Handbook of Solvents - George Wypych - ChemTech - Ventech!

824 Maw-Ling Wang
Table 13.3.19 Effect of added water on the
concentration of 18-crown-6 ether in toluene at
room temperatures
Water,
μL
Equiv. of water, mole
of H2O/mole of crown
Percent crown in
toluene, %
0 0.00 91.5
10 0.14 81.4
15 0.21 77.3
22 0.31 50.0
25 0.35 34.6
30 0.42 17.7
45 1.25 2.5
50 1.39 2.0
80 2.22 1.0
Data obtained from Liotta et al. 62 0.0040 mole of 18-crown-6,
0.027 mol of KCN, 10 mL of toluene
Table 13.3.20 18-Crown-6 catalyzed
reactions of benzyl bromide
with KCN as a function of added
crown ether
Millimoles of
18-crown-6
k×10 5 sec -1
3.0 2.16, 2.47
5.0 3.97, 3.63
7.0 3.86, 3.99
10.0 3.75, 4.00
12.0 3.80, 3.60
Data obtained from Vladea and Simandan; 122
1.0 mL of H 2O, 0.15 mole KBr, 0.15 mol of
KCN, 50 mL of toluene, 25 o C
(B) Reaction catalyzed by PEGs
The structure of polyethylene glycol (HO(CH 2CH 2O) nH, PEG) is similar to that of
crown ether. Polyethylene oxide chains (CH 2CH 2O) form complexes with cations, much
like crown ethers, and these complexes cause the anion to be transferred into the organic
phase and to be activated. 122 Table 13.3.21 shows the binding constant, K, for PEG complexes
with sodium cation depend on both the value of n (i.e., average molecular weight of
PEG or number of (CH 2CH 2O) unit) and on the end-group substituents. 112,114,115 Gokel and
coworkers 31,32 determined the binding strength for Na + in anhydrous methanol solution with
PEGs and obtained the binding constant K=1.4. They concluded that the strength of
complexation is a function of the total number of binding sites present and not the number of
polymer chains, suggesting that a long PEG chain may be involved in binding more than
one cation.
PEGs and their derivatives have been extensively investigated as phase transfer catalysts
and are used in many commercial processes. In the absence of strong acids, PEGs are
nontoxic, inexpensive, and thermally stable. For some reactions such as with hydroxide
transfer, PEGs are excellent catalysts, sometimes better than crown ethers, especially when
used in liquid-solid PTC reactions with potassium salts, and with little or no added water,
and with at least moderately polar organic solutions. PEGs are water soluble and if the organic
phase is not sufficiently polar the PEG will reside almost completely in the aqueous
phase; or with concentrated aqueous solutions of organic salts, the PEG may form a third
catalyst-rich phase, a change that normally leads to a high level of catalytic activity.
PEGs are themselves soluble in water. To obtain partitioning of PEG into an organic
solution may require use of a mono- or diether derivative. Harris and Case 34 found that with