Extractive reactions

This is the commonest mechanism. It simply involves partition of the reactive organic species, e.g. an ester into a separate phase, typically aqueous, where reaction occurs. An example is the two-phase solvolysis of esters by aqueous sodium hydroxide, which has been extensively studied [23]. Some basic principles are worth noting. Since reaction occurs following partition, the reaction rate will depend on the solubility of the reactant in the reacting phase. This leads to some unusual effects. Increasing the 

Addition of a water immiscible solvent will decrease the reaction rate because it will decrease the partition of the reactant into the aqueous phase. In order to be able to predict the effect of agitation on reaction rate it is necessary to understand what other factors, in addition to solubility, control the reaction rate. Depending on the reaction rate reaction may occur in the bulk aqueous phase or in the diffusion film adjacent to the interface. 

Reaction in the bulk phase will occur at a rate which shows some dependence on the volume of the phase in which reaction occurs. The rate of reactions which occur in the diffusion film or at the interface are directly proportional to the interfacial area, and are independent of either phase volume. This is a complex subject and is discussed in detail elsewhere [20, 22]. 

Problems can arise because of differential solubilities of reactants and products. In a recent example studied in the authors laboratory it was desired to hydrolyse a water insoluble ester bearing another group sensitive to hydrolysis, using aqueous sodium hydroxide (equation 12.11). The desired product (II) could react further losing X (equation 12.12). 

Prior to the discovery of the technique the preferred solvent for conversion of an alkyl bromide into the nitrile was dimethylsulphoxide [24]. Starks [25] showed that neat -octyl bromide or chloride could be transformed quantitatively into the nitrile using aqueous sodium cyanide provided that a small amount of a quaternary ammonium or phosphonium salt was present in the system. The quaternary salt may be either wholly or partially partitioned into the organic phase. 

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It is worthwhile to point out that lithium extraction from inverse spinels V[LiM]04, such as V[LiNi]04 and V[LiCo]04 takes place at high voltage, typically between 4 and 5V [153]. Lithium is extracted from the octahedral 16d sites of these spinels with a concomitant oxidation of the divalent nickel or cobalt ions. From a structural point of view, this can be readily understood because lithium must be dislodged from the 16d octahedral sites, which are of low-energy, into neighboring energetically unfavorable 8b tetrahedra, which share all four faces with 16d sites that are occupied by nickel or cobalt and by lithium. Lithium extraction reactions... 

Intelligent engineering can drastically improve process selectivity (see Sharma, 1988, 1990) as illustrated in Chapter 4 of this book. A combination of reaction with an appropriate separation operation is the first option if the reaction is limited by chemical equilibrium. In such combinations one product is removed from the reaction zone continuously, allowing for a higher conversion of raw materials. Extractive reactions involve the addition of a second liquid phase, in which the product is better soluble than the reactants, to the reaction zone. Thus, the product is withdrawn from the reactive phase shifting the reaction mixture to product(s). The same principle can be realized if an additive is introduced into the reaction zone that causes precipitation of the desired product. A combination of reaction with distillation in a single column allows the removal of volatile products from the reaction zone that is then realized in the (fractional) distillation zone. Finally, reaction can be combined with filtration. A typical example of the latter system is the application of catalytic membranes. In all these cases, withdrawal of the product shifts the equilibrium mixture to the product. 

The concept of extractive reaction, which was conceived over 40 years ago, has connections with acid hydrolysis of pentosans in an aqueous medium to give furfural, which readily polymerizes in the presence of an acid. The use of a water-immiscible solvent, such as tetralin allows the labile furfural to be extracted and thus prevents polymerization, increases the yield, and improves the recovery procedures. In the recent past an interesting and useful method has been suggested by Rivalier et al. (1995) for acid-catalysed dehydration of hexoses to 5-hydroxy methyl furfural. Here, a new solid-liquid-liquid extractor reactor has been suggested with zeolites in protonic form like H-Y-faujasite, H-mordenite, H-beta, and H-ZSM-5, in suspension in the aqueous phase and with simultaneous extraction of the intermediate product with a solvent, like methyl Aobutyl ketone, circulating countercurrently. 

The role of biocatalysis in two-phase systems has many parallels with the subject we have covered under extractive reactions. It appears that a two-phase system was originally considered for transformations of water insoluble substances like steroids. Now, a series of treatises are available which teach us that the maximum value of the apparent equilibrium constant for a second-order reaction in a two-phase system can exceed the equilibrium... 

It is useful to combine reaction and separation for equilibrium-limited reactions and also for consecutive reactions, particularly when the desired intermediate products undergo faster undesirable reactions. The concept of extractive reactions for equilibrium-limited and consecutive reactions has been covered in Section 4.2.1. Distillation column reactors provide yet another strategy. 

Suzuki J., Yoshida M., Nakahara C., Sekine K., Kikuchi M., Takamura T., Li mass transfer through a metallic copper film on a carbon fiber during the electrochemical insertion/extraction reaction, Electrochem. and Solid State Lett., (2001), 4 (1), A1-A4. 

River sediment Mono to tetraalkyl tins Adsorption on to G8 silica, tropalone extraction, reaction with Grignard reagent, glc with flame photometric detector low mg kg [79] ... 

Besides the second law method, there is another way of extracting reaction enthalpies from gas-phase equilibrium constants. This alternative involves the determination of a single value of an equilibrium constant at a given temperature and the calculation of the reaction entropy at the same temperature. From equations 2.54 and 2.55, we obtain... 

Such considerations were extended to metal complexes in 1902 by Morse, who studied the distribution of divalent mercury between toluene and water at various Hg and CT concentrations. By taking complex formation in the aqueous phase into consideration Morse could determine the formation constants of HgCr and HgCb from distribution measurements, as well as the distribution constant of the neutral complex HgCl2. The overall extraction reaction can be written... 

Although neither describes the intennediate reaction steps (see Chapters 3 and 4) or kinetics of the reaction (Chapter 5), nor explains why or to what extent HgCb dissolves in the organic solvent (Chapter 2), the extraction reaction is a useful concept in applied solvent exttaction, and values are commonly tabulated in reference works [3-4]. 

Extraction from aqueous solutions into organic solvents can be achieved through different chemical reactions. Some may seem very complicated, but usually occur through a number of rather simple steps we assume this in making a model of the system. The subdivision of an extraction reaction into its simpler steps is useful for understanding how the distribution ratio varies as a function of the type and concentration of the reagents. Often these models allow equilibrium constants to be measured. 

The HTTA -1- TBP system can serve to illustrate the main points of thermodynamics of synergism. The overall extraction reaction is written as ... 

Thermodynamic data for the extraction reactions of Eqs. (4.10a) and (4.10c) allow calculation of the corresponding values for the synergistic reaction of Eq. (4.10d). Measurements of the reaction... 

For the extraction reaction it may suffice to write the reaction of Eq. (4.15d), though it consists of a number of more or less hypothetical steps. As mentioned, equilibrium studies of this system cannot define the individual steps, but supplementary studies by other techniques may reveal the valid ones. Equation (4.15) indicates that the reaction takes place at the boundary (interface) between the aqueous and organic phases. However, it is common to assume that a small amount of B dissolves in the aqueous phase, and the reaction takes place in the steps... 

To analyze these systems, the overall extraction reaction must be broken into its partial reactions, or by introducing Eq. (4.47), to obtain... 

Amine extraction is used also in another important industrial process, the extraction of uranium from sulphuric acid leached ores, which uses trilauryl amine (TLA). In that case, the extraction reaction is... 

Because of the many parameters involved in solvent extraction, chemical as well as physical, it is a difficult task to draw reliable conclusions about the reactions that are responsible for the observed distribution values. In the introductory part of this chapter, section 4.2, the extraction reaction described by was shown to be a product of a number of parameters related to the different steps involved in the formation of the extracted complex. Sections 4.4-4.11 have described these various subreactions, which are described by Eq. (4.46) ... 

In order to solve an algebraic system of n parameters, only n equations are needed (a minimum with no error estimates). When the solvent extraction reaction can be described by Eq. (4.90), there are as many equations as there are experimental points. Commonly, in solvent extraction 10-50 points are needed to cover the whole concentration range of interest, while the number of unknown parameters in simple cases is <5. In evaluating the parameters, it is important to use the complete suite of experimental data, as that gives greater significance to the or values. 

Once a rate law has been defined and the rate constants evaluated, the next step is to correlate it with the most likely mechanism of the extraction reaction. 

When one or more of the chemical reactions is sufficiently slow in comparison with the rate of diffusion to and away from the interface of the various species taking part in an extraction reaction, such that diffusion can be considered instantaneous, the solvent extraction kinetics occur in a kinetic regime. In this case, the extraction rate can be entirely described in terms of chemical reactions. This situation may occur either when the system is very efficiently stirred and when one or more of the chemical reactions proceeds slowly, or when the chemical reactions are moderately fast, but the diffusion coefficients of the transported species are very high and the thickness of the two diffusion films is close to zero. In practice the latter situation never occurs, as diffusion coefficients in liquids generally do not exceed 10 cm s, and the depth of the diffusion films apparently is never less than 10 cm. 

Finally, it has to be emphasized that both the hydrodynamic parameters and the concentrations of the species involved in the extraction reaction simultaneously determine whether the extraction regime is of kinetic, diffusional, or mixed diffusional-kinetic type. It, therefore, is not surprising that different investigators, who studied the same chemical solvent extraction system in different hydrodynamic and concentration conditions, may have interpreted their results in terms of completely different extraction regimes. 

In this section, we describe three simple cases of rates and mechanisms that have been found suitable for the interpretation of extraction kinetic processes in kinetic regimes. These simple cases deal with the exuaction reaction of a monovalent metal cation (solvation water molecules are omitted in the notation) with a weakly acidic solvent extraction reagent, BH. The overall extraction reaction is... 

Case 1 The rate-determining step of the extraction reaction is the aqueous phase complex formation between the metal ion and the anion of the extracting reagent. Even if at very low concentration, BH will always be present in the aqueous phase because of its solubility in water. The rate-determining step of the extraction reaction is as follows ... 

The values of the apparent rate constants of the extraction reaction,... 

The apparent values of the rate constants of the solvent extraction reaction are nsnally evaluated by measuring the rate of extraction of as function of [BH] (at [H ] constant), of [H ] (at [BH] constant), and of [MB] (at [H ] and [BH] constant). The experimental conditions are usually chosen in such a way that the reaction can be assumed pseudo-first-order for [M ]. The apparent rate constants are evaluated from the slope of the straight lines obtained by plotting... 

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