Reaction conditions disproportionation

Mobil s Low Pressure Isomerization Process (MLPI) was developed in the late 1970s (123,124). Two unique features of this process are that it is Operated at low pressures and no hydrogen is used. In this process, EB is converted to benzene and diethylbenzene via disproportionation. The patent beheved to be the basis for the MLPI process (123) discusses the use of H-ZSM-5 zeoHte with an alumina binder. The reaction conditions described are start-of-mn temperatures of 290—380°C, a pressure of 273 kPa and WHSV of 5—8.5/h. The EB conversion is about 25—40% depending on reaction conditions, with xylene losses of 2.5—4%. The PX approach to equiHbrium is about 99 ndash 101%. The first commercial unit was Hcensed in 1978. A total of four commercial plants have been built. 

Under alkaline conditions disproportionation reactions predominate. Thus with Na20 the dioxonitrate(II) first formed, disproportion-ates into the corresponding nitrite(III) and dioxodinitrate(iV-iV)(I) ... 

Olefins could he catalytically converted into shorter and longer-chain olefins through a catalytic disproportionation reaction. For example, propylene could he disproportionated over different catalysts, yielding ethylene and butylenes. Approximate reaction conditions are 400°C and 8 atmospheres ... 

Some limitations of the method arise due to side reactions involving the nitroxide. However, such problems can usually be avoided by the correct choice of nitroxide and reaction conditions. Nitroxides, while stable in the presence of most monomers, may act as oxidants or rcductants under suitable reaction conditions.516 The induced decomposition of certain initiators (e.g. diacyl peroxides) can be a problem (Scheme 3.94).166 177 There is some evidence that nitroxides may disproportionate with alkoxy radicals bearing a-hydrogens,123 Side reactions with thiols have also been identified.4 18... 

Chains with uttdesired functionality from termination by combination or disproportionation cannot be totally avoided. Tn attempts to prepare a monofunctional polymer, any termination by combination will give rise to a difunctional impurity. Similarly, when a difunctional polymer is required, termination by disproportionation will yield a monofunctional impurity. The amount of termination by radical-radical reactions can be minimized by using the lowest practical rate of initiation (and of polymerization). Computer modeling has been used as a means of predicting the sources of chain ends during polymerization and examining their dependence on reaction conditions (Section 7.5.612 0 J The main limitations on accuracy are the precision of rate constants which characterize the polymerization. 

A few studies have appeared on systems based on persistent nitrogen-centered radicals. Yamada et al.2"1 examined the synthesis of block polymers of S and MMA initiated by derivatives of the triphenylverdazyl radical 115. Klapper and coworkers243 have reported on the use of triazolinyl radicals (e.g. 116 and 117). The triazolinyl radicals have been used to control S, methacrylate and acrylate polymerization and for the synthesis of block copolymers based on these monomers [S,243 245 tBA,243 MMA,243 245 BMA,245 DMAEMA,24 5 TMSEMA,247 (DMAEMA-Wbc/fc-MMA),246 (DMAEMA-Woc -S)246 and (TMSEMA-6/ocfc-S)247]. Reaction conditions in these experiments were similar to those used for NMP. The triazolinyl radicals show no tendency to give disproportionation with methacrylate propagating radicals. Dispcrsitics reported arc typically in the range 1.4-1.8.2"43 246... 

When the reaction conditions approach the thermodynamic equilibrium, isomerization follows. The distribution of the double bond is statistical. The molecular formation in the disproportionation stage is also statistical. Normally a run will produce 10-15% by weight of product, which is then suitable for LAB synthesis after distillation. The physical data of these internal olefins are shown in Table 4 [41]. 

The same group showed (Baumeister et al., 1997) that addition of vanadyl compounds as cocatalysts solved a recurring problem in hydrogenations the build-up of arylhydroxylamine intermediates, which can lead to runaway reaction conditions. Vanadyl complexes catalyse the disproportionation of aiylhydroxylamines and, hence, prevent their build-up during... 

Thus partial evaporation of HC1 during the synthesis of [Tc2C18]3 by the method of Eakins et al. [24] and by the autoclave method [22,42,43] creates favorable conditions (from the standpoint of the acidity of the solution and the concentration of technetium ions in it) for the formation of the octachloro-ditechnetates (+2.5), since, on the one hand, the rate of reduction of the technetium(IV) ions increases owing to their hydrolysis [42] and, on the other hand, the stability of the hydrolyzed cluster ions formed increases in relation to the reactions involving disproportionation and oxidation by atmospheric oxygen [9,52,80,87]. We may note that under the conditions of more pronounced hydrolysis, the rate of reduction of technetium(IV) increases so much that the formation of metallic technetium becomes possible. 

This situation is termed pore-mouth poisoning. As poisoning proceeds the inactive shell thickens and, under extreme conditions, the rate of the catalytic reaction may become limited by the rate of diffusion past the poisoned pore mouths. The apparent activation energy of the reaction under these extreme conditions will be typical of the temperature dependence of diffusion coefficients. If the catalyst and reaction conditions in question are characterized by a low effectiveness factor, one may find that poisoning only a small fraction of the surface gives rise to a disproportionate drop in activity. In a sense one observes a form of selective poisoning. 

Dioxaphosphorinanes with R = 7-Pr (3) and Ph (4) have also been obtained from phosphorus-containing diols and aldehydes [79IZV2136, 79DOK(244)610]. Phosphorus-containing diols with P(III) are very labile and are capable of dissociation and disproportionation under the reaction conditions. This apparently explains the difference in the physical constants of diols obtained by different authors. 

Since peroxyl radicals are also removed as a result of disproportionation, the reaction conditions are quasi-stationary for which the equality Vj = 2fc7[InH] [R02 ] I 2A 6[R02 ]2 is more appropriate. In this case, the rate of initiated chain oxidation of RH is equal to ... 

Taskinen and Nummelin (op. cit.) reported many other isomer equilibria in their paper. Most of these used cyclohexane as the solvent and I2 as the catalyst and so are not confounded by solvent effects. However, these authors noted that hydrogen atom transfer induced disproportionation (to form the aromatic benzene) dominates this reaction for the case of 49/50 isomerization and so they needed alternative reaction conditions. 

Pletcher and associates [155, 159, 160] have studied the electrochemical reduction of alkyl bromides in the presence of a wide variety of macrocyclic Ni(II) complexes. Depending on the substrate, the mediator, and the reaction conditions, mixtures of the dimer and the disproportionation products of the alkyl radical intermediate were formed (cf. Section 18.4.1). The same group [161] reported that traces of metal ions (e.g., Cu2+) in the catholyte improved the current density and selectivity for several cathodic processes, and thus the conversion of trichloroacetic acid to chloroacetic acid. Electrochemical reductive coupling of organic halides was accompanied several times by hydrodehalogena-tion, especially when Ni complexes were used as mediators. In many of the reactions examined, dehalogenation of the substrate predominated over coupling [162-165]. 

In the Cannizzaro reaction [110, 111] two aldehyde functionalities disproportionate into the corresponding hydroxyl and carboxyl functions, either as separate compounds or as an ester (Scheme 20.25). The reaction conditions needed are rather harsh, except when R1 or R2 is a phenyl group. Typically, an excess of so-... 

A quantitative model requires knowledge of the diffusivity under reaction conditions and of the intrinsic activities for toluene disproportionation and xylene isomerization. While these are not easily obtained, the methodology has been worked out for the case of paraffin and olefin cracking (5). So far, we have obtained an approximate value for the diffusivity, D, of o-xylene at operation conditions from the rate of sorptive o-xylene uptake at lower temperature and extrapolation to 482°C (Table V). 

Table VI shows the relative reactivities of various asym DAMs. An equi-molecular mixture of two kinds of asym DAMs was fed as a 5% benzene solution and hydrogenolyzed in order to check the effect of the methyl group on the reactivity. Two kinds of asym DAMs having similar reactivities were selected as a combination. The reaction conditions were temperature, 400°C H2/hydrocarbons molar ratio, 2. The contact time was changed since the reactivities of asym DAMs differed considerably according to their structures this made it possible to evaluate the different reactivities. Side reactions such as demethylation, isomerization, and disproportionation were negligible under these reaction conditions. The relative values for the reactivities of the asym DAMs shown in Table VI are determined when the value of 2,5-DMeDPM as a standard material is fixed at 100.

The free radical polymerization of HPMA in the presence of mercaptans involves two different initiation mechanisms (Scheme 2) [26]. One is the initiation by RS radicals from chain transfer agent the other appears to be the direct initiation by the primary isobutyronitrile (IBN) radicals formed by the decomposition of AIBN [27]. The RS are formed by either the free radical transfer reaction of alkyl mercaptans with the IBN radicals or the chain transfer reaction of an active polymer chain with the mercaptans. The initiation by the RS radicals produces the ST polymers with a functional group at one end of the polymer chain. The initiation by IBN radicals leads to nonfunctional polymer chains with an IBN end group. The presence of the polymers with IBN end groups effects the purity and the functionality of ST polymers. As expected, the production of nonfunctionalized polymer chains is affected by reaction conditions. The polymerization is mainly terminated by chain transfer reaction with the mercaptans, but other termination mechanisms, such as disproportionation and recombination, take place depending on the reaction conditions [26]. 

In the presence of NaPF4 (1 equivalent), disproportionation occurs under the same reaction conditions. The following two products are formed in equimolecular amounts ... 

The alkylation with alcohols and amines can lead to alkyl derivatives or a-hydroxy and a-aminoalkyl derivatives according to the nature of the heteroaromatic base and the reaction conditions. The intermediate products in both cases are, however, the a-hydroxy and a-aminoalkyl dihydro derivatives, which can be aromatized by disproportionation or oxidation, while the loss of water or ammonia leads to the alkyl derivatives (Scheme 7). 

The mode of termination varies with monomer and reaction conditions. While styrene macroradicals typically terminate by coupling, methyl methacrylate macroradicals terminate by coupling at temperatures below 60°C, but by disproportionation at higher temperatures. 

As already stated, isomerization on zeolite HY was always accompanied by disproportionation, even at 180 C. With time on stream, Yo,. Np increases, because these heavy products are most efficiently held by the fresh catalyst. It is an interesting result that, at 180 °C, the yield of naphthalene passed through a maximiun as well. Obviously, under appropriate reaction conditions, the disproportiotuOion of methylnaphthalenes in zeolite HY exhibits an induction period, as does the disproportionation of ethylbenzene in large pore zeolites [39,40]. 

The general strategy is based on selective deprotection and oxidation of pairwise cysteine residues, as specified by the orthogonal protection scheme selected for this purpose. Since multiple disulfide bonds are formed step-by-step, reaction conditions are required that prevent breaking or scrambling of the disulfide bonds already formed. Therefore, throughout the synthesis, exposure to thiols or alkaline conditions as well as lengthy reaction times that may cause disulfide disproportionation must be avoided. 

Generally, the reduction is achieved under deaerated conditions to avoid a competitive scavenging of Cjoiv and H atoms by oxygen. These atoms are as homogeneously distributed as the ions and the reducing species, and they are therefore produced at first as isolated entities. Similarly, multivalent ions are reduced by multistep reactions, including disproportionation of intermediate valencies. Such reduction reactions have been observed directly by pulse radiolysis for a variety of metal ions (Fig. 2), mostly in water [28], but also in other solvents where the ionic precursors are soluble. Most of their rate eonstants are known and the reactions are often diffusion controlled. 

This would indicate either that disproportionation is the preferred route of termination for the ketyl radicals or that the pinacol is unstable and dis-proportionates to acyloin and ketone under the reaction conditions. 

Special interest has been devoted to the reaction of oxygen on carbanionic sites.- It has been established that triplet oxygen acts as an electron acceptor, and that the reaction may yield either terminal hydroperoxy or alcohol functions. Depending on the reaction conditions, the latter group which originates from a disproportionation process between a hydro-peroxidate and a carbanlon may be observed. By choosing proper conditions one can get polymers terminated quantitatively with hydroperoxide groups. The latter can be used in a later step as a radical initiator to prepare new block copolymers. 

Pentane Isomerization. Pentane isomerization, although carried out on a much smaller scale, increased the critical supply of aviation gasolines toward the end of the war. Two pentane processes—one developed by Shell and one by Standard (Indiana) —were commercialized before the end of the war. The principal differences between the butane and pentane processes are the use in pentane isomerization of somewhat milder conditions and the use of an inhibitor to suppress side reactions, principally disproportionation. In general, the problems of the butane processes are inherent also in pentane isomerization, but the quality of the feed stocks is less important. Catalyst life is much... 

Various methods have been developed to maximize p-xylene formation. A mild hydrothermal treatment of H-ZSM-5 and the incorporation of Pt greatly reduce disproportionation activities and, consequently, increase para selectivity.355 Suppressing the activity of the external surface can also lead to enhanced para selectivity. By judicious choice of reaction conditions and organometallic reagents... 

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