A Competitive Reaction Sequence

Example 6.7 Determine optimal reactor volumes and operating temperatures for the three ideal reactors a single CSTR, an isothermal PER, and an adiabatic PER. 

Solution The computer programs used for the consecutive reaction examples can be used. All that is needed is to modify the subroutine Reactor. Results are shown in Table 6.5. 

All other things being equal, as they are in this contrived example, the competitive reaction sequence of Equation (6.6) is superior for the manufacture of B than the consecutive sequence of Equation (6.1). The CSTR remains a doubtful choice, but the isothermal PER is now better than the adiabatic PER. The reason for this can be understood by repeating Example 6.5 for the competitive reaction sequence. 

Example 6.8 Find the optimal temperature profile, T(t), that maximizes the concentration of component B in the competitive reaction sequence of Equation (6.6) for a piston flow reactor subject to the constraint that t= 1.8 h. 

Solution The computer program used for Example 6.5 will work with minor changes. It is a good idea to start with a small number of zones until you get some feel for the shape of the profile. This allows you to input a 

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Two step screening was done for the 430,000 library beads. In the first step, the beads stained by lpM NBD-DCA were selected. The fluorescent beads were picked up by a capillary using an inverted fluorescent microscope for observation. The 399 peptide beads obtained were then washed with ethanol. After washing, the beads still fluorescing were excluded since they had bound irreversibly to the NBD-DCA. Thus, only 18 beads were tested in the second step. In the second step, the beads were subjected to a competitive reaction to select only the peptides binding to the DCA moiety of the NBD-DCA. They were suspended in a 5mM DCA solution containing 5% ethanol for 3h and only those five beads for which the fluorescence decreased were selected. Finally, the sequences of the five beads obtained were determined by a standard Edman degradation method. 

Through a designed competition reaction sequence involving the reaction of an alkenylboron dichloride with an allyloxide in the presence of allyltrimethylsilane, the migration of the alkenyl moiety was shown to proceed via a cationic pathway (Scheme 23.71). ... 

Aldehydes are important products at all pressures, but at low pressures, acids are not. Carbon monoxide is an important low pressure product and declines with increasing pressure as acids increase. This is evidence for competition between reaction sequence 18—20 and reaction 21. Increasing pressure favors retention of the parent carbon skeleton, in concordance with the reversibiUty of reaction 2. Propylene becomes an insignificant product as the pressure is increased and the temperature is lowered. Both acetone and isopropyl alcohol initially increase as pressure is raised, but acetone passes through a maximum. This increase in the alcohoLcarbonyl ratio is similar to the response of the methanoLformaldehyde ratio when pressure is increased in methane oxidation. 

The rate of the alkylation reaction depends on the enolate concentration, since it proceeds by a SN2-mechanism. If the concentration of the enolate is low, various competitive side-reactions may take place. As expected, among those are E2-eliminations by reaction of the alkyl halide 2 with base. A second alkylation may take place with mono-alkylated product already formed, to yield a -alkylated malonic ester however such a reaction is generally slower than the alkylation of unsubstituted starting material by a factor of about 10. The monoalkylation is in most cases easy to control. Dialkylated malonic esters with different alkyl substituents—e.g. ethyl and isopropyl—can be prepared by a step by step reaction sequence ... 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

In the highly competitive arena surrounding the Pfizer compounds CP-263,114 and CP-225,917 (Figure 4.2), Nicolaou and co-workers employed a hydrozirconation—iodination sequence to produce vinyl iodide 4 [17]. Lithium—halogen exchange and subsequent conversion to enone 5 sets the stage for a Lewis acid assisted intramolecular Diels—Alder reaction affording polycyclic 6 as the major diastereomer (Scheme 4.3). 

The dicyclopentadienyl metal compounds undergo Friedel-Crafts alkylation and acylation, sulfonation, metalation, arylation, and formyla-tion in the case of ferrocene, dicyclopentadienyl ruthenium, and dicyclopentadienyl osmium, whereas the others are unstable to such reactions ( ). Competition experiments (128) gave the order of electrophilic reactivity as ferrocene > ruthenocene > osmocene and the reverse for nucleophilic substitution of the first two by n-butyl lithium. A similar rate sequence applies to the acid-catalysed cleavage of the cyclopentadienyl silicon bonds in trimethylsilylferrocene and related compounds (129), a process known to occur by electrophilic substitution for aryl-silicon bonds (130). 

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