with \(n \ge 3\).
The common name of an unsubstituted crown ether is 3n–crown–n. eg:
Crown ether molecules can trap metal ions by forming ion-dipole bonds with them, resulting in an entity known as host-guest complex, in which the crown ether molecule is the host and the metal ion is the guest.
Since the diameter of the cavity of a crown ether molecule is more or less a constant, the ability of a crown ether molecule to form a stable host-guest complex with a metal ion is highly selective.
cavity diameter: 18–crown–6 > 15–crown–5
ionic radius; K+ > Na+
18–crown–6 makes a stable host-guest complex with K+ (1), but not with Na+, whereas 15–crown–5 makes a stable host-guest complex with Na+ (2), but not with K+.