Pyrolysis disproportionation

Already in 1955 Ziegler and coworkers had reported that halide-free methyllithium 2 upon pyrolysis disproportionates into methane 7 and dilithiomethane 3 in excellent yields. 

Toluene disproportionation (TDP) is a catalytic process in which 2 moles of toluene are converted to 1 mole of xylene and 1 mole of benzene this process is discussed in greater detail herein. Although the mixed xylenes from TDP are generally more cosdy to produce than those from catalytic reformate or pyrolysis gasoline, thek principal advantage is that they are very pure and contain essentially no EB. 

A breakdown of the mixed xylene supply sources in the United States is summarized in Table 1 (1). As shown in Table 1, the primary source of xylenes in the United States is catalytic reformate. In 1992, over 90% of the isolated xylenes in the United States were derived from this source. Approximately 9% of the recovered xylenes is produced via toluene disproportionation (TDP). In the United States, only negligible amounts of the xylenes are recovered from pyrolysis gasoline and coke oven light oil. In other parts of the world, pyrolysis gasoline is a more important source of xylenes. 

Xylenes. The main appHcation of xylene isomers, primarily p- and 0-xylenes, is in the manufacture of plasticizers and polyester fibers and resins. Demands for xylene isomers and other aromatics such as benzene have steadily been increasing over the last two decades. The major source of xylenes is the catalytic reforming of naphtha and the pyrolysis of naphtha and gas oils. A significant amount of toluene and Cg aromatics, which have lower petrochemical value, is also produced by these processes. More valuable p- or 0-xylene isomers can be manufactured from these low value aromatics in a process complex consisting of transalkylation, eg, the Tatoray process and Mobil s toluene disproportionation (M lDP) and selective toluene disproportionation (MSTDP) processes isomerization, eg, the UOP Isomar process (88) and Mobil s high temperature isomerization (MHTI), low pressure isomerization (MLPI), and vapor-phase isomerization (MVPI) processes (89) and xylene isomer separation, eg, the UOP Parex process (90). 

Petroleum-derived benzene is commercially produced by reforming and separation, thermal or catalytic dealkylation of toluene, and disproportionation. Benzene is also obtained from pyrolysis gasoline formed ia the steam cracking of olefins (35). 

Pyrolysis Thermal decomposition of 1,1,1,2-tetrachloroethane produces tetrachloroethylene (by disproportionation), hydrogen chloride, and trichloroethylene via dehydrochlorination (111). The yield of the latter is increased in the presence of ferric chloride (112). Other catalytic materials include FeCl —KCl mixture (113), AlCl (6), the complex of AlCl with nitrobenzene (114), activated alumina (3), Ca(OH)2 (115,116), and NaCl (94). 

Although ethylene is produced by various methods as follows, only a few are commercially proven thermal cracking of hydrocarbons, catalytic pyrolysis, membrane dehydrogenation of ethane, oxydehydrogenation of ethane, oxidative coupling of methane, methanol to ethylene, dehydration of ethanol, ethylene from coal, disproportionation of propylene, and ethylene as a by-product. 

Pyrolysis of nido- or nrac/ino-carboranes or their reaction in a silent electric discharge also leads to c/o50-species either by loss of Ht or disproportionation ... 

The number of chemical reactions used in CVD is considerable and include thermal decomposition (pyrolysis), reduction, hydrolysis, disproportionation, oxidation, carburization, and nitrida-tion. They can be used either singly or in combination (see Ch. 3 and 4). These reactions can be activated by several methods which are reviewed in Ch. 5. The most important are as follows ... 

The pyrolysis of a number of compounds at temperatures around 600— 800° and at pressures of the order of 10 2 mm. has been shown to give rise to benzyne. These compounds include for example indanetrione 29>, and phthalic anhydride 30 38>. The dimerisation of benzyne to yield biphenylene has been used preparatively 31 33>, an(j the pyrolysis of tetrafluorophthalic anhydride 34>, and tetrachlorophthalic anhydride 3i-33) gave the corresponding octahalobiphenylenes. In the case of the pyrolysis of tetrachlorophthalic anhydride some hexachlorobenzene is also formed, while the pyrolysis of tetrabromophthalic anhydride results in the formation of hexabromobenzene but no octabromobiphenylene. The disproportionation of tetrabromobenzyne to form carbon and bromine is a function of the high temperature involved and, as we shall see later, both tetrabromo- and tetraiodo-benzyne behave normally in solution. 

The recombination and the disproportionation of alkyl radicals play an important role in many other chain reactions, for example, pyrolysis, photolysis, and radiolysis of organic... 

Taylor in 1925 demonstrated that hydrogen atoms generated by the mercury sensitized photodecomposition of hydrogen gas add to ethylene to form ethyl radicals, which were proposed to react with H2 to give the observed ethane and another hydrogen atom. Evidence that polymerization could occur by free radical reactions was found by Taylor and Jones in 1930, by the observation that ethyl radicals formed by the gas phase pyrolysis of diethylmercury or tetraethyllead initiated the polymerization of ethylene, and this process was extended to the solution phase by Cramer. The mechanism of equation (37) (with participation by a third body) was presented for the reaction, - which is in accord with current views, and the mechanism of equation (38) was shown for disproportionation. Staudinger in 1932 wrote a mechanism for free radical polymerization of styrene,but just as did Rice and Rice (equation 32), showed the radical attack on the most substituted carbon (anti-Markovnikov attack). The correct orientation was shown by Flory in 1937. In 1935, O.K. Rice and Sickman reported that ethylene polymerization was also induced by methyl radicals generated from thermolysis of azomethane. 

Rearrangements of clusters, i.e. changes of cluster shape and increase and decrease of the number of cluster metal atoms, have already been mentioned with pyrolysis reactions and heterometallic cluster synthesis in chapter 2.4. Furthermore, cluster rearrangements can occur under conditions which are similar to those used to form simple clusters, e.g. simple redox reactions interconvert four to fifteen atom rhodium clusters (12,14, 280). Hard-base-induced disproportionation reactions lead to many atom clusters of rhenium (17), ruthenium and osmium (233), iron (108), rhodium (22, 88, 277), and iridium (28). And the interaction of metal carbonyl anions and clusters produces bigger clusters of iron (102, 367), ruthenium, and osmium (249). 

Sulfur tetrafluoride undergoes addition to perfluoroalkenes, e.g. 1 and 4, in the presence of cesium fluoride to give bis(perfluoroalkyl)sulfur difluorides and perfluoroalkylsulfur trifluorides.188,198 The ratio of the products is controlled by the reactant ratios. The role of cesium fluoride is ascribed to the formation of perfluorocarbanions which then subsequently attack sulfur tetrafluoride.188 Perfluoroalkyl disulfides, which are formed as side products, arise from the pyrolysis and disproportionation of bis(perfluoroalkyl)sulfur difluoridcs.189... 

The term mixed xylenes describes a mixture containing the three xylene isomers and usually EB. Commercial sources of mixed xylenes include catalytic refonuate. pyrolysis gasoline, toluene disproportionation product, and coke-oven light oil. Ethylbenzene is present in all of these sources except toluene disproportionation product. Catalytic reformate is the product obtained from catalytic reforming processes. 

Two further methods of fragment elimination seem to be generally applicable controlled pyrolysis and hard base induced disproportionation. The thermal Fe(CO)2 fragment removal from 15a which completes the reversible interconversion of 84 and 15a (61) has already been mentioned. Similarly the leaving group HRe(CO)s can be eliminated thermally from 92 yielding the stable unsaturated cluster H3Re3(CO) o 13 (175). 

Various reaction schemes, including pyrolysis, reduction, oxidation, disproportionation, and hydrolysis of the reactants, have been used to produce a large variety of thin films relevant to microelectronics processing. Table I... 

In contrast, in the excited state the primary cleavage mechanism in silacyclobutanes like 5 involves the breaking of a silicon-carbon bond23. The initially formed silyl radicals 15 and 16 are stabilized by an intramolecular disproportionation reaction giving the silenes 17 and 18 and the homoallylsilane 19.17 and 18 were identified by their trapping products (20, 21) with methanol (equation 5)23. From pyrolysis of Z-5 a different set of products from 1,4-diradical disproportionation is obtained, which can be attributed to predominant cleavage of the carbon-carbon bond23. 

A more detailed study of the pyrolysis of H3SiMn(CO)5 in a sealed tube for various periods of time at 450°C showed that the volatile products were H2, CO, SiH. , and CH4 a metallic-looking brown film covered the walls of the tube, and an apparently amorphous grey powder was also present (33). Figure 5 shows that hydrogen and methane are produced in increasing amounts as the reaction proceeds, while CO reaches a steady concentration after about 10 min. Silane reaches a maximum concentration after about 5 min, then decreases to zero after 60 min. silane is known to decompose to silicon and hydrogen above about 425°C (368), and its presence is readily accounted for by the disproportionation reaction (109). 

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