Ionic reactions thermal

Introduction Molecular Orbitals and Frontier Orbitals Ionic Reactions Thermal Pericyclic Reactions Radical Reactions Photochemical Reactions Exceptions. 

A large number of methods have been used to prepare perfluoroepoxides (5). AH of these methods must contend with the great chemical reactivity of the epoxide product, especially with subsequent ionic and thermal reactions which result in the loss of the desired epoxide. 

Although the alkylation of paraffins can be carried out thermally (3), catalytic alkylation is the basis of all processes in commercial use. Early studies of catalytic alkylation led to the formulation of a proposed mechanism based on a chain of ionic reactions (4—6). The reaction steps include the formation of a light tertiary cation, the addition of the cation to an olefin to form a heavier cation, and the production of a heavier paraffin (alkylate) by a hydride transfer from a light isoparaffin. This last step generates another light tertiary cation to continue the chain. 

Effects of Temperature on Ionic Reactions in TD/D2 CH4/ CD4. Observation that the methanium ion proton (deuteron) transfer sequence fails to exhibit a temperature coefficient within experimental uncertainties leads unavoidably to the conclusion that none of the reactions from 1 to 12 requires thermal activation between —78° and 25°C. From Equations I, II, III, appropriate steady state assumptions, and representing both neutralization steps by kX2, we find that... 

Ionic Reactions in TD/D2 Ethane Mixtures. The data in Table III show that deuteron transfer occurs in irradiated mixtures of D2 and ethane as well. Data are shown only for temperatures (<25°C.) at which ionic reactions clearly predominate. Analysis of data concerning thermal atomic and free-radical reactions at higher temperatures will be published elsewhere in the near future. The reaction of D3 + with ethane has been observed directly (1) and postulated (2) by other workers. Both groups have proposed that the sequence initiated by deuteron transfer to ethane proceeds as follows ... 

In some cases, the Q ions have such a low solubility in water that virtually all remain in the organic phase. ° In such cases, the exchange of ions (equilibrium 3) takes place across the interface. Still another mechanism the interfacial mechanism) can operate where OH extracts a proton from an organic substrate. In this mechanism, the OH ions remain in the aqueous phase and the substrate in the organic phase the deprotonation takes place at the interface. Thermal stability of the quaternary ammonium salt is a problem, limiting the use of some catalysts. The trialkylacyl ammonium halide 95 is thermally stable, however, even at high reaction temperatures." The use of molten quaternary ammonium salts as ionic reaction media for substitution reactions has also been reported. " " ... 

The original work was on ionic reactions in normal micelles in water, but subsequently there has been extensive work on reactions in reverse micelles (O Connor et al., 1982, 1984 Kitahara, 1980 O. A. El Seoud et al., 1977 Robinson, et al., 1979). There also has been a great deal of work on photochemical and radiation induced reactions in a variety of colloidal systems, and microemulsions have been used as media for a variety of thermal, electrochemical and photochemical reactions (Mackay, 1981 Fendler, 1982 Thomas, 1984). 

Because an important reason for studying ionic reactions in the gas phase, rather than in the condensed phase, is to eliminate the strong moderating effects of the solvent, it is generally desirable that the buffer gas serve only as a chemically inert physical medium in which the reactants are suspended and thermalized. In the VHP... 

The following reactions all involve one or more ionic reactions, as well as one or more thermal pericyclic reactions.30 In each case, draw out the whole reaction sequence, identify the pericyclic steps, assign them to their classes, and predict the relative stereochemistry at the bonds marked with a wavy line. 

The thermal and mass spectral reactions of the dinitrobenzenes are even more interesting. ) The primary ionic and thermal decomposition reaction is in every case loss of NO2 to give a nitrophenyl cation or radical respectively. In both systems the nitrophenyl radicals may lose another NO2 fragment to give phenylene diradicals. In the case of o-dinitrobenzene loss of two NO2 fragments leads to the formation of benzjme. Benzofurazan, 44, is one of the minor products obtained from thermolysis of o-dinitro-benzene, it is also a minor ion in the electron impact mass spectrum of the compound. 

Polysiloxanes exhibit exceptional properties over an extremely wide range of temperatures because of their unique combination of high thermal stability and low-temperature fiexibility. However, polysiloxanes cannot completely satisfy the needs for high-temperature elastomers that will perform in extreme thermooxidative environments for extended periods. This deficiency originates from the susceptibility of their backbone chains, which are composed completely of polarized siloxyl units, to degradation by ionic reactions when these materials are exposed to temperatures above 200-250 C. At and above such temperatures, polysiloxanes are degraded by... 

From the discussion above, it is clear that there is no evidence for catalysis of persulfate initiation in emulsion polymerization systems. However, many ionic reactions have been shown to be subject to large catalytic effects in the presence of emulsifier micelles (Fendler and Fendler, 1975) so that the question arises as to whether there are any radical reactions that are subject to micellar catalysis and whether this phenomenon plays any part in any emulsion polymerization systems, Prima fade evidence that uiicellar catalysis may be important when emulsified monomer is allowed to polymerize thermally is provided by the work of Asahara et al. (1970, 1973) who find that several emulsifiers decrease the energy of activation for thermal initiation of alkyl methacrylate and styrene, [n particular, the energy of activation for thermal initiation of styrene emulsified with sodium tetrapropylene benzene solfonate was reported as S3 kl mol. much lower than any value determined in bulk. Hui and Hamielec s value of ] IS kj tnol (1972) seems to be representative of the data available on thermal initiation in bulk. The ctmclusions of Asahara et al. are based on observations of the temperature dependence of the degree of polymerization and are open to several objections. 

Until recently little work has been done on the rates of ionic reactions in solution, principally because these are usually so fast as to make measurement difficult. Working with metal ions in non-aqueous solutions at very low temperatures, Bjerrum and Poulsen (1952) found, for example, that NP+ reacted at a rate which was measurable with dimethylglyoxime in methanol at -75°. Awtrey and Connick (1951) applied Hartridge and Roughton s dynamic flow method (1923) to the reaction between SOg - and I3- in aqueous solution. Bell and Clunie (1952) have developed a thermal method for studying reactions occurring in a few seconds. 

When alkyl iodides and ROH were irradiated under CO pressure in the presence of a base such as potassium carbonate, good yields of carboxylic acid esters were obtained (Scheme 4-42) [71]. In the absence of a base, no carbonylation took place. The role of photo-irradiation is to initiate this hybrid radical/ionic reaction by effecting the homolysis of an R-I bond. The thermal initiation process involving allyltin and AIBN has also been found to be useful, as demonstrated by two examples of amide synthesis which are shown in Scheme 4-42 [72], The likely mechanism involves (i) radical initiation via either irradiation or thermal initiation, (ii) radical chain propagation, composed of two reversible type radical reactions (carbonylation and iodine atom transfer) and (iii) ionic quenching to shift... 

The ionic reaction of R2S2 with white phosphorus in a dipolar aprotic solvent (acetone, CH3CN, DMSO) proceeds, unlike the thermal process, exceptionally smoothly under very mild conditions (25 °C) in the presence of a base. 

In our analysis of the synthesis of rings of various sizes, we concluded (Chapter 29) that four-membered rings are uniquely difficult. For this reason, a special method, the photochemical 2 + 2 cycloaddition is often used to make four-membered rings. Some 2 + 2 thermal cydoadditions, particularly of ketenes (Chapter 33), and some ionic reactions (page 272) are also useful. 

In by far the largest number of cases of free radical copolymerization, the reactivity ratios are practically independent of the nature of the starting reaction (thermal, photochemical, radical-forming type) and the site of propagation (bulk, solution, emulsion). Ionic copolymerization, by contrast, leads to quite different parameters (Table 22-13). Thus copolymerizations can be employed as a diagnostic tool for initiators whose mode of action is unknown, differentiating between free radical copolymerization and the nonradical mechanism (Table 22-14). On such evidence, boron alkyls appear to be free radical initiators in the copolymerization of methyl methacrylate with acrylonitrile, whereas lithium alkyls are anionic initiators. 

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