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 ?