Chemistry Reference
In-Depth Information
O
O
O
O
O
O
O
O
O
K
+
O
O
O
O
O
O
X
−
O
O
O
18-Crown-6
Dicyclohexyl-18-crown-6
Crown ether/potassium salt complex
OO
O O
M
+
O
O
OO
OH
A hypothetical model for the complexation of cations by POE nonionic chains
Figure 6.5.
Molecular structures of typical crown ether and POE nonionic surfactant
catalysts and complexes.
There are two types of PTC processes mentioned in the literature. If two liquid
phases are involved, the reaction process is termed liquid-liquid phase transfer cat-
alysis (ll-PTC). If the nonorganic phase is a solid, the process is termed solid-liquid
phase transfer catalysis (sl-PTC). Since the catalyst, routinely referred to in the lit-
erature as the ''quat,'' is the vehicle for the transfer of anions between the two liquid
phases, the mechanistic scheme for ll-PCT can be considered an extraction process.
For sl-PCT, it may be considered a solubilization process, since the normally
organic insoluble reactive anions are solubilized by the formation of more hydro-
phobic ion pairs at the interface. Typical ll-PTC catalysts are quaternary ammonium
salts (R
4
N
þ
X
) and phosphonium salts (R
4
P
þ
X
). Crown ethers, illustrated in
Figure 6.5, and other cation complexing agents, and some nonionic, straight-
chain polyoxyethylene materials have also been found to be efficient catalysts for
sl-PTC processes. Polymer bound catalysts are also effective in some applications.
A variety of mechanistic schemes for PTC can be found in the literature, but most
are variations on the extraction or solubilization mechanisms. Nevertheless, the fun-
damental principles governing these mechanisms are consistent with that illustrated
in Figure 6.4.
Since the concentration of the reacting anions in the organic phase in PTC reac-
tions cannot exceed that of the catalyst, in many cases the reactions can be carried
out without an organic solvent. Indeed, when the starting materials and products
are liquid, they can act as solvents for the reacting species and form the organic
phase when used neat; the reacting anions are introduced directly by the catalyst.
Since the catalyst is usually present at approximately 1 mol%, the reactions proceed
