Disproportionation Processes

Mobil has developed several 1L)P processes. In the 1970s, Mobil developed their LTD (Low Temperature Disproportionation) process (60,61). 

Y. Y. Huang, M. P. Nicoletti, and R. A. Sailor, "The Mobil Selective Toluene Disproportionation Process (MSTDP)," 1990 Petrochemical evieiv DeWitt Company, Houston, Tex., Mar. Ill—70 1990. 

After the cmde BTX is formed, by reforming in this case, a heart cut is sent to extraction. Actually, the xylenes and heavier components are often sent to downstream processes without extraction. The toluene produced is converted to ben2ene, a more valuable petrochemical, by mnning it through a hydrodealkylation unit. This catalytic unit operates at 540—810°C with an excess of hydrogen. Another option is to disproportionate toluene or toluene plus aromatics to a mixture of ben2ene and xylenes using a process such as UOP s Tatoray or Mobil s Selective Toluene Disproportionation Process (STDP) (36). 

Most organic free radicals have very short lifetimes, but certain structural features enhance stability. Radicals without special stabilization rapidly dimerize or disproportionate. The usual disproportionation process for alkyl radicals involves transfer of a hydrogen from the carbon P to the radical site, leading to formation of an alkane and an alkene ... 

Intramolecular chalcogen interactions may also stabilize reactive functional groups enabling the isolation of otherwise unstable species or their use as transient intermediates, especially in the case of selenium and tellurium. For example, tellurium(II) compounds of the type ArTeCl are unstable with respect to disproportionation in the absence of such interactions. The diazene derivative 15.23 is stabilized by a Te N interaction. Presumably, intramolecular coordination hinders the disproportionation process. Other derivatives of the type RTeX that are stabilized by a Te N interaction include 8-(dimethylamino)-l-(naphthyl)tellurium bromide, 2-(bromotelluro)-A-(p-tolyl)benzylamine, and 2-[(dimethylammo)methyl]phenyltellunum iodide. Intramolecular donation from a nitrogen donor can also be used to stabilize the Se-I functionality in related compounds." ... 

Thus, in the latter case, the term ECE has to be abandoned and replaced by disp . (the SET occurs obviously via a disproportionation process). Finally, the strong base R formed after an overall two-electron reaction is protonated by the solvent or by any acidic impurity. Alternative mechanisms could be proposed taking into account that R or Ar may abstract hydrogen atoms from the solvent ... 

In the case of a slow protonation rate (with inefficient proton donors and/or low concentrations of the proton source), the alternative could be an EECC mechanism through a disproportionation process, still at the potential of the first step. 

The relative importance of the disproportionation process (SET between two anion radicals) depends principally on the thermodynamic constant (K). It can be easily determined more or less accurately from the potential difference existing between the first cathodic peak and the second one. (An exact calculation would be possible from the thermodynamic potentials of the two reversible transfers in the absence of proton sources and at reasonable sweep rates so as to inhibit any undesirable chemical reaction.)... 

In aprotic solvents (e.g. in DMF), the reduction does not take place by an ECE-type mechanism. In the case of diphenyl sulphoxide75 it can be shown that a fast disproportionation process occurs at the stage of the substrate anion radical ... 

Disproportionation process those containing Ce or Eu, have an increased catalyst activity and lifetime Isomerization, kinetically controlled in 46... 

The possibility of predicting solid state reactivity from calculated thermochemical data was first addressed with ketodiesters 65a-e, which were substituted with methyl groups to vary the extent of the RSE in the radicals 65-BRl - 65-BR3 involved along the photodecarbonylation pathway (Scheme 7.19). " All ketones reacted in solution to give complex product mixtures from radical combination (66a-e) and disproportionation processes. Calculations revealed RSEs of 8.9 kcal/mol, 15.1 kcal/mol, and 19.8 kcal/mol for radicals 65-BRl (primary enol radical), 65-BR2 (secondary enol radical), and 65-BR3 (tertiary enol radical), respectively. In the... 

Reduction of Ketones and Enones. Although the method has been supplanted for synthetic purposes by hydride donors, the reduction of ketones to alcohols in ammonia or alcohols provides mechanistic insight into dissolving-metal reductions. The outcome of the reaction of ketones with metal reductants is determined by the fate of the initial ketyl radical formed by a single-electron transfer. The radical intermediate, depending on its structure and the reaction medium, may be protonated, disproportionate, or dimerize.209 In hydroxylic solvents such as liquid ammonia or in the presence of an alcohol, the protonation process dominates over dimerization. Net reduction can also occur by a disproportionation process. As is discussed in Section 5.6.3, dimerization can become the dominant process under conditions in which protonation does not occur rapidly. 

Sojka, Z., Pietrzyk, P., Martra, G. et al. (2006) EPR and DFT study of NO interaction with Ni/Si02 catalyst Insight into mechanistic steps of disproportionation process promoted by tripodal surface nickel complex, Catal. Today, 114, 154. 

While this decomposition reaction is slow at room temperature, the disproportionation process can be accelerated by heat, certain catalysts (e.g., metal ions), and light. Traces of alkali metal ions are normally present in aqueous solutions stored in glass bottles, so plastic or wax-coated glass containers reduce the concentration of metal ion catalysts. Brown-colored containers limit the wavelengths of light that can be absorbed by the solution, restricting the initiation of the disproportionation reaction. 

STDP [Selective toluene disproportionation process] A process for converting touene to mixed xylenes, predominately />-xylene. It takes place in the presence of hydrogen over a ZSM-5-type catalyst. Developed by Mobil in the 1980s and first operated by Enichem. 

TDP [Toluene disproportionation process] A general name for catalytic processes for converting toluene to a mixture of xylene isomers and benzene. One proprietary version is MTDP. 

Propylenediamine, 8 485 physical properties, 8486t pK values, 8487t prices of commercial, 8496t typical specifications, 8496t Propylene disproportionation process,... 

FIGURE 6.3. Concentration profiles for two separated waves (Figure 1.25) at a potential located beyond the second wave, a with no disproportionation process, b In the presence of a fast disproportionation. —, A , B —, C. 

The two forms, with protein from Themiste zostericola, undergo spontaneous spectral changes (28, 31) and complete loss of EPR signal (30) by a first-order rate process, with a rate constant independent of protein concentration. This change is ascribed to a remarkable intramolecular disproportionation process within the octamer ... 

Preparative studies on the 5,7-diphenyl derivative suggest that reduction is followed by a disproportionation process leading to the formation of the dihydrodiazepine base and a tetrahydrodiazepine [UP4 80JCS(P2)668]. 

Sulfur Sg is dissolved through a disproportionation process, which can be reversible or not reversible. The only solvent, which dissolves sulfur by a reversible disproportionation process, is liquid ammonia NH3. In this solvent, the dissolution of sulfur gives the reduced form Se in equilibrium with S3, and oxidized forms of sulfur. 

The dissolution of sulfur in primary and secondary amines also gives highly colored solutions [21, 22], but this dissolution is not reversible. The color of these solutions is caused by polysulfides. The presence of reduced forms of sulfur in these solutions suggests the existence of a disproportionation process, but the oxidized form(s) of sulfur have not yet been identified. Therefore, the process of dissolution of sulfur in amines is not clarified. 

The understanding of the reduction process of sulfur requires the identification of the reduced forms of sulfur, that is, polysulfides or S . The phase diagrams of M2S —Sg systems (where M is an alkali-metal cation) have been reviewed [23], as also the properties of polysulfide melts [24]. Many polysulfides have been characterized in the solid state by x-ray diffraction, infrared and Raman spectroscopy [25]. The identification of polysulfides in solution is often rather difficult. The reason is that the dissolution of a M2S polysulfide leads, for most of them, to a disproportionation process of the type ... 

Liquid ammonia is not a classical solvent for sulfur because it dissolves sulfur in the course of a reversible and slow disproportionation process as has been described earlier. Ammonia is a very useful solvent for the investigation of polysulfides because it allows their synthesis with an easy control of their stoichiometry. After evaporation of ammonia and drying under high vacuum, a solid with the overall composition M28 is obtained. The solid is either a polysulfide or a mixture of polysulfide (s) with sulfur, depending... 

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