Hydrolytic exchange

Taking into account the vicarious nature of the nucleophile the authors suggested that ring closure occurs inside alkenic aH-adduct 290 with participation of the nitro group. However, in principle, the reaction might proceed via the formation of nitroso compound 291 with subsequent electrocyclization into oxazine 292 and hydrolytic exchange of the p-tosyl group (Scheme 85). 

This process, historically referred to as hydrolysis because it can proceed even from the weak dissociation reaction of water molecules, causes a marked rise in the solution pH. To avoid confusion with more appropriate usage of the term hydrolysis, exchange reactions involving protons generated from water or carbonic acid and producing alkalinity in solution will be referred to as hydrolytic exchange. 

Unless proton occupancy of exchange sites is taken into account, hydrolytic exchange will seem to create nonstoichiometric exchange between cations. An example is shown in Figure 3.15, where the adsorption of on a Na -smectite appears to release more Na into solution than mole-for-mole exchange could produce. 

Problem Consider a calculation of the pH and extent of hydrolytic exchange that results when 1.0 g of Na -smectite is placed into 1.0 liter of distilled water that is open to the atmosphere. The smectite has a CEC of 0.9 mmole/g. Two reactions involving protons (3.56 and 3.59) must be considered. The first, which describes the dissociation of carbonic acid to form bicarbonate and a proton, has an equilibrium condition given by equation 3.58. The second is the exchange of Na" " from the clay by... 

Here x is the unknown quantity of Na exchanged from the clay by the hydrolytic exchange mechanism. When the equilibrium values from this table are substituted into equation 3.64, the expression... 

Again, as in the last problem, the charge of any HCOf in solution must be balanced by H or by Na released by hydrolytic exchange. Consequently, equation 3.68 is still valid when 10 M NaQ is present. By substituting the expression for [H" ] (equation 3.71) into equation 3.68, a new relationship for is found ... 

By trial and error, the solution to this equation is found to be x = 0.0(X)2. It is clear, then, that almost no hydrolytic exchange occurs if the initial concentration of electrolyte in solution is high that is, Na exchange by H" is suppressed by high Na activity. The value of X, substituted into equation 3.71, gives a solution for [H" ], and reveals that the pH in this system would be 5.65, the same as the pH in aqueous solution when no clay is present. 

Significant replacement of monovalent metal cations on layer silicate clay surfaces by protons can occur if the electrolyte concentration is very low. The long-term result, beyond hydrolytic exchange, is acidic decomposition of the clay structure in part, and release of structural AP or Mg + to solution. These multivalent cations may then readsorb onto exchange sites, influencing the rheological properties of clays in very dilute salts. Some of the anomalous behavior of Na -smectites suspended in... 

The mechanism of hydrolytic exchange, because it generates acidity on the colloids while forming alkalinity in solution, can lead to the spatial separation of acidity on soil solids from alkalinity in solution. The result is an important rule of soil chemistry ... 

It will be seen in Chapter 6 that this principle helps to explain the fact that soils of humid climates become acidified by natural processes. In Chapter 8, hydrolytic exchange will be found to generate alkalinity under environmental conditions that prevent significant leaching. 

There are basically three potential hazards of salt-degraded soils to plants—salinity, sodicity, and alkalinity. The first two hazards are used to classify soils as saline or sodic, or both, as described in Table 8.4. The alkalinity hazard (measured by RSC) is not usually applied directly to the classification of soils, but the pH values reported in Table 8.4 indicate that sodic soils are the most likely to have a problem of high alkaUnity. This arises partly from the hydrolytic exchange reaction, described in Chapter 3, which is enabled by the presence of appreciable exchangeable Na" and low salt concentrations, conditions that are diagnostic of sodic soils. Hydrolytic exchange can be viewed as a two-step reaction ... 

In another study that appeared prior to the advent of CASTing, the traditional combination of epPCR and DNA shuffling was used to enhance the enantioselectivity of the hydrolytic kinetic resolution of p-nitro phenyl glycidyl ether and other epoxides catalyzed by the EH from Agrobacterium radiobacter [59]. Several mutants were obtained with up to 13-fold improved enantioselectivity. The amino acid exchanges took place around the active site. 

Hydrolysis. NMR results show that TBT carboxylates undergo fast chemical exchange. Even the interfacial reaction between TBT carboxylates and chloride is shown to be extremely fast. The hydrolysis is thus not likely to be a rate determining step. Since the diffusivity of water in the matrix is expected to be much greater than that of TBTO, a hydrolytic equilibrium between the tributyltin carboxylate polymer and TBTO will always exist. As the mobile species produced diffuses out, the hydrolysis proceeds at a concentration-dependent rate. Godbee and Joy have developed a model to describe a similar situation in predicting the leacha-bility of radionuclides from cementitious grouts (15). Based on their equation, the rate of release of tin from the surface is ... 

Fig. 9.2. Simplified reaction mechanisms in the hydrolytic decomposition of organic nitrites. Pathway a Base-catalyzed hydrolysis with liberation of nitrite. Pathway b Reversible nitro-syl exchange between organic nitrites and alcohols. Pathway c General acid catalysis with concerted mechanism in the acid hydrolysis of organic nitrites.

---