Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.3 Effects of organic solvents on phase-transfer catalysis 801
in the cation-anion interionic distance for the two ion pairs. In principle, anions do not have
a great affinity for nonpolar solvent and prefer to reside in an aqueous phase.
Ease or difficulty of transfer of most anions into organic-phase solution is also highly
affected by the organic solvent, i.e., interaction of the organic solvent and the reactant. In
general, a polar solvent may be necessary to obtain an appropriate rate of the anion transfer
to the organic phase for a NPTC process. Solvent may be necessary to increase the rate of
the organic-phase reaction. The most common solvent, dichloromethane (CH2Cl2), has been
extensively used as a polar solvent in the PTC work because it readily dissolves most quaternary
salts and other phase-transfer agents, and because it is polar to speed both the transfer
step and the organic-phase reaction step. Although the hydrocarbons suffer from lack of
polarity, they have also been extensively used as solvents for PTC systems. The main reasons
are that they are reasonably safe, inexpensive and easy to recover in a high purity. One
strategy for selecting organic solvent is that a high boiling point solvent is selected for a reaction
in which the product has a low-boiling point. In other cases, a solvent might be chosen
to minimize solubility of phase-transfer agent in the organic phase to force formation of
third phase (catalyst) from which the phase-transfer catalyst may be more easily separated
or extracted.
The phase-transfer catalytic reactions (NPTC, RPTC or IPTC) are usually carried out
in a liquid-liquid two-phase medium. They have been extensively applied to liquid-gas, liquid-solid
two-phase media. 18,21,63,128-130 However, purification of product from catalyst in the
liquid phase of a final solution is difficult to produce a product of high purity. In 1975,
Regen and coworkers 88-92 proposed triphase catalysis (TC) in which the catalyst is immobilized
on a porous solid support (usually polymer). The solid catalyst is easily separated from
the final products after reaction by mechanical separation processes, such as centrifugation
or filtration. The organophilicity and the hydrophilicity of the solid polymer support greatly
influence the content and the imbibed composition of the organic phase and the aqueous
phase within the solid porous polymer support. Hence, the reaction rates are determined by
the concentrations of reactants in both the organic phase and aqueous phase, they are controlled
by the organic solvents. Therefore, it is important to understand the characteristics of
the organic phase in the triphase catalyst as well as the characteristics of the organic solvent
in the liquid-liquid two-phase PTC reaction.
13.3.1 TWO-PHASE PHASE-TRANSFER CATALYTIC REACTIONS
13.3.1.1 Theoretical analysis of the polarity of the organic solvents and the
reactions
The transfer of anions from an aqueous phase to an organic phase may be achieved by
choosing a phase-transfer cation that is not strongly solvated by water, that has organic-like
characteristics, and is compatible with the organic phase for NPTC. The factors that affect
the mass transfer and the distribution of the phase-transfer catalyst cation-anion pair between
the organic and aqueous phases include:
(1) the charge-to-volume ratio of the anion, the polarizability, and the organic
structure of the cation associated with the anion,
(2) the hydrophilic-organophilic balance of the associated cation;
(3) the polarity of the organic phase;
(4) the hydration of the anion;
(5) the presence of aqueous salts and/or aqueous hydroxide ions.