Crown Ether
Crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups. The most common crown ethers are oligomers of ethylene oxide, the repeating unit being ethyleneoxy, i.e., -CH2CH2O-. Important members of this series are the tetramer (n = 4), the pentamer (n = 5), and the hexamer (n = 6). The term "crown" refers to the resemblance between the structure of a crown ether bound to a cation, and a crown sitting on a person's head. The first number in a crown ether's name refers to the number of atoms in the cycle, and the second number refers to the number of those atoms that are oxygen. Crown ethers are much broader than the oligomers of ethylene oxide; an important group are derived from catechol.
Crown ethers strongly bind certain cations, forming complexes. The oxygen atoms are well situated to coordinate with a cation located at the interior of the ring, whereas the exterior of the ring is hydrophobic. The resulting cations often form salts that are soluble in nonpolar solvents, and for this reason crown ethers are useful in phase transfer catalysis. The denticity of the polyether influences the affinity of the crown ether for various cations. For example, 18-crown-6 has high affinity for potassium cation, 15-crown-5 for sodium cation, and 12-crown-4 for lithium cation. The high affinity of 18-crown-6 for potassium ions contributes towards its toxicity. Crown ethers are not the only macrocyclic ligands that have affinity for the potassium cation. Ionophores such as valinomycin also display a marked preference for the potassium cation over other cations.
Read more about Crown Ether: History, Affinity For Cations, Azacrowns
Famous quotes containing the words crown and/or ether:
“So much of the trouble is because I am a woman. To me it seems a very terrible thing to be a woman. There is one crown which perhaps is worth it all—a great love, a quiet home, and children. We all know that is all that is worthwhile, and yet we must peg away, showing off our wares on the market if we have money, or manufacturing careers for ourselves if we haven’t.”
—Ruth Benedict (1887–1948)
“Is this then a touch? quivering me to a new identity,
Flames and ether making a rush for my veins,
Treacherous tip of me reaching and crowding to help them,
My flesh and blood playing out lightning to strike what is hardly
different from myself,
On all sides prurient provokers stiffening my limbs,”
—Walt Whitman (1819–1892)