Reactivity of hydrocarbons

Reactivity of Hydrocarbons.—In their work on steam dealkylation over Rh catalysts Kikuchi, Mori, and their co-workers determined the relative 

Cyclohexane trimethylbutane n-butane, n-decane n-heptane ethane benzene methane. 

Ethylbenzene, iso-octane n-heptane, methylcyclohexane toluene p-, m-xylene CT-xylene. 

Ethylbenzene m-, p-xylene, mesitylene toluene, n-heptane, iso-octane, methylcyclohexane or-xylene. 

The results obtained by Nielsenfor steam reforming over a Ni catalyst are also included in Table 5. He found that branched hydrocarbons were more reactive than normal paraffins, although other workers had found this not to be so these differences are discussed in ref. 4. He reported low reactivity for benzene compared with paraffins which is credible, reflecting the higher stability of the aromatic ring. 

Chemical inertness of alkanes is reflected in one of their old names, paraffins , from the Latin parum affinis (without affinity). However, saturated hydrocarbons can be involved very easily in a total oxidation with air (simply speaking, burning) to produce thermodynamically very stable products water and carbon dioxide. It should be emphasized that at room temperature alkanes are absolutely inert toward air, if a catalyst is absent. At the same time, some active reagents, e.g., atoms, free radicals, and carbenes, can react with saturated hydrocarbons at room and lower temperatures. These compounds are easily transformed into various products under elevated (above 1000 °C) temperatures, in the absence of other reagents. 

Some important reactions of alkanes have been developed, e.g., autoxi-dation by molecular oxygen at elevated temperatures, which proceeds via a radical chain mechanism. The main feature of this and many other reactions is a lack of selectivity. Reactions with radicals give rise to the formation of many products all possible isomers may be obtained. As far as burning is concerned, this process can be very selective, producing solely carbon dioxide, but apart from being an important source of energy, is useless from the viewpoint of the synthesis of valuable organic products. Chemical inertness of alkanes is due to 

we should conclude, that alkanes are extremely inert toward normal (i.e., not very reactive) reagents in reactions that proceed more or less selectively. In many respects, alkanes, especially lower ones (methane, ethane) are similar to molecular hydrogen. Indeed, like alkanes, dihydrogen while being inert towards dioxygen at ambient temperatures can be burned in air to produce thermodynamically stable water. The values of the C-H and H-H dissociation energy 

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Haag W O, Lago R M and Weisz P B 1982 Transport and reactivity of hydrocarbon molecules in a shape-selective zeolite Faraday Disouss. Chem. Soo. 72 317-30... 

P.B. (1982) Transport and reactivity of hydrocarbon molecules in shape selective zeolites. J. Chem. Soc., Farad. Disc., 72, 317. 

Table III. Relative Reactivities of Hydrocarbons Toward Various Radicals"...

Reactivity ratios for all the combinations of butadiene, styrene, Tetralin, and cumene give consistent sets of reactivities for these hydrocarbons in the approximate ratios 30 14 5.5 1 at 50°C. These ratios are nearly independent of the alkyl-peroxy radical involved. Co-oxidations of Tetralin-Decalin mixtures show that steric effects can affect relative reactivities of hydrocarbons by a factor up to 2. Polar effects of similar magnitude may arise when hydrocarbons are cooxidized with other organic compounds. Many of the previously published reactivity ratios appear to be subject to considerable experimental errors. Large abnormalities in oxidation rates of hydrocarbon mixtures are expected with only a few hydrocarbons in which reaction is confined to tertiary carbon-hydrogen bonds. Several measures of relative reactivities of hydrocarbons in oxidations are compared. 

Tphe original objectives of this work were to determine how much the relative reactivity of two hydrocarbons toward alkylperoxy radicals, R02, depends on the substituent R—, and whether there are any important abnormalities in co-oxidations of hydrocarbons other than the retardation effect first described by Russell (30). Two papers by Russell and Williamson (31, 32) have since answered the first objective qualitatively, but their work is unsatisfactory quantitatively. The several papers by Howard, Ingold, and co-workers (20, 21, 23, 24, 29) which appeared since this manuscript was first prepared have culminated (24) in a new and excellent method for a quantitative treatment of the first objective. The present paper has therefore been modified to compare, experimentally and theoretically, the different methods of measuring relative reactivities of hydrocarbons in autoxidations. It shows that large deviations from linear rate relations are unusual in oxidations of mixtures of hydrocarbons. 

Since the third objective of this paper is to compare, theoretically and experimentally, different methods of measuring reactivities of hydrocarbons in autoxidation, the most pertinent work is reviewed below. 

A subsequent paper (23) gives propagation and termination constants for numerous additional hydrocarbons and deals mostly with relative reactivities of active hydrogen atoms and with effects of structure on termination constants. A comparison of relative reactivities of hydrocarbons toward alkylperoxy, tert-butoxy, and phenyl radicals uses a different alkyl in each alkylperoxy radical in spite of the differences in reactivity among different alkylperoxy (29) radicals. 

Reactivity of Hydrocarbons. - Each homologous series in a liquid fuel can exhibit different kinetics upon reforming under similar reaction conditions. For example, aromatic compounds are the most difficult to reform and require higher temperatures and lower space velocities. Aromatics also contribute significantly to carbon formation, compared to paraffins and naphthenes. At the same reaction conditions, the H2 production rates are typically in the order aromatics naphthenes. ° The relative reactivities of various higher hydrocarbons are summarized in Table 12. 

The reactivity of hydrocarbons in citrus oils has brought considerable attention to citrus processing. Processors are only allowed to emit certain levels of volatile organic compounds... 

F. Comparative Reactivities of Hydrocarbon Species Adsorbed on Different Metal Surfaces... 

It is noteworthy that the reactivity of hydrocarbons towards CrOs(Ph3PO) parallels their reactivity toward R02- radicals for hydrogen abstraction. This suggests that Crvl-peroxo complexes almost exclusively act as hydrogen atom abstractors, in their homolytically dissociated form Crv—OO (equation 39).80... 

III. The reactivities of hydrocarbons toward the styrene radical. Disc. Faraday Soc. 2, 328 (1947). 

Structure, determination of organic reactivity, 35, 67 Structure and mechanism, in curbene chemistry, 7, 153 Structure and mechanism, in organic electrochemistry, 12, 1 Structure and reactivity of carbencs having aryl substitutents, 22, 311 Structure and reactivity of hydrocarbon radical cations, 38, 87 Structure of electronically excited molecules, 1, 365... 

W. W. Pritzkow, V Y. Suprun, Reactivity of Hydrocarbons and Their Individual C-H Bonds in Respect to Oxidation Processes Including Peroxy Radicals, Russ. Chem. Rev. 1996, 65, 497-503. 

Relative Reactivities of Hydrocarbons toward Cobalt Oxidation... 

However, it is difficult to reconcile the observed relative reactivities of hydrocarbons with a mechanism involving electron transfer as the rate-determining process. For example, n-butane is more reactive than isobutane despite its higher ionization potential (see Table VII). Similarly, cyclohexane undergoes facile oxidation by Co(III) acetate under conditions in which benzene, which has a significantly lower ionization potential (Table VII), is completely inert. Perhaps the answer to these apparent anomalies is to be found in the reversibility of the electron transfer step. Thus, k-j may be much larger than k2 for substrates, such as benzene, that cannot form a stable radical by proton loss from the radical cation [Eqs. (224) and (225)]. With alkanes and alkyl-substituted arenes, on the other hand, proton loss in Eq. (225) is expected to be fast. 

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