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Crown ethers are giant molecules belonging to the ether family. They are named in a general formate x-Crown-y, where x is the total number of atoms present in the ring and y is the number of ether oxygen atoms. The cations bind in the center of the ring by ether oxygen atoms.

15-Crown-5 ether complex Na⁺is a 15-membered ring containing 6 ether oxygen atoms. It binds strongly withNa⁺ ion present at the center of the ring.

12-Crown-4 ether complex Li⁺ is a 12-membered ring containing 4 ether oxygen atoms. It binds strongly with Li⁺ion present at the center of the ring.

Models of crown ethers

Based on ionic radii, in 15-Crown-5, the ion-to-oxygen distance is about 40% longer than that of 12-Crown-4 because the ring size is bigger in 15-Crown-5.