Handbook of Solvents - George Wypych - ChemTech - Ventech!

798 Maw-Ling Wang
13.3 EFFECTS OF ORGANIC SOLVENTS ON PHASE-TRANSFER
CATALYSIS
Maw-Ling Wang
Department of Chemical Engineering
National Chung Cheng University, Taiwan, ROC
The reaction of two immiscible reactants is slow due to their low solubilities and limited
contact surface area. The conventional way to improve the reaction rate or to elevate the
conversion of reactants is to increase the agitation speed, temperature, or use the protic or
aprotic solvent to dissolve the reactants. The increase in agitation speed can increase the
contact surface area between two phases only to a certain value. Thus, the reaction rate or
the conversion is limited by the increase in the agitation speed. Usually, the rate of reaction
is increased by raising the temperature. However, byproducts are accompanied by elevating
the solution temperature. The separation of product from byproducts or catalyst makes the
cost to increase. Although protic solvent (CH 3OH, or CH 3COOH) can dissolve reactants,
solvation and hydrogen bonding make the activity of the nucleophilic anion decrease significantly.
Thus, the reaction rate using protic solvent is retarded. For the other case, the reaction
rate is largely increased using aprotic solvent. The application of aprotic solvent is also
limited because of cost and recovery difficulty. For this, the problem of two-phase reaction
is not overcome until the development of phase-transfer catalysis (PTC). Phase-transfer catalytic
reactions provide an effective method in organic synthesis from two immiscible reactants
in recent development. 93,103,111,113
In 1951, Jarrouse 47 found that the reaction of aqueous-soluble sodium cyanide (NaCN)
and organic-soluble 1-chlorooctane (1-C 8H 17Cl) is dramatically enhanced by adding a small
amount of quaternary ammonium salt (R 4N + X - ,orQ + X - ,Q + :R 4N + ). The reaction is almost
complete and a 95% conversion is obtained within two hours when a catalytic amount of
tetra-n-butylammonium chloride ((C 4H 9) 4N + Cl - ,orQ + Cl - ,Q + :(C 4H 9) 4N + ) is added. The
mechanism of the reaction of sodium cyanide and 1-chlorooctane in organic solvent/water
two-phase medium is expressed as
[13.3.1]
As shown in Equation [13.3.1], sodium cyanide (NaCN) and 1-chlorooctane
(1-C 8H 17Cl) are soluble in aqueous phase and organic phase, respectively. In the aqueous
phase, NaCN first reacts with tetra-n-butylammonium chloride ((C 4H 9) 4N + Cl - ,Q + Cl - )to
produce organic-soluble tetra-n-butylammonium cyanide ((C 4H 9) 4N + CN - ,Q + CN - ). Then,
this tetra-n-butylammonium cyanide (QCN) further reacts with 1-chlorooctane (1-C 8H 17Cl)
to produce 1-cyanooctane (C 8H 17CN) in the organic phase. Tetra-n-butylammonium chloride
((C 4H 9) 4N + Cl - ), which is also produced from the organic-phase reaction, transfers to the
aqueous phase, prepared for further regeneration. It is obvious that PTC reaction 107 involves