Alkanes radical reactions

Kinetic Models Used for Designs. Numerous free-radical reactions occur during cracking therefore, many simplified models have been used. For example, the reaction order for overall feed decomposition based on simple reactions for alkanes has been generalized (37). 

Instead of radical reactions, models based on molecular reactions have been proposed for the cracking of simple alkanes and Hquid feeds like naphtha and gas oil (40—42). However, the vaUdity of these models is limited, and caimot be extrapolated outside the range with confidence. With sophisticated algorithms and high speed computers available, this molecular reaction approach is not recommended. 

Initiation step (Section 4.17) A process which causes a reaction, usually a free-radical reaction, to begin but which by itself is not the principal source of products. The initiation step in the halogenation of an alkane is the dissociation of a halogen molecule to two halogen atoms. 

Despite the limitations of radical chlorination of alkanes, the reaction is still useful for synthesizing certain halogenated compounds. For which of the following compounds does radical chlorination give a single moiiochloro product ... 

The enhanced selectivity of alkane bromination over chlorination can be explained by turning once again to the Hammond postulate. In comparing the abstractions of an alkane hydrogen by Cl- and Br- radicals, reaction with Br- is less exergonic. As a result, the transition state for bromination resembles the alkyl radical more closely than does the transition state for chlorination, and the stability of that radical is therefore more important for bromination than for chlorination. 

This allylic bromination with NBS is analogous to the alkane halogenation reaction discussed in the previous section and occurs by a radical chain reaction pathway. As in alkane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br- radical, which cycles back... 

Alkyl halides can be reduced to alkanes by a radical reaction with tributyltin hydride, (C4H9)3SnH, in the presence of light (hv). Propose a radical chain mechanism by which the reaction might occur. The initiation step is the light-induced homolytic cleavage of the Sn— H bond to yield a tributyltin radical. 

The results of kinetic studies suggest that alkane substitution reactions typically proceed by a radical chain mechanism (Section 13.9). The initiation step in the chlorination of methane is the dissociation of chlorine ... 

Reactions involving radicals occur widely in the gas phase the combustion of any organic compound is nearly always a radical reaction, and the oxidative breakdown of alkanes in internal combus-... 

Another characteristic of many radical reactions is that, once initiated, they often proceed with great rapidity owing to the establishment of fast chain reactions of low energy requirement, e.g. in the halogenation of alkanes (3, cf. p. 323) ... 

Dihydroperoxides were found to be the primary products of branched alkanes oxidation. The intramolecular peroxyl radical reaction was proposed F. F. Rust [55]... 

Finally, when we are running out of cyclohexane, the process terminates by the interaction of two radical species, e.g. two chlorine atoms, two cyclohexyl radicals, or one of each species. The combination of two chlorine atoms is probably the least likely of the termination steps, since the Cl-Cl bond would be the weakest of those possible, and it was light-induced fission of this bond that started off the radical reaction. Of course, once we have formed cyclohexyl chloride, there is no reason why this should not itself get drawn into the radical propagation steps, resulting in various dichlorocyclohexane products, or indeed polychlorinated compounds. Chlorination of an alkane will give many different products, even when the amount of chlorine used is limited to molar ratios, and in the laboratory it is not going to be a particularly useful process. 

Many times, multistep synthesis problems start with an alkane. An efficient way of adding a functional group to that alkane is to brominate (or chlorinate) the alkane through a free-radical reaction. 

Oxygenation of alkanes with hydrogen peroxide in homogeneous reactions can be obtained also with Keggin-type iron-substituted polyoxometalates. Evidence against radical reactions has been offered. 

The kinetics and mechanisms of nitrate radical reactions with alkanes and a variety of other organics relevant to the atmosphere are discussed in detail in two excellent reviews by Wayne et al. (1991) and Atkinson (1991). The kinetics of the N03-alkane reactions are summarized in Table 6.3, where it can be seen that, with the exception of methane, they are in the range 10 lX-10 lf cm3 molecule-1 s-1. 

While these reactions are much slower than the corresponding OH reactions, the nighttime peak concentrations of NO, under some conditions are much larger than those of OH during the day, 400 ppt vs 0.4 ppt. Even given the differences in concentration, however, as seen from the lifetimes in Table 6.1, the nitrate radical reaction is still relatively slow. While the removal of the alkanes by NO, is thus not expected to be very significant under most tropospheric conditions, reaction (20) can contribute to HNO, formation and the removal of NOx from the atmosphere. 

In 1983 Barton and co-workers reported the use of iron-based reagents in the functionalization of alkanes (equation 72). The elucidation of the mechanisms involved a long dispute, but the role of free radical reactions appears to be well established. ... 

The principal polyolefins are low-density polyethylene (ldpe), high-density polyethylene (hope), linear low-density polyethylene (lldpe), polypropylene (PP), polyisobutylene (PIB), poly-1-butene (PB), copolymers of ethylene and propylene (EP), and proprietary copolymers of ethylene and alpha olefins. Since all these polymers are aliphatic hydrocarbons, the amorphous polymers are soluble in aliphatic hydrocarbon solvents with similar solubility parameters. Like other alkanes, they are resistant to attack by most ionic and most polar chemicals their usual reactions are limited to combustion, chemical oxidation, chlorination, nitration, and free-radical reactions. 

Radical reactions are often called chain reactions. All chain reactions have three steps chain initiation, chain propagation and chain termination. For example, the halogenation of alkane is a free radical chain reaction. 

It is clear from a recent review of the mechanisms of metal-catalyzed oxidations of hydrocarbons (89) that by far the most extensive studies have been on the oxidation of alkenes and aromatic compounds relatively little work on alkane oxidation is to be found. The studies on these reactions show that, if the reactivity is enhanced by a hard metal, it is often because the metal becomes involved in the free-radical reactions and generates further free radicals by the chain decomposition of hydroperoxides (39) ... 

The Ni(I) complex of tetramethylated cyclam, [Ni(TMC)]+, generated from the corresponding Ni(II) complex by electrochemical or photochemical methods, reacts with alkyl halides (RX) (133,140-143). It is a radical reaction, generating R transients and/or Ni-alkyls, which then decay to form alkanes, alkenes, and dimeric or cyclic organics. 

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