Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.3 Effects of organic solvents on phase-transfer catalysis 809
Table 13.3.6. Solubilities of potassium salts in CH 3CN at 25 o C in the presence and
absence of 18-crown-6 ether
Potassium
salt
Solubility of potassium salt
in 0.15 M crown in CH 3CN (A) in CH 3OH (B) Enhancement factor (A/B)
KF 4.30×10 -3
KCl 5.55×10 -2
KBr 1.35×10 -1
KI 2.02×10 -1
KCN 1.29×10 -1
KOAc 1.02×10 -1
KSCN 8.50×10 -1
Data adopted from the work of Liotta 60
Table 13.3.7. Rates of reaction of benzyl
chloride with potassium cyanide at 85 o Cin
the presence and absence of 18-crown-6
ether as a function of added water
Water
k×10 5 s -1
(crown)
k×10 5 s -1
(no crown)
0.0 3.2 0.0
0.36 9.2 0.0
0.50 9.4 0.0
1.00 11.6 0.0
2.00 14.7 0.0
10.0 10.2 0.0
20.0 5.8 1.3
40.0 3.9 1.9
75.0 4.8 3.2
Data obtained from the work of Liotta; 61 0.05 mol of
benzyl chloride, 0.01 mol of 18-crown-6 ether, 0.15 mol
of KBr, and 0.015 mol of KCN
3.18×10 -4
2.43×10 -4
2.08×10 -1
1.05×10 -1
1.19×10 -3
5.00×10 -4
7.55×10 -1
13.52
228.40
64.90
1.92
108.40
204
1.13
The rates of reaction of benzyl bromide
and benzyl chloride with potassium
cyanide were studied as a function of added
water in the presence and absence of crown
ether in toluene at 85 o C, 61 as shown in Table
13.3.7. The reaction is highly affected by
the addition of 18-crown-6 ether. In addition
to enhancing the reaction rate, it is important
to note that in the absence of added
water, the rates followed zero-order kinetics,
while in the presence of added water,
the rates followed first order kinetics.
(C) Synthesis of ether compound catalyzed
by polyethylene glycols (NPTC)
Similar to quaternary ammonium
salts, polyethylene glycols (PEGs) act as
the phase transfer catalyst. There are two
majors effects of PEG on the two-phase reactions.
First, part of the PEG, existing in
the organic phase, forms a complex with
metal cation. The formation of a complex
leads to an increase in the solubility of sodium
alkoxide (RONa) or sodium
phenoxide (PhONa) for the synthesis of
ether in the organic phase. Hence, the reac-
tion rate in the organic phase is promoted. Second, PEG acts as an excellent organic solvent,
but it can also dissolve in water. Thus, part of the alkyl halide that is dissolved by PEG is
brought into the aqueous phase from the organic phase. The dissolved alkyl halide directly