Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.3 Effects of organic solvents on phase-transfer catalysis 813
ties. The unpaired electrons on the oxygen atom associate with the anions. A relatively
lower conversion is obtained in MeOH or EtOH solvent. This result indicates that the acidic
hydrogen bond does not have a strong catalytic capability.
The aprotic solvents, which do not possess hydrogen bond, are highly polar. Therefore,
the aprotic solvents possess high alkalinity and nucleophilicity required to obtain a
high conversion of o-phenylene diamine in the synthesis of mercaptobenzimidazole (MBI).
A larger conversion is obtained when using a protic solvent or aprotic solvent of high polarity.
However, the structure of DMF, which is an amide, is similar to that the tertiary amine.
It possesses similar catalytic property to dimethylaminopyridine (DMAP). The effect of
DMF on the conversion of o-phenylene diamine is more pronounced than that of DMSO.
The Arrhenius rate equations in various solvents for the reaction of o-phenylene diamine
and carbon disulfide catalyzed by tributylamine are as follows:
DMF: k app = 1.06x10 15 exp(-1.20x10 4 /T)
DMSO: k app = 7.82x10 8 exp(-7.78x10 3 /T)
MeCN: k app = 1.39x10 13 exp(-1.09x10 4 /T) [13.3.10]
MeOH: k app = 9.62x10 14 exp(-1.24x10 4 /T)
EtOH: k app = 3.84x10 10 exp(-9.29x10 3 /T)
THF: k app = 3.25x10 38 exp(-2.99x10 4 /T)
k app is the apparent rate constant in which the reaction follows pseudo-first-order rate law.
In two-phase phase-transfer catalytic reactions, the solvents significantly affect the reaction
rate. The main reason is that the distribution of regenerating catalyst QX and the active
catalyst QY between two-phases is highly dependent upon the polarity of the solvent. It
is desirable for most of the intermediate products to stay in the organic phase and react with
the organic-phase reactant. Therefore, a solvent with high polarity will be preferred for the
reaction.
(C) Pyridine 1-oxide (PNO) as IPTC
The substitution reaction of benzoyl chloride (PhCOCl) and sodium acetate
(CH 3COONa) using pyridine 1-oxide (PNO) as the inverse phase-transfer catalyst (IPTC)
in a two-phase system of organic solvent and water was investigated by Wang, Ou and
Jwo. 148-150 They found that the polarity of the organic solvent strongly affected conversion
of benzoyl chloride, the yield of the main product (acetic benzoic anhydride
(PhCOOCOCH 3)), and the reaction rate. The reaction follows a pseudo-first-order kinetic
rate law. Dichloromethane, chloroform, tetrachloromethane and cyclohexanone (C 6H 10O)
were used as the organic modifier in the two-phase reaction system. The results are given in
Table 13.3.10. A linear reaction rate was observed for a more polar organic solvent. The order
of relative reactivities in these solvents is cyclohexanone > dichloromethane > chloroform
> tetrachloromethane, consistent with their polarities. Kuo and Jwo 54 obtained similar
results. Wang, Ou and Jwo 148 also found that the conversion was substantially increased
with initial concentration of PNO increasing in the aqueous phase with CH 2Cl 2 present as