Free radicals from alkanes

Bond dissociation energies have already shown (Sec. 3.24) that the amount of energy required to form free radicals from alkanes decreases in the same order CH3 > r > 2° > 3°. If we combine these two sets of data—ionization potentials and bond dissociation energies—we see (Fig. 5.9) that, relative to the various alkanes concerned, the order of stability of carbonium ions is ... 

The usual way to achieve heterosubstitution of saturated hydrocarbons is by free-radical reactions. Halogenation, sulfochlorination, and nitration are among the most important transformations. Superacid-catalyzed electrophilic substitutions have also been developed. This clearly indicates that alkanes, once considered to be highly unreactive compounds (paraffins), can be readily functionalized not only in free-radical from but also via electrophilic activation. Electrophilic substitution, in turn, is the major transformation of aromatic hydrocarbons. 

Cation (Section 1 2) Positively charged ion Cellobiose (Section 25 14) A disacchande in which two glu cose units are joined by a 3(1 4) linkage Cellobiose is oh tamed by the hydrolysis of cellulose Cellulose (Section 25 15) A polysaccharide in which thou sands of glucose units are joined by 3(1 4) linkages Center of symmetry (Section 7 3) A point in the center of a structure located so that a line drawn from it to any element of the structure when extended an equal distance in the op posite direction encounters an identical element Benzene for example has a center of symmetry Cham reaction (Section 4 17) Reaction mechanism m which a sequence of individual steps repeats itself many times usu ally because a reactive intermediate consumed m one step is regenerated m a subsequent step The halogenation of alkanes is a chain reaction proceeding via free radical intermediates... 

In contrast, the ultrasonic irradiation of organic Hquids has been less studied. SusHck and co-workers estabHshed that virtually all organic Hquids wiU generate free radicals upon ultrasonic irradiation, as long as the total vapor pressure is low enough to allow effective bubble coUapse (49). The sonolysis of simple hydrocarbons (for example, alkanes) creates the same kinds of products associated with very high temperature pyrolysis (50). Most of these products (H2, CH4, and the smaller 1-alkenes) derive from a weU-understood radical chain mechanism. 

Hydroperoxides have been obtained from the autoxidation of alkanes, aralkanes, alkenes, ketones, enols, hydrazones, aromatic amines, amides, ethers, acetals, alcohols, and organomineral compounds, eg, Grignard reagents (10,45). In autoxidations involving hydrazones, double-bond migration occurs with the formation of hydroperoxy—azo compounds via free-radical chain processes (10,59) (eq. 20). 

Most organic free radicals have very short lifetimes, but certain structural features enhance stability. Radicals without special stabilization rapidly dimerize or disproportionate. The usual disproportionation process for alkyl radicals involves transfer of a hydrogen from the carbon P to the radical site, leading to formation of an alkane and an alkene ... 

The simplest paraffin (alkane) and the most widely used feedstock for producing ethylene is ethane. As mentioned earlier, ethane is obtained from natural gas liquids. Cracking ethane can be visualized as a free radical dehydrogenation reaction, where hydrogen is a coproduct ... 

Reactions and Properties of Phosphonic and Phosphinic Acids and their Derivatives.-A free radical mechanism has been proposed to account for the cleavage of the phosphorus-carbon bond in the alkylphosphonic acids (155) by E coli to give a mixture of alkane (methane only, from methylphosphonic acid) and terminal alkene. 

It is now clearly demonstrated through the use of free radical traps that all organic liquids will undergo cavitation and generate bond homolysis, if the ambient temperature is sufficiently low (i.e., in order to reduce the solvent system s vapor pressure) (89,90,161,162). The sonolysis of alkanes is quite similar to very high temperature pyrolysis, yielding the products expected (H2, CH4, 1-alkenes, and acetylene) from the well-understood Rice radical chain mechanism (89). Other recent reports compare the sonolysis and pyrolysis of biacetyl (which gives primarily acetone) (163) and the sonolysis and radiolysis of menthone (164). Nonaqueous chemistry can be complex, however, as in the tarry polymerization of several substituted benzenes (165). 

It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism [2], In order to explain the different product distributions obtained in terms of experimental conditions (temperature, pressure), two mechanisms were proposed. The first one was by Kossiakoff and Rice [3], This R-K model comes from the studies of low molecular weight alkanes at high temperature (> 600 °C) and atmospheric pressure. In these conditions, the unimolecular reactions are favoured. The alkyl radicals undergo successive decomposition by [3-scission, the main primary products are methane, ethane and 1-alkenes [4], The second one was proposed by Fabuss, Smith and Satterfield [5]. It is adapted to low temperature (< 450 °C) but high pressure (> 100 bar). In this case, the bimolecular reactions are favoured (radical addition, hydrogen abstraction). Thus, an equimolar distribution ofn-alkanes and 1-alkenes is obtained. 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

It is well-known that many organic excited states (e.g. the triplet state of benzophenone) can effectively abstract hydrogen atoms from organic compounds such as alkanes and alcohols. This behaviour is not commonly found for metal-containing compounds - a notable exception being the lowest excited state of uranyl ion which abstracts H atoms from alcohols, sugars etc., with the resultant formation of free radicals and U(V) compounds. Recent work has shown that it is very effective in inducing strand breaks in DNA (see Sect. 8). 

Olah showed that nitrations can be split into the three categories of electrophilic, nucleophilic and free radical nitration. Free radical nitrations are extensively used for the industrial synthesis of low molecular weight nitroalkanes from aliphatic hydrocarbons. Nucleophilic nitration is the basis for a number of important methods for the synthesis of nitro and polynitro alkanes. Generally speaking only electrophilic nitration is of preparative importance for the... 

When two free radicals combine, they form a molecule with aU covalent bonds that is a stable and unreactive molecule we call P . It consists of n monomer units (M) linked together with an initiator unit (A) at each end, AM, A. From ethylene, this polymer is the molecule A CH2)2nA. Note that the product molecule is simply a linear alkane with no double bonds remaining. This molecule is totally unreactive, and it is called the dead polymer, ff n is large enough, we neglect the end groups and caU it simply (C2iin)n-... 

CIS-[Ru(H20)2(dinso) ] is made from as-RuClj(dmso) and Ag(BF ) in aq. EtOH. The system c/s-[Ru(H20)j(dmso) ] Vaq. Na(ClO) or TBHP/CH Cl oxidised alkanes such as adamantane, cyclo-octane, -heptane and -hexane to the corresponding alcohols and ketones as did [Ru(Hj0) PWjj(0)3g ] . A free-radical mechanism may be involved for the TBHP oxidations, but those with (C10) probably involve oxoruthenate(VI) or oxoruthenate(IV) intermediates [823], The oxidative destruction of a-chlorinated alkenes by CM-[Ru(HjO)2(dmso) ] Vaq. Oxone /Me(CH3) jN(HSO ) MCj to carboxylic acids and ultimately to CO and HCl was reported [946],... 

This technique was quickly adopted by others and it was soon found by F.O. Rice and co-workers that the pyrolysis of many organic compounds at 800 to 1000°C removed metallic mirrors, implicating the formation of free radicals. The cleavage of larger free radicals into smaller radicals and olefins under these conditions, was also proposed (equation 22), as well as chain reactions in which radicals abstract hydrogen from alkanes. Reactions of alkyl halides with metal atoms in the gas phase were also found by M. Polanyi and co-workers to yield alkyl radicals (equation 23). 

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 catalysis of the selective oxidation of alkanes is a commercially important process that utilizes cobalt carboxylate catalysts at elevated (165°C, 10 atm air) temperatures and pressures (98). Recently, it has been demonstrated that [Co(NCCH3)4][(PF6)2], prepared in situ from CoCl2 and AgPF6 in acetonitrile, was active in the selective oxidation of alkanes (adamantane and cyclohexane) under somewhat milder conditions (75°C, 3 atm air) (99). Further, under these milder conditions, the commercial catalyst system exhibited no measurable activity. Experiments were reported that indicated that the mechanism of the reaction involves a free radical chain mechanism in which the cobalt complex acts both as a chain initiator and as a hydroperoxide decomposition catalyst. 

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