Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.3 Effects of organic solvents on phase-transfer catalysis 823
Figure 13.3.9. Effect of the water content on the conversion; 2.28 g
of 2,4-dibromophenol, 0.2 g of TBAB catalyst, 0.7 g of allyl bromide,
1.0 g of KOH 50 mL of chlorobenzene, 50 o C. (Adapted from
Ref. [132], by permission.)
alkaline solution of KOH/chlorobenzene
two-phase medium under
PTC conditions, 132 the conversion
increases with the increase in the
concentration of ArOQ in the
aqueous phase (or decreasing content
of water). However, this
change is small, reflecting a small
mass transfer resistance, as shown
in Figure 13.3.9. 132 The influence
of the amount of water on the conversion
in the reaction of carbon
disulfide and o-phenylene
diamine catalyzed by tertiary
amine in a two-phase medium was
studied. The conversion decreased
with the increase in the
amount of water. Therefore, the
value of apparent rate constant
(k app), in which the reaction follows
pseudo-first-order-rate law,
decreases with the increase in the
volume of water. 128 Wang and Chang 144-146 found that the conversion increases with the increase
in the volume ratio of water to organic solution up to 1/5. The conversion is independent
of the volume ratio of water to organic phase (chlorobenzene), greater than 1/5. The
reason is that the reaction is carried out in a large amount of KOH (solid form). Probably,
the omega phase is generated for the volume ratio of water to chlorobenzene less than 1/5.
However, this change in the conversion vs. the volume ratio of water to chlorobenzene is
not significant.
(A) Omega phase reaction
It is found that 92% of the 18-crown-6 ether added to a salt (KCN and KCl) and toluene
system resided in the organic phase. However, all but approximately 1-2% of the crown
ether was translocated onto the surface of the salt upon addition of small quantities of water.
The results of Liotta et al. 62 are given in Table 13.3.19. The initial water added to the system
coats the surface of the salt particles and it was this aqueous salt coating that extracted the
crown from the organic phase. Liotta et al. 61,62 called this new region of the reaction system
the omega phase. The 8% of the crown located on the surface of the salt particles prior to the
addition of water was probably due to the presence of water already present in the salt. The
distribution of 18-crown-6 ether between the organic phase and the omega phase was determined.
62 The amount of crown ether in the organic phase remained low and relatively constant
(0.06-0.07 milimoles of 18-crown-6 ether in organic phase). The omega phase
adsorbed most of added crown ether. For the accompanying pseudo first-order kinetics reaction
of benzyl bromide with potassium cyanide, the results are given in Table 13.3.20. 122
There is a slight increase in the rate as the number of millimoles of 18-crown-6 ether increases,
but the rate remains essentially constant with the increase of crown ether.