Phase Transfer Catalysis - Publications of the IAS Fellows

practical purposes the quat exists as an ion-pair and not as
free ions in the organic phase. In solid-liquid systems, where
the restriction of immiscibility with water is relaxed, polar
solvents like acetonitrile can be used to give high reaction
rates. Another attractive option is to use the organic substrate
as the organic phase. Besides giving higher conversions
due to higher concentrations of the reactant (pure), solvent
recovery steps are eliminated leading to huge reductions in
process costs (Bram and Sansoulet, 1985, and references
therein).
For a quat to be an efficient PT catalyst, it is necessary
that the extraction coefficient of Y- be higher than that of
X- (Gordon and Kutina, 1977). Bar et al. (1984) have theoretically
derived conditions for which a foreign counterion Zassociated
with the phase-transfer catalyst can poison the reaction
between RX and m. Similarly, experimental results
have shown that catalysts in the iodide form are poor PT
catalysts, since they lead to catalyst poisoning due to the tendency
of the quaternary salt to associate strongly with iodide
ion, whereby transfer of the nucleophilic anion is not initiated
(Gordon and Kutina, 1977). Similarly, Sasson and Zahalka
(1983) reported catalyst poisoning effects in the PT catalyzed
esterification of alkyl chlorides by hydrophilic formate
ion, with the poisoning effect reduced by the use of highly
concentrated formate solutions. This phenomenon (catalyst
poisoning) can only be solved by using large excesses of Y-
(Landini et al., 1974), or by replacing the aqueous phase with
fresh Y- batches several times. The use of a saturated aqueous
phase of the attacking hydrophilic nucleophile, which
precipitates the salt of the leaving group, also provides a simple
solution to catalyst poisoning (Sasson and Zahalka, 1983).
Mechanisms of PTC
PTC under neutral conditions
A large number of PT catalyzed reactions involving simple
displacement reactions are carried out under neutral conditions,
although some variants of the typical PTC mechanism
are possible. In the general case, the role of the PT catalyst
(Q+X-) is to function as a vehicle to transfer the anion (Y-)
of the metal salt (W Y-) from the aqueous or solid phase
into the organic phase where it reacts with the organic substrate
RX, giving the desired product RY and regenerating
Q+X-, which can continue the PTC cycle.
Liquid-Liquid PTC. For a typical LLPTC cycle involving a
nucleophilic substitution reaction under neutral conditions,
Figure 2a shows the PTC cycle for cases where the PTC is
partitioned between the organic and aqueous phases. Starks’
extraction mechanism for a nucleophilic substitution reaction
suggests that the quaternary salt must dissolve in the aqueous
phase in order to pick up the nucleophile from the aqueous
phase and then ferry the Q+ Y- into the organic phase where
reaction occurs. Here, the ion-exchange step between the
PTC and the nucleophile occurs in the aqueous phase bulk,
followed by transfer of the PTC-reactive anion pair into the
organic phase, where reaction ensues. However, according to
another parallel mechanism, the Brandstorm-Montanari
mechanism (Figure 2b), dissolution of the qua< in the aqueous
phase is not necessary. The quaternary salt could be too
lipophilic to dissolve in the aqueous phase and yet function
as a good PT catalyst. In this case, the PT catalyst resides
AIChE Journal March 1998
Aqueous