Ch16 : Crown Ethers

Chapter 16: Ethers, Epoxides and Sulfides

Crown Ethers

Oxygen atoms are capable of acting as Lewis bases due to the presence of the lone pairs (e.g. in hydrogen bonding between water molecules or hydration of a metal ion in aqueous solution, see below).
The interaction with metal ions with ethers is also important:

In regular ethers, only weak complexes are formed.
However, in certain polyethers (where mulitple interactions are possible) the complexes are much stronger.


18-Crown-6	complex	hydration of a metal ion

These ethers are called "crown ethers" due to their shape.
They are based on repeating -OCH₂CH₂- units, derived from ethylene glycol : HOCH₂CH₂OH
The name 18-crown-6 indicates that there are 18 atoms in the ring, 6 of which are oxygen.
These compounds are important co-solvents.
The interior of the cavity is water like, whereas the exterior is hydrocarbon like.
So a metal ion inside the cavity can be "carried" into an organic solvent.
This allows ionic systems such as KF to be dissolved in organic solvents and used as reagents where the metal ion is in a complex, but the anion is unsolvated or naked and therefore quite reactive.
Varying the size of the crown ether varies the cavity size and some metal ions fit better than others.
For example, 18-crown-6 is an good fit for K+

QUESTIONS

	In the crown ether 12-crown-4, how many atoms are there in the ring and how many are oxygen atoms ?	ANSWER
	Where else have we talked about "naked" anions ?	ANSWER