoms provide the main cation-binding sites in crown ethers.
Nobel Prize in Chemistry 1987: The Nobel Prize awarded to Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen for host–guest chemistry. The prize recognized crown ether discovery alongside broader advances in molecular recognition.
Potassium permanganate: An inorganic salt, KMnO₄, containing potassium ions and the strongly oxidizing permanganate ion. Crown ether complexation can dissolve this ionic oxidant in organic solvents.
Cryptand: A three-dimensional host molecule with donor atoms arranged to enclose a guest ion in a cavity. Cryptands encapsulate ions more completely than the usually flatter crown ether rings.

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