Combustion, of alkanes

Combustion of alkanes is an example of oxidation-reduction Although It IS possible to calculate oxidation numbers of carbon m organic mole cules It IS more convenient to regard oxidation of an organic substance as an increase m its oxygen content or a decrease m its hydrogen content... 

Acetylene can be produced by pyrolysis of coal and by fuel-rich combustion of alkanes, a process that also produces soot or carbon black, the major ingredient in automobile tires. Acetylene can also be made with nearly stoichiometric efiiciency by decomposing calcium carbide, CaC, which is produced by pyrolyzing limestone and coke,... 

Mention should be made of studies of slow, controlled combustion of alkanes, where formation of oxetanes can be detected. For example, oxetane is observed during combustion of propane, while 2-f-butyl-3-methyloxetane and 2-isopropyl-3,3-dimethyloxetane are observed from combustion of isooctane. While the yields are extremely low, the presence of these compounds, along with the other products found, have provided evidence for the mechanism of combustion. The oxetanes are believed to result from rearrangement of peroxy radicals in the radical chain process (equation 114) (70MI51300,73MI51301). 

Problem 4.18 (a) Why are alkanes inert (b) Why do the C—C rather than the C—H bonds break when alkanes are pyrolyzed (c) Although combustion of alkanes is a strongly exothermic process, it does not occur at moderate temperatures. Explain. 4... 

Alkanes such as ethane are relatively unreactive and reactions involving alkanes require high-energy atoms or free radicals. Three general types of reactions involving alkanes are combustion, halogenation, and pyrolysis. The most common reactions of alkanes involve combustion. Combustion of alkanes has been the primary source of heat for human civilizations throughout... 

Two factors that influence the heats of combustion of alkanes are, in order of decreasing importance, (1) the number of carbon atoms and (2) the extent of chain branching. Pentane, isopentane, and neopentane are all C5H12 hexane is C6H14. Hexane has the largest heat of combustion. Branching leads to a lower heat of combustion neopentane is the most branched and has the lowest heat of combustion. 

When comparing heats of combustion of alkanes, two factors are of importance ... 

The heats of combustion of alkanes increase as the number of carbon atoms increases. 

Common alkanes are highly flammable, and the more volatile lower molecular mass alkanes form explosive mixtures with air. Furthermore, combustion of alkanes in an oxygen-deficient atmosphere or in an automobile engine produces significant quantities of carbon monoxide, CO, the toxic properties of which are discussed in Section 11.2.2. 

Unfortunately, the burning of gasoline and fuel oil pollutes the air and depletes the petroleum resources needed for lubricants and chemical feedstocks. Solar and nuclear heat sources cause less pollution, and they do not deplete these important natural resources. Facilities that use these more environment-friendly heat sources are currently more expensive than those that rely on the combustion of alkanes. 

The combustion of alkanes and other hydrocarbons obtained from fossil fuels adds a tremendous amount of CO2 to the atmosphere each year. Quantitatively, data show a 20% increase in the atmospheric concentration of CO2 in the last 46 years (from 315 parts per million in 1958 to 377 parts per million in 2004 Figure 4.19). Although the composition of the atmosphere has changed over the lifetime of the earth, this may be the first time that the actions of humankind have altered that composition significantly and so quickly. 

The combustion of alkanes, the concentration of atmospheric carbon dioxide, and global warming (Section 4.14B) An introduction to lipids, biomolecules whose properties can be explained by understanding alkane chemistry cholesterol in the cell membrane (Section 4.15)... 

More recent work on the generation of global reaction schemes for the combustion of alkane hydrocarbons up to isobutane in flames, has been carried out by Jones and Lindstedt [207]. Their global model included two fuel consumption steps ... 

Much of the outstanding chemical investigation of the low-temperature combustion of alkanes was performed on the C5 and alkanes, in the late 1960s and early 1970s, mainly by Cullis and co-workers [137, 189-193] and Fish and co-workers [99, 194-199]. The quality and extent of the chemical analyses in these studies has rarely been equalled in subsequent work, and the data obtained provide very strong evidence, not only for the importance of alkylperoxy radical formation and isomerization in the low-temperature chemistry, but also the role of these reactions in the development of cool-flames and two-stage ignitions. However, one constraint on quantitative application is that much of the information was obtained under markedly non-isothermal conditions in the absence of any record of the reactant temperature. Moreover, the effects of convection on the movement of cool-flame combustion waves within an unstirred reaction vessel were not appreciated at the time [52], which casts doubt on some of the mechanistic interpretations of the evolution of multiple cool-flames that were made [195]. 

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