Handbook of Solvents - George Wypych - ChemTech - Ventech!

826 Maw-Ling Wang
quaternary salt catalyst is not highly soluble in either the aqueous or the organic phase, but
instead forms a third phase. 154 For example, tetrabutylammonium salts in the presence of
highly concentrated aqueous solutions, and with toluene as solvent for the organic phase,
form a third (quaternary compound) phase. 126 Jin et al. 48 investigated the relationship between
the properties and the catalytic activity of the liquid-liquid-liquid three-phase
phase-transfer catalytic system. The condition of formation of the third phase are investigated
by changing the kind of PTC and organic solvent. With tetra-n-butylammonium bromide
Bu 4N + Br - is used as the phase transfer catalyst, the third phase is formed with both
dodecane and toluene as the organic solvents. On the other hand, when
tetra-n-hexylammonium bromide (Hex) 4N + Br - is used as the phase transfer catalyst, the
third phase forms with dodecane but not with toluene. In such situations, most of the reaction
actually occurs in the third phase with both aqueous and organic reagent transferring to
this phase for conversion. Third-phase reactions of this type may be faster than simple PTC
reactions. Because formation of the third phase offers simplified catalyst removal and recovery
procedures, third-phase catalyst is highly attractive for commercial operations.
In the conversion of benzyl chloride to benzyl bromide using tetra-n-butylammonium
bromide as the catalyst, a third phase was also observed. 126 More rapid reaction rates were
obtained in the presence of this additional phase. The kinetics associated with the base-catalyzed
isomerization of p-allylanisole in the presence of a variety of polyethylene glycols
(PEGs) has been reported. 79 The reaction followed first-order kinetics in p-allylanisole and
the reaction system was characterized by three phases consisting of an organic solvent
phase, an aqueous base phase, and a complex liquid phase consisting of PEG and potassium
hydroxide. It was suggested that the isomerization reaction took place in the complex third
phase.
PEG forms a third phase between toluene and aqueous KOH when some methanol is
added. 39 Some methanol must be added to reduce the amount of PEG that otherwise would
dissolve in the toluene phase. Thus, dehydrobromination of 2-bromooctane in toluene, using
PEG as catalyst, could be accomplished by removal and replacement of the organic and
aqueous phases after completion of reaction, and recycle of the catalyst phase. After four
cycles no catalyst was lost. Reaction rates for KOH dehydrohalogenation of 2-bromooctane
in toluene with PEG catalysts were increased by a maximum factor of 126 by addition of
methanol. 38,40 The base efficiency (moles base per mole of catalyst) PEGs with molecular
weights 3000 and 20,000 exceeded unity and reached a maximum of 12 on addition of
methanol. Hydrogenative dehalogenation of polychlorinated aromatic halides can be accomplished
by hypophosphite reduction using a quaternary ammonium salt as a
phase-transfer catalyst in conjunction with a palladium-on-carbon co-catalyst. 68 The quaternary
salt, being insoluble in both reactant phases, coats the Pd/C catalyst forming a third
phase. The strongly alkaline medium and the phase-transfer agent are synergistic.
In principle, formation of a third catalyst could be accomplished either by (1) use of a
phase-transfer catalyst that has limited solubility in both the organic and aqueous phase, or
(2) by use of a special solvent, perhaps a fluorocarbon, having great affinity for the catalyst
but only modest affinity for both of the reactants. Tetra-n-butyl-ammonium salts frequently
form third layer when used in conjunction with an organic phase that has little polarity (e.g.,
neat 1-chlorooctane, toluene, or cyclohexane as solvent, but only dichloromethane) and
with a concentrated aqueous solution of inorganic salts.