Heterogeneous solvent environments

Alkylation of 2-naphthoxide ion (Eq. (6)) occurs mainly on carbon in aqueous solvents and mainly on oxygen in aprotic solvents. The product distribution is often used as a probe of the solvent environment in heterogeneous reactions. Brown and Jenkins 54) found that 40-100 % RS spacer chain catalysts 15 and 16 gave up to 98 % O-benzylation of 2-naphthoxide ion with benzyl bromide. The shorter spacer chain catalyst 16 gave 85% O-alkylation, and a conventional benzyltrimethylammonium ion resin 2 gave about 70 % O-alkylation. Because of low activity, product distribution data were obtained with varied amounts of catalyst and were extrapolated to equimolar amounts of catalyst and substrate to obtain the catalyzed O/C product ratios. Interpretation of the data also was complicated by independent evidence that catalysts 15 adsorbed 2-naphthoxide ion, in addition to that bound by ion exchange54). Essentially the same results were obtained with catalysts 24 which lack the ester link in the spacer chain 106). 

Chemically modified silica gel is well known to provide very attractive molecular environments in which a wide variety of physico-chemical processes can be examined. One of the advantages of using the chemically modified silica gel is the easiness of the control of the molecular environments, for instance, by varying the extent of chemical modification or by exposing it to different kinds of solvents or gases. By combining these features of the chemically modified silica gel with the technique of the lifetime measurements under a microscope developed recently in our laboratory (1), we can expect to get a much closer view of the heterogeneous molecular environments provided by the interfaces. 

The polarity of a liquid-liquid interface is an important factor to consider for heterogeneous reaction kinetics, as the solvent environments at the interface are different from those in bulk media. In 1998, Wang et al. reported a second... 

The heterogeneity of the reaction medium is also important in determining the molecular weight and in solution polymerization of maeromonomers. The magnitude of the effect varies according to the solvent quality. PS macromonomer chains in good solvents (e.g. toluene) have au extended conformation whereas in poor solvents (e.g. melhylcyclohexane) chains are tightly coiled.89 As a consequence, the radical center may see ail environment that is medium dependent (see also Sections 7.6.5 and 8.3.7). 

Fig. 9.20 Grafted heterogeneous polymer brushes in different environments and principle of switching (a) Structure in a nonselective solvent, (b) in a selective solvent for polymer PI (e.g. PS), and (c) in a selective solvent for polymer P2 (e.g. P4VP) (modified from ref. [214]).

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