Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.3 Effects of organic solvents on phase-transfer catalysis 811
Table 13.3.8. Effect of the aprotic solvent on the initial reaction rate of the allylation of
phenoxide
Solvents Dielectric constant (20 o C)
Initial reaction rate×10 3 Mh -1
(-r) i,B
(-r) i,PEG
(-r)i,PEG/(-r)i,B
n-decane 1.991 171.77 355.62 2.07
cyclohexane 2.023 127.5 277.68 2.16
ethyl ether 4.335 98 205.83 2.10
cyclobenzene 5.708 75.25 174.56 2.32
dichloromethane 9.080 45.61 86.31 1.89
Data obtained from the work of Wang and Chang 142
13.3.1.3 Effects of the organic solvents on the reactions in other catalysts
(A) Quaternary ammonium salts as NPTC
The effects of the organic solvents on the reaction rate are given in Table 13.3.9. 121 The
rates of the reactions of the tetra-n-butylammonium and potassium salts of phenoxide with
1-chlorobutane and 1-bromobutane in pure solvents and solvent mixtures varying in dielectric
constant from 2.2 to 39 were obtained by Uglestad et al. 121
Table 13.3.9. Effect of organic solvent on the reaction of tetra-n-butylammonium salts
of phenoxide and potassium salts of phenoxide with halobutane
Reactants
Solvents
Rate constant×10 5 , Lmol -1 s -1
1-C4H9Cl K
1-C4H9Cl 1-C4H9Br 1-C4H9Br Dielectric
constant
() ε
+- OC6H5 Bu4N +- OC6H5 K +- OC6H5 Bu4N +- OC6H5 Dioxane 10% 0.01 330 2.2
CH3CN 50% 0.0025 2.8 0.22 400 6
CH3CN 0.084 4.0 12 600
Acetonitrile 0.33 2.2 40 300 39
Data obtained from Uglestad et al., 121 0.2MC 6H 5O - , 0.05 or 0.1 M C 4H 9X, 25 o C
The rates of reaction of potassium phenoxide vary over three orders of magnitude with
the changes in the dielectric constant of solvent, whereas the corresponding rates with the
tetra-n-butylammonium salt vary by approximately a factor of 6.
(B) Tertiary amines as NPTC
Quaternary ammonium salts, PEGs and crown ethers are the common compounds,
employed as PTC. The inexpensive tertiary amines have also been used as the phase transfer
catalysts (PTC) in recent years. The synthetic process for producing 2mercaptobenzimidazole
(MBI) is a reaction of o-phenylene diamine (C 6H 4(NH 2) 2) and carbon
disulfide (CS 2) in a two-phase medium affected by appropriate choice of solvent. 128-130