Pyrolysis of a-olefins

The Chugaev reaction [169], involving pyrolysis of a xanthate by an Ej mechanism, is a classical method for olefins. 

HOFMANN DEGRADATION. Formation of an olefin and a tertiary amine hy pyrolysis of a quaternary ammonium hydroxide useful for the preparation of some cyclic olefins and for opening nitrogen-containing ring compounds. 

This chapter comprises part of an experimental program undertaken to provide precise kinetic and product distribution data for the steam pyrolysis of a number of pure olefins, diolefins, and naphthenes over a wide range of temperatures and residence times, including those encountered in industrial practice. Runs were carried out in the bench-scale flow reactor (1), which has been shown (2) to provide yield and conversions data that are in excellent agreement with pilot plant and commercial data. 

The pyrolysis of A -oxides of 3a- and 3/S-dimethylamino-5ct-cholestane (e.g. 17) confirms the accepted ws-character of the elimination reaction, which leads to olefins [42]. The two A/ -oxides afforded similar yields of olefins [32], in contrast to the conformationally specific reactions of the corresponding quaternary salts. 

Ethylene has been separated from ethane by a silver nitrate solution passing countercurrent in a hollow fiber poly-sulfone.165 This separation has also been performed with the silver nitrate solution between two sheets of a polysilox-ane.166 A hydrated silver ion-exchanged Nafion film separated 1,5-hexadiene from 1-hexene with separation factors of 50-80.167 Polyethylene, graft-polymerized with acrylic acid, then converted to its silver salt, favored isobutylene over isobutane by a factor of 10. Olefins, such as ethylene, can be separated from paraffins by electroinduced facilitated transport using a Nafion membrane containing copper ions and platinum.168 A carbon molecular sieve made by pyrolysis of a polyimide, followed by enlargement of the pores with water at 400 C selected propylene over propane with an a-valve greater than 100 at 35°C.169... 

Hydrogenation pyrolysis has been applied to the determination of the composition of copolymers of a-olefins, the sequence of monomer units and the manner in which they are added (head-to-head and head-to-tail) [253]. Mikhailov et al. [251] used Py—GC to investigate the structure of low- and high-density polyethylenes and copolymers of ethylene with propylene. The pyrolysis products were hydrogenated. The method made it possible to examine alkanes up to Cjo, which facilitates the investigation of the polymer chain structure. The isoalkanes identified corresponded to the branched polyethylene structure. It has been established that the ethyl and butyl side-chains occur most frequently in polyethylenes. 

In the first step, propylene is dimerized to 2-methylpent-l-ene by passage over a catalyst of tri-n-propylaluminium at about 200°C and 200 atmospheres. This product is then isomerized to 2-methylpent-2-ene by heating at 150—300°C in the presence of a silica-alumina catalyst. The final step in the process is the pyrolysis of the olefin to isoprene at 650—800°C in the presence of a free radical initiator such as hydrogen bromide. The isomerization step is necessary because pyrolysis of 2-methylpent-l-ene gives much poorer yields of isoprene than pyrolysis of 2-methylpent-2-ene. 

Tetracyanoethylene is colorless but forms intensely colored complexes with olefins or aromatic hydrocarbons, eg, benzene solutions are yellow, xylene solutions are orange, and mesitylene solutions are red. The colors arise from complexes of a Lewis acid—base type, with partial transfer of a TT-electron from the aromatic hydrocarbon to TCNE (8). TCNE is conveniendy prepared in the laboratory from malononitrile [109-77-3] (1) by debromination of dibromoma1 ononitrile [1855-23-0] (2) with copper powder (9). The debromination can also be done by pyrolysis at ca 500°C (10). 

Pyrolysis. The pyrolysis of simple esters of the formula RCOOCR R CHR 2 to form the free acid and an alkene is a general reaction that is used for producing olefins ... 

Catalytic Pyrolysis. This should not be confused with fluid catalytic cracking, which is used in petroleum refining (see Catalysts, regeneration). Catalytic pyrolysis is aimed at producing primarily ethylene. There are many patents and research articles covering the last 20 years (84—89). Catalytic research until 1988 has been summarized (86). Almost all catalysts produce higher amounts of CO and CO2 than normally obtained with conventional pyrolysis. This indicates that the water gas reaction is also very active with these catalysts, and usually this leads to some deterioration of the olefin yield. Significant amounts of coke have been found in these catalysts, and thus there is a further reduction in olefin yield with on-stream time. Most of these catalysts are based on low surface area alumina catalysts (86). A notable exception is the catalyst developed in the former USSR (89). This catalyst primarily contains vanadium as the active material on pumice (89), and is claimed to produce low levels of carbon oxides. 

The pyrolysis of sodium chlorodinuoroacetate is still a widely used, classical method for generating difluorocarbene, especially with enol and allyl acetates [48, 49, 50, 51] (equation 21) A convenient alternative that avoids the hygroscopic salt uses methyl chlorodifluoroacetate with 2 equivalents of a lithium chlonde-hexa-methylphosphoric triamide complex at 75-80 °C in triglyme [52], Yields are excellent with electron-rich olefins but are less satisfactory with moderately nucleophilic alkenes (4-5% yields for 2-bulenes)... 

The pyrolysis of perfluoro carboxylic salts can result both in mono and bimolecular products At 210-220 °C, silver salts give mostly the coupled products, at 160-165 °C in A -methylpyrrolidinone, the corresponding copper salts also give the simple decarboxylated compounds in nearly equal amounts The decomposition of the copper salts m the presence of lodobenzene at 105-125 °C results m a phenyl derivative, in addition to the olefin and coupled product [94] (equations 60-62)... 

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... 

For the regioselectivity similar rules as for the ester pyrolysis do apply. With simple, alkylsubstituted amine oxides a statistical mixture of regioisomeric olefins is obtained. On the other hand with cycloalkyl amine oxides the regioselectivity is determined by the ability to pass through a planar, five-membered transition state. This has been demonstrated for the elimination reaction of menthyl dimethylamine oxide 10 and neomenthyl dimethylamine oxide 11 ... 

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