Pyrolysis multistep

Hertler16 was the first to report the preparation ofpoly(tetrafluoro-p-xylylene) by a multistep synthesis as shown in Scheme 2. Pyrolysis (330°C, 0.025 Torr) of dibromotetrafluoro-p-xylene (B CgFL,) over zinc led to deposition of the polymer film in a cold trap. 

The procedure described is that of Linn, Webster, and Benson.3 Carbonyl cyanide has previously been prepared by the pyrolysis of the diacetyl derivative of diisonitrosoacetone, a multistep process that suffers from low yield, lack of reproducibility, and risk of explosion.2 The present procedure provides a convenient high-yield synthesis of carbonyl cyanide. 

Relatively phase pure, high ceramic yield SiC was obtained by this method however, some disadvantages include (a) a multistep, low yield synthesis, (b) heat treatments to ca 350 °C prior to pyrolysis and (c) the high cost of UAIH4. 

Interrante and coworkers developed two polysilaethylene precursor systems, which upon pyrolysis gave nearly phase-pure SiC (95-99% SiC) in high ceramic yield (75-90%) (Scheme ll)116. The first procedure begins with a multistep synthesis of 1,1,3,3-tetrachloro-l,3-disilacyclobutane (monomer), 5. Alternately, gas phase pyrolysis will convert 6 to 5. 

This type of model works well at high applied heat flux levels, where the pyrolysis front is thin. Simplicity is its advantage it is not necessary to specify any parameters related to the decomposition kinetics. A large body of flame spread modeling work has applied this type of model, but there is a tendency to focus with great detail on gas-phase phenomena (i.e., full Navier-Stokes, detailed radiation models, multistep combustion reactions) and treat the condensed-phase fuel generation process in an approximate manner. 

There are also cases where addition of thiyi radicals to unsaturated carbon-carbon bonds is the crucial step for thiophene ring formation. Flash vacuum pyrolysis (FVP) of the phosphorus ylide 90 affords initially the alkyne 91, which produces thiyi radical with loss of methyl radical. Cyclization of the resulting radical affords thienothiophene 92 as the final product with loss of one more methyl radical (Scheme 20) <1995SL53>. FVP of ylide 93 results in a multistep cascade reaction leading to 7-(2-benzothienyl)benzofuran 94 (Scheme 21) <2001SL228>. 

Each of these designs has its particular characteristics. Thus, the filament-type permits multistep pyrolysis, which enables the discriminative analysis of formulations in a compounded material. The Curie-point type offers the most precisely controlled equilibrium temperature, but the heating conditions depend greatly on the shape of the sample holder. On the other hand, a precise temperature regulation of the microfurnace type is not easy, but it is most suited for thermally labile compounds such as biopolymers. 

Pyrolysis of suitably 3-substituted benzotriazinones 19 provides access to various fused heterocycles. Thus, pyrolysis of 19j at 250°C gives l,3-diphenyl-277-pyrrolo[3,4-f]isoquinolin-5-one 69 <1992JHC1309>, and a multistep sequence transforms 19k into the dione 70 <1982JCM295>. 

Continuous Reactions As a reaction vessel for a unimolecular reaction such as an ester pyrolysis, one thinks first of a pyrolysis tube, which is the simplest sort of continuous reactor. There are certain advantages to running bimolecular laboratory reactions in a similar manner Reaction times can be shorter, yields are higher (especially when heat-sensitive substances are involved), and less solvent is required. For large scale operations such as the first reactions in a long multistep synthesis, continuous reactors are worth considering. Two reactions are used to illustrate the technique. In the first, reactants are added from the top the volatile product distills out, and the nonvolatile product collects at the bottom. In the second, the nonvolatile reactant is added from the top and the volatile reactant from the side the products collect as before. 

Fig. 4-116 Pattern of the Multistep Model for Pyrolysis of Oil Shale according to [4-49]...

A computational study of the kinetics and mechanism of the gas-phase pyrolysis of allylmethylamine has been performed. Two mechanisms were investigated, one involving a six-membered cyclic TS and the other being a multistep, free radical mechanism. Both DFT and high-level calculations show that propene and imine formation are in accordance with a concerted cyclic mechanism. 

Liu et al. prepared catalysts by a multistep preparation [115]. In the first step oti either carbon or silica supports, metal complexes were formed by the reactirai of Co ( 03)2 and FeS04 with ethylendiamine. These precursors were heat-treated twice at 800°C in inert gas whereas after the first pyrolysis, an acid leaching was performed in order to remove soluble metal species. The authors came to the conclusion that in the surface region accessible for ORR, aU metal-containing species should have been removed and could therefore be excluded as components of any... 

The difluorocarbene radicals produced by PTFE pyrolysis undergo a number of autogenous gas-phase reactions. These range from simple recombination to multistep... 

Vegetable oils and fats are usually identified as their fatty acid derivatives or their triglycerides. The well-established methods usually depend on GC as a means of identification. Pyrolysis derivatisation procedures (THM-GC) developed more recently [636] provide a method for characterising these materials. Microgram quantities of the triglycerides are reacted with tetramethylammonium hydroxide (TMAH) at high temperature to yield fatty acid methyl esters without employing multistep procedures. 

More recently other methods have been reported for the synthesis of hexafluoropropylene. One technique involves the pyrolysis of a mixture of tetrafluoroethylene and carbon dioxide at atmospheric pressure at 700-900°C.P ] Conversions of 20-80% and HFP yields of better than 80% were obtained. The unreacted tetrafluoroethylene and carbon dioxide were distilled from the product and recycled. HFP can be synthesized from hexachloropropylene via a multistep process beginning with fluorination.t ] Later steps convert the initial products to CF3-CFCI-CF3 which is dehalogenated to HFP. Other techniques report on the synthesis of hexafluoropropylene from the mixture of a variety of linear and cyclic three-carbon hydrocarbons with a partially halogenated three-carbon acyclic hydrocarbon.t " ]... 

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