The pyrolysis of chlorinated hydrocarbons

I was admitted to Imperial College in October 1938 when I was 20 years old. Because of the good marks I had obtained in the entrance examinations I was allowed to proceed directly into the second year. I completed the BSc with First Class Honors in 1940. Because of what turned out to be a minor problem with my heart, I had earlier been called to the Army and rejected. In any case, I was in a reserved profession and expected to work on a subject of national importance. 

For several weeks these two dichlorides decomposed at the same rate by the unimolecular mechanism. But one day, without warning, the ethylene dichloride started to decompose much faster than the 1,1-dichloroethane, such that I could obtain the same conversion at 100 °C lower temperature. After reflection, I realized that the ethylene dichloride used had been recovered from the dry ice trap and then redistilled before use. Normally the 1,1- and 1,2-dichloroethanes were purified by careful fractional distillation. Clearly, my recovered sample contained a catalyst, or lacked an inhibitor. The latter seemed more probable, so I treated the 1,2-dichloroethane with chromic acid or potassium permanganate, shaking overnight. After redistillation, the purified dichloride still gave variable results, some days decomposing very fast and some not. The simply distilled material always had a constant rate of decomposition. The rate for 1,1-dichloroethane was always constant and independent of comparable chemical treatment. 

The inhibitor in commercial ethylene dichloride of the 1940 era was ethylene chlorohydrin. The two form an azeotrope and so all my laborious efforts at purification by distillation failed. 

CH3 - CC13 CHC12 - CHC12 CC13 CH2C1 CC13 - CHClj CH3 - CHCI 11 12 13 14 15 

1-Dichloroethane 3 always decomposes by the unimolecular mechanism even when oxygen, chlorine, or other radical generators are added. This is because radical attack on 3 gives the derived radical 9 which cannot carry the chain. 

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The work on the pyrolysis of chlorinated hydrocarbons, especially the catalyzed synthesis of vinyl chloride, was patented by the Distillers Company and subsequently sold to the Dow Chemical Corp. Perhaps I justified my research career within the first few months I have never met anyone who could, or would, tell me if ethylene dichloride pyrolysis, which... 

My mind was prepared from my knowledge of the pyrolysis of chlorinated hydrocarbons (Chapter 1) and related subjects. The work of the late Professor E. W. R. Steacie, who eventually became Director of the National Research Council of Canada, had showed that the pyrolysis of alkyl nitrites in the gas phase gave NO and an alkoxy radical in a unimolecular reaction. The temperatures required for this reaction were much too high for... 

Barton, Derek Harold Richard FRS (1918-98) A British chemist noted for his contribution to the pyrolysis of chlorinated hydrocarbons and many other areas of organic chemistry. After gaining his doctomte from Imperial College, London, he held many academic positions and visiting professorships including Re us professor of chemistry at... 

Further studies on the pyrolysis of chlorinated and brominated hydrocarbons have been reported by Maccoll et al. for 3-bromopentane , menthyl and neo-menthyl chlorides , r-alkyl chlorides , dimethylallyl chlorides , a-chloro-o-xylene , and substituted 1-phenylethyl chlorides . Other workers have reported on the thermal reactions of ethyl chloride , monochloropentanes , 1-... 

Another process for silicon carbide fibers, developed by Verbeek and Winter of Bayer AG [45], also is based on polymeric precursors which contain [SiCH2] units, although linear polysilmethylenes are not involved. The pyrolysis of tetramethylsilane at 700°C, with provision for recycling of unconverted (CHg Si and lower boiling products, gave a polycarbosilane resin, yellow to red-brown in color, which was soluble in aromatic and in chlorinated hydrocarbons. Such resins could be melt-spun but required a cure-step to render them infusible before they were pyrolyzed to ceramic... 

All this was later put on a sound basis as a result of more precise measurements of rate constants and of activation energies. However, it did not require precise measurements to predict which chlorinated hydrocarbons would decompose by a radical chain mechanism and which by the unimolecular mechanism. Clearly, if the chlorinated hydrocarbon, or the product from the pyrolysis of the chlorinated hydrocarbon reacted with chlorine atoms to break the chain then the chain mechanism would not exist. Such chlorinated hydrocarbons would decompose by the unimolecular mechanism. Mono-chlorinated derivatives of propane, butane, cyclohexane, etc. would afford propylene, butenes, cyclohexene, etc. All these olefins are inhibitors of chlorine radical chain reactions because of the attack of chlorine atoms at their allylic positions to give the corresponding stabilized allylic radicals which do not carry the chain. 

The pyrolysis products obtained from a variety of mixed plastics containing PVC are investigated. While hydrochloric acid is the major chlorinated product produced by PVC pyrolysis, other chlorinated hydrocarbons are produced. However, the composition and yield of these compounds are very much dependent upon the other polymers present in the plastic mixture. In... 

The BP process [7] is based on a sand fluidized-bed pyrolysis reactor. The cracking temperature is kept at 400-600°C. Low-molecular hydrocarbons can be obtained. The process mainly involves converting waste plastics into normal linear hydrocarbons, the average molecular weight of which is 300-500. Most plastics can be treated by this process. Polyolefins are decomposed into small molecules with the same linear structure. PS is converted into styrene monomers and PET into mixture of hydrocarbons, carbon monoxide and carbon dioxide. A maximum of 2% PVC is allowed in this process, and the content of chlorine in the products is lower than 5 ppm. The distribution of alkene products in this process is like that in petroleum pyrolysis. The BP process was industrialized in 1997. 

The slow combustion of methylene chloride is a degenerately branched chain reaction it proceeds by a mechanism similar to that involved in the pyrolysis of the same compound which takes place at a slightly higher temperature [153]. The primary chains are the same and several of the chlorinated hydrocarbon minor products are identical. Oxygen is only involved in the conversion of the intermediate dichloroethylene to the final products hydrogen chloride and carbon monoxide. 

Pyrolysis of chlorinated unsaturated polyhydrocarbons is in some respects similar to that of the parent polyhydrocarbon, and in some others similar to halogenated saturated hydrocarbon type polymers. The elimination of the hydrohalogenated acid takes place relatively easily, and the polymers from this class are not resistant to heating. In practice, additives that enhance the resistance to heating frequently are added. Since the elimination of the acid seems to accelerate thermal decomposition, similarly to the case of PVC, metallic oxides that scavenge the acid increase the resistance to heating. The main pyrolysis products of each polymer are not modified by the addition of these additives. 

Before a discussion on the reactions forming carbosilanes can be undertaken, the following comments should be made carbosilanes from SiMe are compounds that contain Si atoms with Me groups and to a lesser degree H atoms as substituents. The Si—Si moiety in the molecular skeleton is only observed in exceptional cases. From the methylchlorosilanes, compounds are formed in which Me or Cl groups will occupy those valencies of silicon not bound within the molecular skeleton. Carbon-chlorinated carbosilanes are not formed. The amount of higher-molecular-weight pyrolysis products increases remarkably by thermal work up of the pyrolysis mixture, but hydrocarbon polymers are not formed. 

By-products from EDC pyrolysis typically include acetjiene, ethylene, methyl chloride, ethyl chloride, 1,3-butadiene, vinylacetylene, benzene, chloroprene, vinyUdene chloride, 1,1-dichloroethane, chloroform, carbon tetrachloride, 1,1,1-trichloroethane [71-55-6] and other chlorinated hydrocarbons (78). Most of these impurities remain with the unconverted EDC, and are subsequendy removed in EDC purification as light and heavy ends. The lightest compounds, ethylene and acetylene, are taken off with the HCl and end up in the oxychlorination reactor feed. The acetylene can be selectively hydrogenated to ethylene. The compounds that have boiling points near that of vinyl chloride, ie, methyl chloride and 1,3-butadiene, will codistiU with the vinyl chloride product. Chlorine or carbon tetrachloride addition to the pyrolysis reactor feed has been used to suppress methyl chloride formation, whereas 1,3-butadiene, which interferes with PVC polymerization, can be removed by treatment with chlorine or HCl, or by selective hydrogenation. 

Reaction Conditions. Typical iadustrial practice of this reaction involves mixing vapor-phase propylene and vapor-phase chlorine in a static mixer, foEowed immediately by passing the admixed reactants into a reactor vessel that operates at 69—240 kPa (10—35 psig) and permits virtual complete chlorine conversion, which requires 1—4 s residence time. The overaE reactions are aE highly exothermic and as the reaction proceeds, usuaEy adiabaticaEy, the temperature rises. OptimaEy, the reaction temperature should not exceed 510°C since, above this temperature, pyrolysis of the chlorinated hydrocarbons results in decreased yield and excessive coke formation (27). 

In 1869 Berthelot- reported the production of styrene by dehydrogenation of ethylbenzene. This method is the basis of present day commercial methods. Over the year many other methods were developed, such as the decarboxylation of acids, dehydration of alcohols, pyrolysis of acetylene, pyrolysis of hydrocarbons and the chlorination and dehydrogenation of ethylbenzene." ... 

Catalytic reduction of thiophenes over cobalt catalysts leads to thiolane derivatives, or hydrocarbons. " Noncatalytic reductions of thiophenes by sodium or lithium in liquid ammonia leads, via the isomeric dihydrothiophenes, to complete destructions of the ring system, ultimately giving butenethiols and olefins. " Exhaustive chlorination of thiophene in the presence of iodine yields 2,2,3,4,5,5,-hexachloro-3-thiolene, Pyrolysis of thiophene at 850°C gives a... 

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