Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.3 Effects of organic solvents on phase-transfer catalysis 827
The catalyst may form a third layer during the PTC reaction, when using high salt concentrations
and nonpolar organic solvents. Wang and Weng 123-126,154 found that a third layer
phase was built when using a low polarity organic solvent in the PTC reaction system. This
has been observed for PEGs, quaternary onium salts and crown ether. In such cases, catalyst
recovery involves a simple phase separation. In industrial kettles (with limited visibility in
the reactor) or when a “rag” layer distorts an otherwise sharp phase boundary, a phase separation
operation may not be simple. Nevertheless, choosing conditions in which the catalyst
separates as a third layer is usually advantageous. A dehydrohalgenation was performed
four times consecutively, with no loss in catalytic activity by simply replacing the organic
phase and replenishing the aqueous base phase after each use, leaving the third phase containing
PEG in the reactor.
Table 13.3.23 Effect of organic solvent
on current efficiency
Organic solvent Current efficiency, %
CH2Cl2 80
CH2ClCH2Cl 75
CHCl3 86
CH3COOC2H5 64
Data obtained from the work of Do and Chou; 23 0.5
MofC 6H 5CH 2OH in the organic phase; 25 o C, 600
rpm, 20 mAcm -2 of current density, 70 mL of organic
phase, 70 mL of aqueous phase, 0.005 M of
Bu 4NHSO 4, 1.0 M of NaCl, pH=6.9, electricity
passed 2 Fmol -1 of benzyl alcohol
(B) Electrochemical and PTC reaction
The combination of phase transfer catalysis
(PTC) with other processes, such as supercritical
fluid extraction 22 and electrochemical
process 5-7,23,120 have been investigated in detail.
Do and Chou 23 carried out the anodic oxidation
of benzyl alcohol in the two-phase system containing
both the redox mediator, ClO - /Cl - , and a
phase-transfer catalyst (PTC). The reaction
mechanism and the factors which affect the efficiency
of benzaldehyde production were explored.
The current efficiency is mainly
governed by the pH value and the nature of the
organic solvent as well as the types and the
concentration of phase transfer catalyst (PTC).
When ethyl acetate and chlorinated hydrocarbons
were used as solvents, the current efficiencies
were between 64 and 86%,
respectively as shown in Table 13.3.23. 23 With ethyl acetate as solvent, the current efficiency
was less than for chlorinated hydrocarbons. The results indicate that the chlorinated
hydrocarbons have a higher extraction capacity for Bu 4N + ClO - from the aqueous phase than
has ethyl acetate. Similar results were obtained by Dehmlow and Dehmlow. 16 The current
efficiencies change slightly when different chlorinated hydrocarbons were used as organic
solvents.
Tsai and Chou 120 carried out the indirect electrooxidation of cyclohexanol by using a
double mediator consisting of ruthenium and chlorine redoxes in the multiphase system.
Table 13.3.24 shows that the current efficiency had the highest value at 83% using carbon
tetrachloride as organic solvent. 120 The current density decreased in order, carbon tetrachloride
> chloroform > toluene > cyclohexane. The selectivity was 100% except when toluene
was used as organic solvent. For this case, the concentration of cyclohexanol in carbon tetrachloride
is higher than that of the other solvents.