Handbook of Solvents - George Wypych - ChemTech - Ventech!

802 Maw-Ling Wang
Both cation and anion of phase-transfer catalyst can affect the distribution of the PTC
between two phases, and hence the reaction rate. The partitioning equilibrium of the anion
between organic and aqueous phases can be qualitatively estimated from the free energies
of the anion transfer from water to organic phase. A large positive free energy of transfer
from the aqueous phase to the organic phase clearly indicates that the anion prefers to reside
in the aqueous phase. For example, the free energies of transfer of Cl - ,Br - , and I - from water
to acetonitrile are +11.6, +8.1 and +4.8 Kcal/mol, respectively. 110 Thus, the transfer from
aqueous to organic phase becomes less unfavorable as one proceeds from chloride to bromide
to iodide. These trends may be understood in terms of the change in charge-to-volume
ratios of the halide ions. Because chloride has the largest charge-to-volume ratio, it is the
least polarizable and the most strongly hydrated. In contrast, iodide has a relatively diffuse
charge and is less strongly hydrated.
A successful phase-transfer catalytic reaction occurs when the process is able to transfer
the anions from the aqueous phase to the organic phase or vice versa for the reaction to
proceed, and the transferred anions are active and prepared for reaction. An active catalyst
needs to be sufficiently distributed in the organic phase for the reaction to occur. The distribution
of catalysts and the associated anion in the organic phase strongly depends on the
structure of the quaternary cation and the hydration of anion being transferred into the aqueous
phase. Therefore, the following results are used for the reference in selective NPTC catalyst.
(1) Tetramethylammonium cation with a simple anion (CH 3) 4N + Y - (Y=Cl - ,Br - ,CN - ,
etc.) is not easily distributed in most organic solutions. Therefore, (CH 3) 4N + Y - are usually
not good PTCs. The only ways to increase the distribution of (CH 3) 4N + Y - is to couple the
cation with a large organic anion 13,49,78,95,155,156 or to use an organic solvent of high purity.
17,107,109,110,152
(2) Tetraethylammonium ((C 2H 5) 4N + Y - ) and tetrapropylammonium ((C 3H 7) 4N + Y - )
salts are also poor catalysts for transferring small anions into most organic solutions. 51,105
(3) Tetrabutylammonium salts show high efficiencies as phase-transfer catalysts.
They are readily available in high purity on a commercial scale.
(4) Quaternary ammonium cations, R 4N + , R=C 5H 11 to C 10H 21 easily extract anions into
organic phase and exhibit higher catalytic activities.
(5) Higher tetraalkylammonium salts, R 4N + X, R: (C 12H 25) 4N + and higher groups, can
easily extract anion into an organic phase. However, the interchange of anions between organic
and aqueous phases is slow and the reaction rate decreases compared with quaternary
salts where R=C 5H 11 to C 10H 21.
Table 13.3.1 shows the effect of catalyst structure on the rate of PTC reaction of
thiophenoxide with 1-bromooctane.
In addition to the preference of anion to reside in the aqueous or organic phase, a distribution
ratio (or partition coefficient), α, of phase-transfer catalyst (QX) cation between
aqueous and organic phase is defined as
α =[QX] org/[QX] aq
[13.3.4]
Use of solvents having higher polarity facilitates distribution of quaternary salts into
organic solvents. Hence, it also allows use of smaller quaternary salts as catalysts. With di-