Fragmentation process aromatic hydrocarbons

Anodic oxidation in inert solvents is the most widespread method of cation-radical preparation, with the aim of investigating their stability and electron structure. However, saturated hydrocarbons cannot be oxidized in an accessible potential region. There is one exception for molecules with the weakened C—H bond, but this does not pertain to the cation-radical problem. Anodic oxidation of unsaturated hydrocarbons proceeds more easily. As usual, this oxidation is assumed to be a process including one-electron detachment from the n system with the cation-radical formation. This is the very first step of this oxidation. Certainly, the cation-radical formed is not inevitably stable. Under anodic reaction conditions, it can expel the second electron and give rise to a dication or lose a proton and form a neutral (free) radical. The latter can be either stable or complete its life at the expense of dimerization, fragmentation, etc. Nevertheless, electrochemical oxidation of aromatic hydrocarbons leads to cation-radicals, the nature of which is reliably established (Mann and Barnes 1970 Chapter 3). 

A number of papers report investigations of the pyrolytic cleavage of aromatic hydrocarbons. The oxidation and pyrolysis of anisole at 1000 K have revealed first-order decay in oxygen exclusively via homolysis of the O—CH3 bond to afford phenol, cresols, methylcyclopentadiene, and CO as the major products.256 A study of PAH radical anion salts revealed that CH4 and H2 are evolved from carbene formation and anionic polymerization of the radical species, respectively.257 Pyrolysis of allylpropar-gyltosylamine was studied at temperatures of 460-500 °C and pressures of 10-16 Torr. The product mixture was dominated by hydrocarbon fragments but also contained SO2 from a proposed thermolysis of an intermediate aldimine by radical processes.258... 

Di-(l-naphthylmethyl)sulphone forms an excimer but does not react to give an intramolecular cycloaddition product like the corresponding ether but rather fragments to give sulphur dioxide and (l-naphthyl)methyl radicals (Amiri and Mellor, 1978). I-Naphthylacetyl chloride has a very low quantum yield of fluorescence and this is possibly due to exciplex formation between the acyl group and the naphthalene nucleus (Tamaki, 1979). Irradiation leads to decarbonylation. It is known that acyl chlorides quench the fluorescence of aromatic hydrocarbons and that this process leads to acylation of the aromatic hydrocarbon (Tamaki, 1978a). The decarboxylation of anhydrides of phenylacetic acids [171] has been interpreted as shown in (53), involving... 

By comparing the results for chlorinated polypropylene with those for polypropylene, it can be concluded that the two materials undergo very different pyrolytic reactions. Typical for polypropylene is the formation of fragments of the polymeric backbone with formation of monomer, dimer, etc., or with cleavage of the backbone in random places and formation of compounds with 3n, 3n-1, and 3n+1 carbon atoms (see Section 6.1). Pyrolysis of the chlorinated compound leads to a significant amount of HCI and also char. Very few chlorinated compounds are identified in the pyrolysate, since the elimination of HCI leaves very few chlorine atoms bound to carbons. Some aromatic hydrocarbons are formed by a mechanism similar to that of poly(vinyl chloride) pyrolysis. The elimination of HCI leads to the formation of double bonds, and the breaking of the carbon backbone leads to cyclization and formation of aromatic compounds. The reactions involved in this process are shown below for the case of formation of 1,3-dimethylbenzene ... 

In addition to the desired species, the fullerene soot contains a multitude of insoluble components like soot particles or graphitic fragments. Moreover, there may be polycyclic aromatic hydrocarbons also. The separation from insoluble contaminants may be affected by extraction because the fuUerenes, and most of all the small ones, exhibit significant solubility in some organic solvents (refer to Section 2.4.1.1). Toluene is most commonly used for this process. Other possible solvents include chlorinated aromahc species, carbon disulphide, benzene and hexane. As an alternative, the fullerene part of the raw material may be separated by sublimation. In any case a mixture of the generated fuUerenes is obtained which has to be resolved into its constituents. In most cases chromatography is used to this purpose. 

The common mechanism for nitration of aromatic hydrocarbons consists of replacement of ff " by N02 and In this process, oxidation-reduction plays no part. However, there are reactions of nitric acid and its reduction products that do produce nitro compounds and do involve complex oxidations and reductions. Examples are (a) the conversion of alkanes to smaller nltroalkanes via fragmentations, (b) the conversion of alkanes to nltroalkanes,... 

Experimentally, it is uniformly observed that an aromatic hydrocarbon will suffer deprotonation from the benzylic position when irradiated in the presence of an SET sensitizer provided that (1) the sensitizer is a strong enough acceptor in the excited state to cause electron transfer from the substrate (2) the reaction is carried out in a polar solvent which does not interfere which any step of the process (acetonitrile is routinely used, but dimethylsulfoxide, which is competitively oxidized is not suitable the role of nucleophilic or acidic solvents will be discussed later) and (3) the BDE of the radical cation, evaluated as above, is negative or only slightly positive and there are no competing fragmentations with a sufficiently low BDE(RX ), with X sH (see Section 3.2). 

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