Charged Catalysts Quaternary Ions

We have discussed above in general terms the requisites and expectations associated with the basic principles of phase transfer catalysis. In fact, these ideas accord well with what is now known about the mechanism of many phase transfer processes. Detailed work has been carried out by several groups, and their conclusions are in substantial agreement. 

Starks [10] examined the reaction of cyanide ion with n-octyl bromide (Eq. 1.7) and found that 1) the reaction occurred in the organic phase 2) the displacement was first order in alkyl halide and first order in catalyst (QX) 3) the rate of reaction 

Herriott and Picker [19] addressed the same question in a somewhat different fashion. They examined the two phase reaction of secondary-octyl bromide with hydroxide ion. Based on Ingold s prediction, if the reaction occurred in the organic phase, elimination products would predominate, whereas reaction in the aqueous phase would favor substitution. From the predominance of elimination products, it was inferred that reaction occurred in the organic phase. Beyond a minimum value, stirring rate was found not to affect the reaction rate, a fact which allows one to exclude interfacial phenomena as important factors. The independence of catalytic effectiveness on the general shape of the catalyst helped exclude micellar effects from consideration. 

Herriott and Picker have also reported a careful study of catalyst efficiencies [19b]. The system examined was the reaction of thiophenoxide ion with -bromooctane (Eq. 1.8). The second order rate constants for the reaction as a function of catalyst are shown in Table 1.1. Note that the comparison is valid only for the benzene-water 

It is interesting to note that the very widely used Makosza catalyst , benzyl triethyl ammonium chloride, does not show high efficiency in this study. 4) Phosphonium ions are somewhat more effective and thermally stable than the corresponding ammonium catalysts and both are better than arsonium systems. 5) Substitution of the quaternary ion by alkyl rather than aryl groups yields more effective catalysts. 6) Reaction rates are generally greater in orf/io-dichlorobenzene (and presumably in other chlorocarbon media) than in benzene, and botli are better than heptane. In connection with this latter point, Ugelstad and coworkers have studied the reactions of quaternary ammonium phenoxide ions with alkyl halides in a variety of media and concluded that the 

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Phase transfer processes rely on the catalytic effect of quaternary onium or crown type compounds to solubilize in organic solutions otherwise insoluble anionic nucleophiles and bases. The solubility of the ion pairs depends on lipophilic solvation of the ammonium or phosphonium cations or crown ether complexes and the associated anions (except for small amounts of water) are relatively less solvated. Because the anions are remote from the cationic charge and are relatively solvation free they are quite reactive. Their increased reactivity and solubility in nonpolar media allows numerous reactions to be conducted in organic solvents at or near room temperature. Both liquid-liquid and solid-liquid phase transfer processes are known the former ordinarily utilize quaternary ion catalysts whereas the latter have ordinarily utilized crowns or cryptates. Crowns and cryptates can be used in liquid-liquid processes, but fewer successful examples of quaternary ion catalysis of solid-liquid processes are available. In most of the cases where amines are reported to catalyze phase transfer reactions, in situ quat formation has either been demonstrated or can be presumed. 

The resulting catalyst was highly active for cyanide and acetate ion displacements on 1-bromobutane. As expected, soluble low molecular weight quaternary ammonium salts and a soluble quaternized linear poly(ethyleneimine) were even more active, presumably because they had no mass transfer and intraparticle diffusional limitations. These catalysts had a much higher density of charged sites (at least within the micro domains of the poly(ethyleneimine)) than any of the other active quaternary ammonium ion catalysts reported for nucleophilic displacement reactions. 

Several quaternary ammonium compounds are used in organic chemistry as phase-transfer catalysts. The mechanism of the catalytic process can be represented by a combination of phase-transfer and ion-exchange equilibria. In the case of substitution reactions in two-phase systems, the negatively charged nucleophile is extracted by the positive ammonium ion from the aqueous phase into the organic phase where substitution takes place (Makosza and rafin, 1965, Makosza, 1969, Dockx, 1973). 

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