Free radical thermal reaction

Ethylene Dichloride Pyrolysis to Vinyl Chloride. Thermal pyrolysis or cracking of EDC to vinyl chloride and HCl occurs as a homogenous, first-order, free-radical chain reaction. The accepted general mechanism involves the four steps shown in equations 10—13 ... 

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

Thermal chlorination of ethane is generally carried out at 250—500°C. At ca 400°C, a free-radical chain reaction takes place ... 

Grafting reactions onto a polymer backbone with a polymeric initiator have recently been reported by Hazer [56-60]. Active polystyrene [56], active polymethyl methacrylate [57], or macroazoinitiator [58,59] was mixed with a biopolyester polyhydroxynonanaate [60] (PHN) or polybutadiene to be carried out by thermal grafting reactions. The grafting reactions of PHN with polymer radicals may proceed by H-abstraction from the tertier carbon atom in the same manner as free radical modification reactions of polypropylene or polyhy-droxybutyratevalerate [61,62]. 

The relative ease with which aryl benzyl sulfoxides undergo homolytic dissociation (Rayner et al., 1966) as compared to aryl benzyl sulfides or sulfones is supportive of this idea that ArSO radicals are easier to form than ArS or ArS02 radicals. Another interesting set of observations is the following. Booms and Cram (1972) found that optically active arene-sulfinamides ArS(0)NRPh (R = H or CH3) racemize thermally very readily at room temperature and that this racemization is the result of a free radical chain reaction (160) that is initiated by the dissociation of some of the sulfinamide into an ArSO and a PhNR radical (159). While the length of the inhibition... 

Thermal cracking is a free radical chain reaction. The mechanism is given in Fig. 7.8. 

Furthermore this chapter deals chiefly with polymerizations which are catalyzed by acid-acting catalysts. A comprehensive discussion of not only the thermal but even the photochemical and free radical-initiated polymerizations is outside its scope. The free radical-initiated reactions include those which are induced by metal alkylies, peroxides, oxygen and certain other substances. They depend on free radical initiation of a chain reaction whether or not these free radicals should be considered to be catalysts has been questioned because the radicals enter into the reaction chain and are part of the reaction product. 

Thermal dehydrochlorination of 1,2-dichloroethane188-190 272 273 takes place at temperatures above 450°C and at pressures about 25-30 atm. A gas-phase free-radical chain reaction with chlorine radical as the chain-transfer agent is operative. Careful purification of 1,2-dichloroethane is required to get high-purity vinyl chloride. Numerous byproducts and coke are produced in the process. The amount of these increases with increasing conversion and temperature. Conversion levels, therefore, are kept at about 50-60%. Vinyl chloride selectivities in the range of 93-96% are usually achieved. 

A general free-radical chain reaction with the thermal or photoinduced dissociation of halogen as the initiation step is operative in radical halogenation [Eqs. (10.12)—(10.14)]. 

Initiation normally requires molecules with weak bonds to undergo homolytic cleavage to produce free radicals. Since bond homolysis even of weak bonds is endothermic, energy in the form of heat (A) or light (hv) is usually required in die initiation phase. However, some type of initiation is required to get any free-radical reaction to proceed. That is, you must first produce free radicals from closed-shell molecules in order to get free-radical reactions to occur. Benzoyl peroxide contains a weak 0-0 bond that undergoes thermal cleavage and decarboxylation (probably a concerted process) to produce phenyl radicals which can initiate free-radical chain reactions. 

In the thermal-catalytic method a peroxide catalyst is usually used to initiate the free radical chain reaction. The main disadvantages are the higher temperatures required for carrying out the polymerizations, the potential hazard of explosion on addition of catalyst to the monomer, and disposal of excess catalyzed monomer after impregnating. Combinations of heat, radiation, and catalyst have been experimented with to reduce the radiation and catalyst requirements and to increase the rate of polymerization. In thermal polymerization a muffle furnace, infrared heating, and microwave heating can be used to provide the thermal energy. 

The concept of using the functional groups of electrode surfaces themselves to attach reagents by means of covalent bonding offers synthetic diversity and has been developed for mono- and multi-layer modifications. The electrode surface can be activated by reagents such as organosilanes [5] which can be used to covalently bond electroactive species to the activated electrode surface. Recently, thermally induced free-radical polymerization reactions at the surfaces of silica gel have been demonstrated [21]. This procedure has been applied to Pt and carbon electrode surfaces. These thermally initiated polymer macromolecules have the surface Of the electrode as one of their terminal groups. Preliminary studies indicate that the... 

Simple homolysis of the C-I bond by heating or by light is the most straightforward approach and was the first used for adding perfluoroalkyl iodides to olefins. One presumes that both the thermal and the photochemically induced addition reactions of perfluoroalkyl radicals proceed via free radical chain reactions as depicted in the Scheme below. However, the conditions of these reactions are rarely ideal for preparative purposes because high temperatures are required for the thermolytic process and long photolysis times are required for the photolytic method [60]. 

The photo-initiated addition process appears to have general applicability, although it can require extensive photolysis times [194-196]. Indeed, photolytic generation of RF- from RFI has been the method used to add Rf- to C60 and C70, not for synthetic purpose, but to examine epr spectra of the resulting radical species [197-199]. A good comprehensive review of the early work on thermal and photochemically-induced free radical addition reactions to olefins can be found in Sosnovsky s book [60]. 

Consumption of thermal stabilizers in 1967 amounted to 59 million pounds (see Table I). Commercial stabilizers may be classified into four groups (1) metal salts of inorganic and organic acids (2) organometallic compounds (3) epoxides (4) inhibitors of free radical chain reactions. 

The thermal pyrolysis of hydrocarbons proceeds by free radical chain reaction processes. These processes are exceedingly complex and this overview concentrates on the details as it impacts on the technology and economics of olefin production. 

It is generally accepted that thermal cracking is a free radical chain reaction (a free radical is an atom or group of atoms with an unpaired electron). Free radicals react with hydrocarbons and produce new hydrocarbons and new free radicals ... 

Thermal oxidation is also autocatalytic and considered as metal-catalyzed because it is very difficult to eliminate trace metals (from fats and oils or food) that act as catalysts and may occur as proposed in Equation 4. Redox metals of variable valency may also catalyze decomposition of hydroperoxides (Scheme 2, Equations [6] and [7]). Direct photooxidation is caused by free radicals produced by ultraviolet radiation that catalyzes the decomposition of hydroperoxides and peroxides. This oxidation proceeds as a free radical chain reaction. Although there should be direct irradiation from ultraviolet light for the hpid substrate, which is usually uncommon under normal practices, the presence of metals and metal complexes of oxygen can become activated and generate free radicals or singlet oxygen. 

Dame , and co-workers (193, 194) revealed the formation of free radicals from secondary nitramines as the result of 7 and ultra-violet radiation. Recently Dubovitskii and Korsunskii (4) have reviewed nitramines from the point of view of the kinetic value of their thermal decomposition and came to the conclusion of free radical-chain reaction of breaking the N-NO2 bond. Also a monograph on octogen has appeared [5]. 

Free radical propagation reactions have been studied extensively by ESR. The primary reagent radical is generated in an initiation step, which may be a thermal chemical reaction, photolysis, radiolysis, etc. Reaction to give the secondary radical then follows. ESR data for a very wide variety of radical species have been obtained using this approach. Typical primary radicals are OH, alkoxyl radicals (e.g., Bu O ) and hydrated electrons formed in the reactions shown ... 

The key feature of the use of a dormant species may be seen in the following general scheme (Scheme 1.32) that involves complexation of the propagating species by means of a stable nitroxide radical (Hawker et al, 2001). The P -0 bond of the alkoxy amine P -0-NR is thermally labile at the polymerization temperature, so this becomes the site for the insertion of monomer. Propagation then occurs at a rate that is much slower than for a simple free-radical addition reaction since the propagating radical concentration (which is governed by the position of the equilibrium with the alkoxy amine... 

Free Radicals - An atom or group of atoms with an odd or unpaired electron. Free radicals are highly reactive and participate in free radical chain reactions such as combustion and pol5mier oxidation reactions. Scission of a covalent bond by thermal degradation or radiation in air can produce a molecular fragment named a free radical. Most free radicals are highly reactive because of their unpaired electrons, and have short half lives. 

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