Parallel reactions examples

Thus, a relatively high value of rD/ru is favored by relatively high T. This is another illustration of the conclusion (made also for parallel reactions, Example 18-2) that, of two reactions under consideration, higher T favors the reaction with the higher activation en-... 

We will now give three examplc.s of multiple reactions with heat effects Example 12-5 discusses parallel reactions. Example 12-6 discusses series reactions, and Example 12-7 discusses complex reactions. 

An example of a parallel reaction system occurs in the production of ethylene oxide ... 

An example of such recychng in a parallel reaction system is in the Oxo process for the production of C4 alcohols. Propylene and synthesis gas (a mixture of carbon monoxide and hydrogen) are first reacted to ra- and isobutyraldehydes using a cobalt-based catalyst. Two parallel reactions occur ... 

Step 4 of the thermal treatment process (see Fig. 2) involves desorption, pyrolysis, and char formation. Much Hterature exists on the pyrolysis of coal (qv) and on different pyrolysis models for coal. These models are useful starting points for describing pyrolysis in kilns. For example, the devolatilization of coal is frequently modeled as competing chemical reactions (24). Another approach for modeling devolatilization uses a set of independent, first-order parallel reactions represented by a Gaussian distribution of activation energies (25). 

When potassium fluoride is combined with a variety of quaternary ammonium salts its reaction rate is accelerated and the overall yields of a vanety of halogen displacements are improved [57, p 112ff. Variables like catalyst type and moisture content of the alkali metal fluoride need to be optimized. In addition, the maximum yield is a function of two parallel reactions direct fluorination and catalyst decomposition due to its low thermal stability in the presence of fluoride ion [5,8, 59, 60] One example is trimethylsilyl fluoride, which can be prepared from the chloride by using either 18-crown-6 (Procedure 3, p 192) or Aliquot 336 in wet chlorobenzene, as illustrated in equation 35 [61],... 

Change of reaction conditions to minimize kinetic complications. For example, if two parallel reactions have substantially different activation energies, their relative rates will depend upon the temperature. The reaction solvent, pH, and concentrations are other experimental variables that may be manipulated for this purpose. 

Parallel reactions give rate equations having sums of rate terms. Each term provides the transition state composition for a reaction path. Eor example, some acid-catalyzed reactions have the rate equation... 

That the reaction with a lower rate constant is taking place preferentially and that the rate increases during the reaction are phenomena that can also occur with parallel reactions. As an example, Wauquier and Jungers (48), when studying competitive hydrogenation of a series of couples of aromatic hydrocarbons on Raney-nickel, have observed these phenomena for the couple tetraline-p-xylene (Table I). The experimental result was... 

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

Thus we have Example 5 from Table 4.1. Equation 4 gives a better description of the overall reaction, but equation 5 highlights the essential chemical process, and can also stand for the parallel reactions where sodium chloride is replaced by potassium chloride, or at r other soluble chloride. The chemistiy student is expected to appreciate how both equations 4 and 5 can represent the same chemical processes. 

Parameter setup for Example 7.4. Competing parallel reactions... 

Added to this, the mass transfer can also influence the selectivity. For example, consider a system of two parallel reactions in which the second reaction produces an unwanted by product and is slow relative to the primary reaction. The dissolving gas species will tend to react in the liquid film and not reach the bulk liquid in significant quantity for further reaction to occur there to form the by product. Thus, in this case, the selectivity would be expected to be enhanced by the mass transfer between the phases. In other cases, little or no influence can be expected. 

Montanari el al., for example, studied a Co—H-MFI sample through FT-IR spectroscopy of in situ adsorption and coadsorption of probe molecules [o-toluonitrile (oTN), CO and NO] and CH4-SCR process tests under IR operando conditions. The oTN adsorption and the oTN and NO coadsorption showed that both Co2+ and Co3+ species are present on the catalyst surface. Co3+ species are located inside the zeolitic channels while Co2+ ions are distributed both at the external and at the internal surfaces. The operando study showed the activity of Co3+ sites in the reaction. The existence of three parallel reactions, CH4-SCR, CH4 total oxidation and NO to NOz oxidation, was also confirmed. Isocyanate species and nitrate-like species appear to be intermediates of CH4-SCR and NO oxidation, respectively. A mechanism for CH4-SCR has been proposed. On the contrary, Co2+ substitutional sites, very evident and predominant in the catalyst, which are very hardly reducible, seemed not to play a key role in the SCR process [173],... 

Competitive Parallel Reactions. Competitive parallel reactions are those in which two or more reactant species compete for yet another reactant for example,... 

This illustration has provided us with a concrete example that indicates in quantitative form the validity of the general rule of thumb that we have stated for analyzing parallel reactions. High concentrations favor the higher-order reaction, and low concentrations favor the lower-order reaction. 

This reaction set may be regarded as parallel reactions with respect to consumption of species B and as a series reaction with respect to species A, V, and W. Common examples include the nitration and halogenation of benzene and other organic compounds to form polysubstituted compounds. To characterize the qualitative behavior of such systems, it is useful to consider reactions 9.3.3 and 9.3.4 as mechanistic equations and to analyze the effects of different contacting patterns on the yield of species V. We shall follow the treatment of Levenspiel (7). 

Independent Parallel Reactions of Different Species on the Same Catalyst. One often requires a catalyst that promotes the reactions of one component of a feedstock but does not promote the reactions of other constituents of the mixture. For example, one might desire to dehydrogenate six-membered rings, but not five-membered rings. This type of selectivity behavior may be represented by mechanistic equations of the form... 

Microwave-assisted reactions allow rapid product generation in high yield under uniform conditions. Therefore, they should be ideally suited for parallel synthesis applications. The first example of parallel reactions carried out under microwave irradiation conditions involved the nucleophilic substitution of an alkyl iodide with 60 diverse piperidine or piperazine derivatives (Scheme 4.22) [76]. Reactions were carried out in a multimode microwave reactor in individual sealed polypropylene vials using acetonitrile as solvent. Screening of the resulting 2-aminothiazole library in a herpes simplex virus-1 (HSV-1) assay led to three confirmed hits, demonstrating the potential of this method for rapid lead optimization. 

The reactions of intramolecular isomerization occur and are important in the oxidation of natural and synthetic rubbers. The peroxyl radical addition to the double bond occurs very rapidly. For example, the peroxyl radical adds to the double bond of 2-methylpropene by 25 times more rapidly than abstraction of hydrogen atom from this hydrocarbon (see Chapter 4). Therefore, the oxidation of polymers having double bonds proceeds as a chain process with parallel reactions of P02 with double and C—H bonds including the intramolecular isomerization of the type [12] ... 

This parallel reaction set was used, for example, by Johnson and Prud homme (2003a) to investigate the quality of mixing in a confined impinging-jets reactor. 

Figure 5.3 Concentration profiles for parallel reaction network in Example 5-6... 

A reaction network, as a model of a reacting system, mas7 consist of steps involving same ar all of opposing reactions, which may or may not be considered to be at equilibrium, parallel reactions, and series reactions. Some examples ate dted in Section 5.1. 

The following example illustrates a combination of semibatch and semicontinuous operation for an irreversible reaction, with one reactant added intermittently and the other flowing (bubbling) continuously, that is, a combination of Figures 12.3(a) and 12.4(a). Chen (1983, pp. 168-211, 456-460) gives several examples of other situations, including reversible, series-reversible, and series-parallel reactions, and nonisothermal and autothermal operation. 

Example 18-2 illustrates the effect of two process variables (concentration and temperature) on the instantaneous fractional yield for two parallel reactions. 

Figure 18.4 Vessel configuration for parallel reactions in Example 18-6 (a) single PER (b) three PFRs of same total volume in series with FBo split evenly among them...

Micromixing may also have a major impact upon the yield and selectivity of complex reaction networks. Consider, for example, the following parallel reaction network, where both a desired product (D) and an undesired product (U) may be formed ... 

The now familiar alternatives of visual and potentiometric detection are available. A number of organic dyes form coloured chelates with many metal ions. These coloured chelates are often discernible to the eye at concentrations of 10 6-10 7 mol dm 3 and can function as visual indicators. Most metal ion indicators will also undergo parallel reactions with protons bringing about similar colour changes. Hence, a careful consideration of pH is prudent when selecting an indicator. Some typical indicators appear in Table 5.9. Of these, eriochrome black T, which forms red complexes with over twenty metal ions, is amongst the most widely used. Its behaviour will serve as a general example of indicator function. 

An alternative strategy for selective intermolecular G-H insertions has been the use of rhodium carbenoid systems that are more stable than the conventional carbenoids derived from ethyl diazoacetate. Garbenoids derived from aryldiazoacetates and vinyldiazoacetates, so-called donor/acceptor-substituted carbenoids, have been found to display a very different reactivity profile compared to the traditional carbenoids.44 A clear example of this effect is the rhodium pivalate-catalyzed G-H insertion into cyclohexane.77 The reaction with ethyl diazoacetate gave the product only in 10% yield, while the parallel reaction with ethyl phenyldiazoacetate gave the product in 94% yield (Equation (10)). In the first case, carbene dimerization was the dominant reaction, while this was not observed with the donor/acceptor-substituted carbenoids. 

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