Dissociation-extraction extractive reaction

Table 25.1 lists several combinations of reaction and separation. The sequencing of the two in the nomenclature of the different combinations clearly reveals their orientations. This chapter is primarily concerned with reactive extraction (also termed dissociation-extraction), extractive reaction, reactive distillation (or dissociation-extractive-distillation), and distillative reaction (or distillation column reactors). Crystallization is almost always used for separation and seldom for enhancing a reaction. A notable exception is when one of the reactants is a sparingly dissolving solid and the size of the crystallizing solid is less than the thickness of the film surrounding the reactant. Then the crystallizing microphase enhances the rate of dissolution and hence the rate of reaction, a situation that was considered in Chapter 23. 

In addition to the Hquid—Hquid reaction processes, there are many cases in both analytical and industrial chemistry where the main objective of separation is achieved by extraction using a chemical extractant. The technique of dissociation extraction is very valuable for separating mixtures of weakly acidic or basic organic compounds such as 2,4-dichlorophenol [120-83-2] and 2,5-dichlorophenol [583-78-8] which are difficult to separate by... 

This has been studied much less frequently and appears to be a rather more complex reaction. The first results obtained, for the butyl-lithium, styrene reaction in benzene have already been described. In a similar way the addition of butyllithium to 1,1-diphenylethylene shows identical kinetic behaviour in benzene (26). Even the proton extraction reaction with fluorene shows the typical one-sixth order in butyllithium (27). It appears therefore that in benzene solution at least, lithium alkyls react via a small equilibrium concentration of unassociated alkyl. This will of course not be true for reactions with polar molecules for reasons which will be apparent later. No definite information can be obtained on the dissociation process. It is possible that the hexamer dissociates completely on removal of one molecule or that a whole series of penta-mers, tetramers etc. exist in equilibrium. As long as equilibrium is maintained, the hexamer is the major species present and only monomeric butyllithium is reactive, the reaction order will be one-sixth. A plausible... 

Dissociation extraction is the process of using chemical reaction to force a solute to transfer from one liquid phase to another. One example is the use of a sodium hydroxide solution to extract phenohcs, acids, or mercaptans from a hyclrocarbon stream. The opposite transfer can be forced by adding an acid to a sodium phenate stream to spring the phenolic back to a free phenol that can be extracted into an organic solvent. Similarly, primary, secondary, and tertiary amines can be protonated with a strong acid to transfer the amine into a water solution, for example, as an amine hydrochloride salt. Conversely, a strong base can be added to convert the amine salt back to free base, which can be extracted into a solvent. This procedure is quite common in pharmaceutical production. 

Fractionation dissociation extraction involves both the chemical reaction and the fractionation scheme for the separation of components by their difference in dissociation constants as described by Colby [in Hanson (ed.). Recent Advances in Liquid-Liquid Extraction, Pergamon, New York, 1971, chap. 4]. 

First we explain the basic principles of reactive (i.e., dissociation) extraction and then outline the more economical regenerative dissociation-extraction technique. We follow this up with a brief description of extractive reaction as an extension of what was described in Chapter 18 on biphasing. 

This example demonstrates a particularly useful feature of organic synthesis engineering. Any by-product mixture from a process that cannot be easily separated can be converted by reaction to a more useful mixture, whose components can then be separated relatively easily by dissociation-extraction. 

Biphasing by deliberate addition of a second liquid phase was considered in Chapter 18. We extend the treatment in this section to aspects of biphasing that involve extractive reaction. Thus we focus the discussion on extractive reactions that depend on the dissociation equilibria of the components involved, and we restrict the treatment to a few instructive examples. 

Distillation combined with reaction has been successfully used for separating close boiling mixtures. When used in this separation mode, the technique is frequently referred to as dissociation-extractive distillation. It can also be used in the reaction mode by continuous separation of the reaction products from the reactants. The equipment used in the latter case is often referred to as a distillation column reactor (DCR). The chief advantage of this method is that the reactants can be used in stoichiometric quantities, with attendant elimination of recycling costs. 

Reaction-separation parameter defined by Equation 25.38. Molar ratio of free acids [HA]s/[HB]s in the organic phase at equilibrium in dissociation-extraction. 

This three volume series contains equilibrium data on liquid-liquid distribution reactions. The equilibria of concern Include distribution, dissociation and aggregation of the extractant, reactions of the extractant with diluents and with other solvents, extraction of the water, equilibria of the extraction metal Ions, and extraction of metal ions with the extractant as the ligand. Volumes 1 and 2 cover organophosphorous extractions and alkylammonium salt extractants, respectively. Volume 3 deals with distribution reactions of carboxylic and sulfonic acid extractants, and the distribution of Inorganic acids, salts, and complexes between aqueous solutions and both Inert solvents and solvents which have oxygen donor atoms. Each reaction entry Includes the equilibrium constant, the temperature for which the data exists, the conditions, and a reference(s) to the source literature. 

As mentioned at the beginning, it is also possible to impose a chemical reaction on the distillation of two closely boiling compounds to achieve effective separation of the components. This method is based on exploiting the dissociation processes involved in the acid-base reactions imposed on separation by distillation of the components of two closely boiling liquids. This is referred to as dissociation-extractive distillation. 

The cation of the dissociated amine nitrate in the aqueous phase reacts with Ln(N03)J. The resulting complex is transferred into the organic phase according to the extraction reaction... 

Although extraction of lipids from membranes can be induced in atomic force apparatus (Leckband et al., 1994) and biomembrane force probe (Evans et al., 1991) experiments, spontaneous dissociation of a lipid from a membrane occurs very rarely because it involves an energy barrier of about 20 kcal/mol (Cevc and Marsh, 1987). However, lipids are known to be extracted from membranes by various enzymes. One such enzyme is phospholipase A2 (PLA2), which complexes with membrane surfaces, destabilizes a phospholipid, extracts it from the membrane, and catalyzes the hydrolysis reaction of the srir2-acyl chain of the lipid, producing lysophospholipids and fatty acids (Slotboom et al., 1982 Dennis, 1983 Jain et al., 1995). SMD simulations were employed to investigate the extraction of a lipid molecule from a DLPE monolayer by human synovial PLA2 (see Eig. 6b), and to compare this process to the extraction of a lipid from a lipid monolayer into the aqueous phase (Stepaniants et al., 1997). 

In aqueous solution at 100° the change is reversible and equilibrium is reached when 95 per cent, of the ammonium cyanate has changed into urea. Urea is less soluble in water than is ammonium sulphate, hence if the solution is evaporated, urea commences to separate, the equilibrium is disturbed, more ammonium cyanate is converted into urea to maintain the equilibrium and evfflitually the change into urea becomes almost complete. The urea is isolated from the residue by extraction with boiling methyl or ethyl alcohol. The mechanism of the reaction which is generally accepted involves the dissociation of the ammonium cyanate into ammonia and cyanic acid, and the addition of ammonia to the latter ... 

Chemical Equilibria. In many cases, mass transfer between two Hquid phases is accompanied by a chemical change. The transferring species can dissociate or polymerize depending on the nature of the solvent, or a reaction may occur between the transferring species and an extractant present in one phase. An example of the former case is the distribution of benzoic acid [65-85-0] between water and benzene. In the aqueous phase, the acid is partially dissociated ... 

The extraction system which was measured by the HSS method for the first time was the extraction kinetics of Ni(II) and Zn(II) with -alkyl substituted dithizone (HL) [14]. The observed extraction rate constants linearly depended on both concentrations of the metal ion [M j and the dissociated form of the ligand [L j. This seemed to suggest that the rate determining reaction was the aqueous phase complexation which formed a 1 1 complex. However, the observed extraction rate constant k was not decreased with the distribution constant Kj of the ligands as expected from the aqueous phase mechanism. 

These experimental results strongly suggested a significant contribution of other processes than just the aqueous reaction. On this issue, the HSS method revealed that the dissociated form of the -alkyl-dithizone did adsorb at the interface generated by vigorous agitation [5]. Therefore, the extraction rate was analyzed by introducing the interfacial reaction... 

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