Radicals branched alkanes

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

The peculiarities of the oxidation of PP, whose molecules have alternating tertiary C—H bonds in the (3-position, are of special interest. Such branched alkanes are oxidized with the formation of polyatomic hydroperoxides produced by the intramolecular isomerization of the peroxyl radical [88],... 

If you recall that combustion is a free radical process, we can easily see why cyclic and branched alkanes bum more easily (and more smoothly) than straight-chain alkanes. The reason is that more stable free radicals are formed. This results in less knocking and a higher octane rating. Examples of free radical stability are the following ... 

The radical-forming reactions are suggested to take place mostly after an Si T type ISC the reactions have nonactivated character. The homolytic split to H atom and alkyl radical has a considerable yield in the photolysis of n-alkanes and cycloalkanes, while the scission to two radicals is characteristic of the decay of excited branched alkane molecules. 

Very little skeletal rearrangement occurs via pyrolysis, a fact inherent in the failure of free radicals to readily isomerize by hydrogen atom or alkyl group migration. As a result, little branched alkanes are produced. Aromatization through the dehydrogenation of cyclohexanes and condensation to form polynuclear aromatics can take place. Additionally, olefin polymerization also can occur as a secondary process. 

Raw materials. The paraffins used in the manufacture of paraffin sulphonates are essentially the same as those used in the pro duction of LAB but favouring the higher end of the molecular weight range. For paraffin sulphonate manufacture, it is essential to use a normal paraffin, free of any aromatics because branched alkane (especially tertiary) and aromatic species will act as radical traps and reduce the reaction yield. 

Cation and radical stabilities help to explain the mass spectra of branched alkanes as well. Figure 12-19 shows the mass spectrum of 2-methylpentane. Fragmentation of a branched alkane commonly occurs at a branch carbon atom to give the most highly substituted cation and radical. Fragmentation of 2-methylpentane at the branched carbon atom can give a secondary carbocation in either of two ways ... 

Air oxidation of /i-butane to maleic anhydride is possible over vanadium phos(4tate and, remaiicably, a 60% selectivity is obtained at 85% conversion. In the gas phase oxidation, in conffast to the situation found in the liquid, n-allcanes are oxidized more rapidly than branched chain alkanes. This is because secondary radicals are more readily able to sustain a chain for branched alkanes the relatively stable tertiary radical is preferentially formed but fails to continue the chain process. Vanadium(V)/ manganese(II)/AcOH has been used as a catalyst for the autoxidation of cyclohexane to adipic acid, giving 25-30% yields after only 4 h. ... 

Fluorinated alkenes are able to insert into weak C-H bonds of various compounds branched alkanes, haloforms. alcohols, ethers, aldehydes and their corresponding ketals. These reactions usually involve the use of UV irradiation or radical-initiation catalysts, such as peroxides or azobisisonitriles. Variable amounts of telomcric products are also formed. Under the influence of ) -irradiation ( °Co source), one-to-one adducts are obtained predominantly. Attack of the intermediate radical occurs preferentially on the less-hindered carbon of the fluorinated alkene. 

The names of alkanes with radical branches use the name from Table 18-2 for the longest continuous chain of carbon atoms. The radicals are named using the -yl ending presented in Table 18-3, and their positions along the carbon chain are denoted by a number. For example,... 

The oxidation of a branched C4 alkane, isobutane, was carried out to probe the mechanism of the C-H bond activation of alkanes on the VPO catalysts [103]. Maleic anhydride was among the products of oxidation of this branched alkane. In the case of isobutane 29 (Figure 15), the surface-bound peroxo radical would show discrimination in activating first the weaker tertiary C-H bond. The... 

Appropriate modification of the ESR spectrometer and generation of free radicals by flash photolysis enables time-resolved (TR) ESR spectroscopy [22]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They can be enhanced as much as one-hundredfold and appear as absorption, emission, or a combination of both. Effects of this type are a result of chemically induced dynamic electron polarization (CIDEP) these spectra indicate the intermediacy of radicals whose sublevel populations deviate substantially from equilibrium populations. Significantly, the splitting pattern characteristic of the spin-density distribution of the intermediate remains unaffected thus, the CIDEP enhancement not only facilitates the detection of short-lived radicals at low concentrations, but also aids their identification. Time-resolved ESR techniques cannot be expected to be of much use for electron-transfer reactions from alkanes, because their oxidation potentials are prohibitively high. Even branched alkanes have oxidation potentials well above the excited-state reduction potential of typical photo-... 

In the next section we discuss the radical cations generated upon electron transfer from simple -alkanes to holes generated in various matrices upon radiolysis. The ESR spectra observed in these matrices reveal many interesting structural details. We will begin with the special case of the long-elusive methane radical cation, followed by the radical cations of -alkanes and those of branched alkanes. 

Many other branched alkanes undergo electron-transfer reactions in cryogenic matrices. Although the ESR spectra of the resulting radical cations reveal interesting structural features [60-62], detailed discussion would exceed the scope of this review. 

Significant applications of y-radiolysis in the study of electron-transfer processes and radical cations in zeolites have emanated from the radiation group at Argonne National Laboratories [73]. For example, Barnabas et al. studied the fate of highly branched alkanes, 2,3-dimethylbutane, 2,2,3-trimethylbutane, and 2,2,3,3-tetramethylbutane, upon y-radiolysis in pentasil zeolite (ZSM-5) [74, 75]. 

Similar to linear and branched alkanes, cycloalkanes also give rise to radical cations in zeolites, spontaneously or upon y-radiolysis. This brief discussion of selected examples is intended only to give a flavor of the work being done. Thus, a 13-line radical cation spectrum (a = 0.17 mT, g = 2.003) obtained upon incorporation of 1-methylcyclohexane, 43, into zeolites [71] was identified as 1,2-dimethylcyclopentene radical cation, 44 + (two sets of protons with hyperfine couphng constants in the ratio of ca 2 1 a = 1.67 mT, 2 CH3 a = 3.42 mT, 4H) [72]. The formation of 44 + was rationalized by protonation of the 3° carbon of 43, followed by loss of H2. Loss of a proton from a rearranged carbocation may generate 44, which is oxidized to 44 + by a Lewis site. 

Both types of anti-knock are more effective in paraffinic fuels then in olefinic or aromatic fuels, and can even promote knock when added to some alcohols. In Fig. 7.9 the response of some pure hydrocarbons to the addition of 3ml/US gal of tetra-ethyl lead is shown, in terms of Performance Number. Almost all the alkanes lie on a steeper line than the alkenes. The exceptions are low octane number alkenes, which are largely straight alkane chains, and a few highly-branched alkanes (which also have high sensitivity, see Section 7.2.3). Notwithstanding the subtleties of lead additives, a broad explanation in chemical kinetic terms is that the antiknock acts to increase radical termination rates and, consequently, has proportionately less effect in those fuels where the termination rates are already high. 

The first two represent the high-temperature chemistry and the second two the branch chain of the low-temperature chemistry. Griffiths [85] has criticized the model for losing the essential feature of alkane autoignition chemistry, a switch from radical branching to non-branching reactions as the temperature increases, which is responsible for the negative temperature coefficient. The model relies on thermal feedback mechanisms only. 

Fig. 5. Pyrolysis of lumped component of branched alkanes C15H32. Selectivity of methyl radical and small alkenes as a function of the probability of methyl substitution.

H-abstraction reactions of cyc/o-alkanes follow the same rules and apply the same reference kinetic parameters as the analogous reactions of normal and branched alkanes. For example, Fig. 6 shows the main cyclo-hexyl radical pyrolysis pathways. For simplicity s sake, most of the dehydrogenation reactions are not reported. 

Fig. 4. 26 Possible reaction schemes involved in alkane formation from thermal degradation of kerogen (a) radical reactions in w-alkane generation (b) carbocation reactions in methyl-branched alkane generation (after Kissin 1987).

Steric effects in oxidation are found primarily in the H-atom donor as, for example, isobutane versus 2,4-dimethylpentane where both co-oxidation and added hydroperoxide measurements indicate isobutane to be 1.4 times as reactive toward either R02 radical [38,39], For a series of branched alkanes, the value of kp was more sensitive to changes in steric bulk on the adjacent carbon than on the same carbon but all values were within a factor three [39], Increasing size in f-R02 radicals has little effect on the value of kp for H-atom transfer from primary, secondary or tertiary sites [69]. However, kp for a given hydrocarbon does increase by a factor of about 5—10 as R02 is changed from a tertiary to secondary or primary ROa [78], Moreover, in the oxidation of aromatic compounds, the ratio of kp s for H-atom transfer to parent R02- and to f-Bu02-correlate with meta substituent constants which suggests that this difference in reactivity is due mainly to polar effects [86]. 

When a branched alkane is used as the substrate, a radical usually attacks the C-H bonds in accordance with normal selectivity, i.e., the reactivity of C-... 

It is important, however, that not all the radicals formed are liberated in the solution, since the oxidation of (+)-3-methylheptane by chromic acid involves the formation of (+)-3-methyl-3-heptanol with retention of configuration to the extent of 70-85%. The normal selectivity has been observed in the hydroxylation of branched alkanes. 

In cyclohexane and decalins, reaction (1) is endothermic by 0.1-0.4 eV [60] and it seems reasonable that the excitation of the hole may facilitate the proton transfer. Fragmentation of matrix-isolated hydrocarbon radical cations upon excitation with 2-4 eV photons was observed by EPR (see review [61]). For cycloalkanes, the main photoreaction is reaction (3). For radical cations of methyl-branched alkanes, the loss of CH4 was also observed, while the radical... 

For branched alkanes, the fragmentation patterns could be very complex [3, 111]. The prompt yield of the -H radicals is always minor the highest yields are of the radicals formed by scission of skeletal C-C bonds next to the branches. For example, in radiolysis of isooctane only 15% of the radicals are of the -H type, the rest bemg tert-butyl and 2-propyl radicals [111]. In radiolysis of 2,3-dimethylbutane, 70% of radicals are 2-propyl and 30% are -H radicals (2,3-dimethyl-2-butyl). It is not known what species dissociate (singlets triplets excited holes excited radicals ) and what controls these fragmentation patterns. 

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