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 ? Where else have we talked about "naked" anions ?
