Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.3 Effects of organic solvents on phase-transfer catalysis 831
agent. Resin solvation changes during the course of the reaction when the attached substrate
changes its polarity or other physicochemical properties.
The basic steps involved in reactions with resin-supported PTC catalysts differ from
ordinary two-phase PTC reactions in one important respect: ordinary PTC reactions require
only one reagent to be transferred from their normal phase to the phase of the second reactant.
Use of resin-supported catalysts requires that both reagents diffuse to active PTC sites
on the catalyst surface, or for reactions with slow intrinsic rates, both reagents must also diffuse
to the active sites inside the resin bulk phase. The need for diffusion processes with
solid catalysts also means that both reagents are required to diffuse to and penetrate the stagnant
outer layer of liquid(s) (the Nernst layer), coating the catalyst particle.
Ford and Tomoi 28 carried out the reaction of 1-bromooctane with aqueous sodium cyanide,
catalyzed by tributylphosphonium groups bound into beads of an insoluble styrene-divinylbenzene
resin,
polymer −C8H5PBu
3Br
1− C8H17Br( org ) + NaCN(
aq ) ⎯⎯⎯⎯⎯⎯⎯→1− C H CH + NaBr
8 17 ( org ) ( aq ) [13.3.16]
The reaction includes the following steps:
(a) Diffusion of aqueous sodium cyanide through the bulk phase and through the resin
bulk to active sites
(b) Equilibrium exchange of CN - for Br - at the active sites
(c) Diffusion of Br - out of the catalyst particle and into the aqueous bulk phase
(d) Diffusion of RBr (1-C 8H 17Br (org)) through the organic bulk phase and through the
bulk resin phase to active sites. Some reactions may occur at sites on the catalyst surface,
but since the number of surface sites is small compared to the number of sites within the
bulk of the catalyst, most of the reaction occurs inside the catalyst bulk.
(e) Chemical reaction (intrinsic reaction) between RBr and Resin-PR 3 + CN - at active
sites to produce RCN and Br -
(f) Diffusion of RCN out of the catalyst particle and into the organic phase.
A schematic diagram of the general
resin-bound PTC catalyst is given in Figure
13.3.10. As indicated in Figure 13.3.10,
spacer chains can increase some reactions
by removing the active site away from the
polymer chain, and from other active sites.
When active sites, particularly quaternary
onium salts, are located close to one an-
Figure 13.3.10. Schematic diagram of the general
resin-bound PTC catalysts.
other, they join to form doublets, triplets,
and higher aggregates that are less active
catalyst centers, and that tend to present an
“aqueous” face to the reactants. Thus, the use of spacer chains increases the rates of some
reactions, such as nucleophilic displacements, two-to-four-fold. 1,9,73 When the spacer chain
also contains complexable ether oxygen atoms, using 15-crown-5 ether as the PTC functional
group, catalyst activity is even greater, as observed in halide exchange of KI with
1-bromooctane. 10
Preparation of phase transfer catalyst (PTC) functional groups bound to insoluble resins
and their activity for catalyzing two-phase reactions has been extensively stud-