Radical generation pyrolysis

Pyrolysis. Pyrolysis of 1,2-dichloroethane in the temperature range of 340—515°C gives vinyl chloride, hydrogen chloride, and traces of acetylene (1,18) and 2-chlorobutadiene. Reaction rate is accelerated by chlorine (19), bromine, bromotrichloromethane, carbon tetrachloride (20), and other free-radical generators. Catalytic dehydrochlorination of 1,2-dichloroethane on activated alumina (3), metal carbonate, and sulfate salts (5) has been reported, and lasers have been used to initiate the cracking reaction, although not at a low enough temperature to show economic benefits. 

Koplitz and co-workers have studied the photolysis of C2H5 via the A2A (3s) state by using 248-nm photolysis radiation and hot ethyl radicals generated from photolysis of ethyl halides.125,126 Chen and co-workers have investigated the photodissociation of jet-cooled ethyl (produced by flash pyrolysis of ethyl iodide and n-propylnitrite) in the region of 245-264 nm... 

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. 

In this connection investigations are to be mentioned in which a mass-spectrometric analysis has been made of neutral radicals, e.g., CHjCO, split off from acetone by u.v. photons in the ordinary range.27-28 In the first a flash lamp has been used and the radicals were ionized as usual by electron impact. In the second the same radical ionized at a field emission electrode. Recently, several alkyl radicals generated by pyrolysis have been studied. Their values of lv and of the photoionization cross sections could be obtained in the mass spectrometer under monochromatic vacuum u.v. irradiation.29... 

Thne-of-flight (TOF) mass spectrometric analysis of the pyrolysis fragments of di-t-butyl peroxide suggests t-BuCO as the primary product, followed by decomposition of this radical into CHj.253 Elsewhere, the kinetics of the pyrolysis of dimethyl, diethyl, and di-t-butyl peroxides in a modified adiabatic bomb calorimeter have been investigated.254 The lifetime of acyloxy radicals, generated by the photolysis or thermolysis of acetyl propionyl peroxide, have been studied. Chemical nuclear polarization has been used to determine the rate constant for the decarboxylation of these radical intermediates.255... 

Hua et al. (1995) proposed a supercritical water region in addition to two reaction regions such as the gas phase in the center of a collapsing cavitation bubble and a thin shell of superheated liquid surrounding the vapor phase. Chemical transformations are initiated predominantly by pyrolysis at the bubble interface or in the gas phase and attack by hydroxyl radicals generated from the decomposition of water. Depending on its physical properties, a molecule can simultaneously or sequentially react in both the gas and interfacial liquid regions. 

The addition reactions take place at a carbon-carbon multiple bond, or carbon-hetero atom multiple bond. Because of this peculiarity, the addition reactions are not common as the first step in pyrolysis. The generation of double bonds during pyrolysis can, however, continue with addition reactions. The additions can be electrophilic, nucleophilic, involving free radicals, with a cyclic mechanism, or additions to conjugated systems such as Diels-Alder reaction. This type of reaction may explain, for example, the formation of benzene (or other aromatic hydrocarbons) following the radicalic elimination during the pyrolysis of alkanes. In these reactions, after the first step with the formation of unsaturated hydrocarbons, a Diels-Alder reaction may occur, followed by further hydrogen elimination ... 

The anhydride units interrupt the unzipping process characteristic for PMMA pyrolysis. A similar effect may have the free radicals generated during the decomposition of one polymer and interacting with the other one in a blend. 

As an example of application of automatic generation, Table I gives the complete set of the primary propagation reactions of -decyl radicals isomerization, -decomposition and dehydrogenation reactions. These reactions are produced directly by the MAMA program which was specifically developed for pyrolysis mechanism generation (Dente and Ranzi, 1983 Dente et al., 2005 Pierucci et al., 2005). 

The vacuum UV photochemistry of the allyl radical, generated by flash pyrolysis, has been investigated in a study using synchrotron radiation, in which mass spectra recorded over a wide energy range showed the appearance of the C3H3+ fragment from dissociative photoionization above 10 eV. ... 

As so much energy can be introduced at once, one of the most common ways of setting up a pair of radicals is by photochemical excitation. The excited state has a chemistry of its own, as we have seen in the earlier part of this chapter, but one outcome for the excited state is for a bond to break homolytically, with the two radicals (or the diradical if it is a bond in a ring that breaks) either produced in, or able rapidly to revert to, their electronic ground states. When this happens, the radicals usually show the same patterns of reactivity and selectivity as the corresponding radicals generated by pyrolysis of a strained or weak single bond. 

The gas-phase chemistry of Si-containing radicals forms one important sub-mechanism which is attracting considerable attention. The two most important Si radicals generated in either discharge, pyrolysis, or photochemical CVD sources are silylene, SiH2, and silyl, SiHs. 

Radical generated by pyrolysis of dibenzyl ether at 770 K and by phot, of DTBP in the presence of dibenzyl ether at temperatures up to 473 K. Study of equilibrium between dibenzyl and benzyl radicals under pressure in toluene at 878 K. 

The formal kinetics A B C occurs commonly in practice. Consider the reactivity of the free methyl radical, generated by azomethane pyrolysis, with an alkane, RH ... 

---