Molecular elimination reactions hydrocarbons

One of the early controversies in the field of pyrolysis revolved about the question of the importance of concerted, molecular elimination reactions of hydrocarbons to give olefins and alkanes as products. Numerous experiments (51) have given a decisive answer to this problem in favor of the completely radical mechanism for alkane pyrolysis. At the same time, ironically, very clear cut evidence has been elicited to show that both mono- and polyolefins can react quite readily in concerted, molecu-... 

The occurrence of molecular eliminations in photolysis is something that does not occur to a significant extent in the pyrolysis of hydrocarbons. Most frequently the molecular decomposition is accompanied by free-radical decomposition, with the fraction of each often dependent on the wavelength of the light used to initiate reaction. Such a case is the photolysis of methane where the following reactions occur ... 

The photolysis of n-butane follows a pattern similar to that of propane, with many corresponding reactions. As found for previous hydrocarbons the photolysis includes both molecular and free-radical processes. The molecular elimination of Hj and Dj from C4H10-C4D10 mixtures was first shown by Sauer and Dorfman, who concluded that at 1470 A more than 90 % of the hydrogen came from molecular processes. On the basis of a study of the decomposition of excited -butane molecules generated by electron impact , they attributed hydrogen, methane, ethylene, and other hydrocarbon products to molecular processes, and concluded that free-radical reactions were minimal. 

Figure 4 indicates that the yields of olefins produced by radiolysis are independent of the absorbed dose. These unsaturated products are pentene, light hydrocarbons, and eventually decenes produced by Reaction 15. Cage disproportionation of alkyl radicals produced by C—C bond scission and molecular fragmentation account for the formation of light olefins. Pentene would be formed either by elimination of molecular hydrogen (Reaction 5) or by disproportionation of radicals when the temperature of the sample is raised (Reactions 9 and 16). The latter process is thus a postirradiation production of pentene. 

Circa 35 rather well-defined stoichiometric homometallic and heterobimetallic binuclear elimination reactions have been identified to date. The synthetic products range from molecular hydrogen to hydrocarbons and even more functionalized organics. 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

The trans elimination can take place if the basic sites of the alumina attack the hydrogen from one side of the plane and the hydroxyl group is removed from the opposite side of the plane by the acidic sites of the alumina. This may be possible if the reaction occurs within the pores of molecular dimensions (46) or within the crevices of the aluminas. Crevice sites on silica-alumina catalyst have been proposed by Burwell and co-workers (57) on the basis of racemization and exchange reactions of hydrocarbons. 

Molecular reactions are generally more difficult to treat because of the complexity of the possible transition states. The most widely studied complex molecular reaction class is HX elimination from halogenated hydrocarbons. These reactions proceed primarily via the formation of polar, four-centered tight transition states, and examples include... 

FIGURE 11.11 Products formed in the reactions of ground state carbon atoms with unsaturated hydrocarbon molecules under single collision conditions via hydrogen atom elimination. Note that for the reaction of carbon atoms with acetylene, the tricarbon plus molecular hydrogen elimination channel was observed, too. 

In the photolysis of 0-deuterated alcohols some (about 15 - 20%) H2 is also formed. This could be caused by elimination of carbon bound hydrogen atoms (e.g., reaction 4), or the formation of molecular hydrogen and a carbene. Carbenes have been shown to be intermediates in the photolysis of hydrocarbons... 

In many cases the elimination of water takes place so easily that the use of concentrated sulphuric add is unnecessary, since the diluted add answers the purpose. With the higher members of the series the reaction is complicated by the fact that the simple alkylenes polymerise under the influence of sulphuric add. Thus there is formed, besides butylene, C4Hj, hydrocarbons having respectively twice and three times its molecular weight, e.g. ... 

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