Nitration, radicals

Chemistry. Free-radical nitrations consist of rather compHcated nitration and oxidation reactions (31). When nitric acid is used in vapor-phase nitrations, the reaction of equation 5 is the main initiating step where NO2 is a free radical, either -N02 or -ON02. Temperatures of >ca 350° are required to obtain a significant amount of initiation, and equation 5 is the rate-controlling step for the overall reaction. Reactions 6 and 7 are chain-propagating steps. 

An important side reaction in all free-radical nitrations is reaction 10, in which unstable alkyl nitrites are formed (eq. 10). They decompose to form nitric oxide and alkoxy radicals (eq. 11) which form oxygenated compounds and low molecular weight alkyl radicals which can form low molecular weight nitroparaffins by reactions 7 or 9. The oxygenated hydrocarbons often react further to produce even lighter oxygenated products, carbon oxides, and water. 

Amongst these are the studies of Lowen et al (Ref 37), Urbanski et al (Ref 58) and Brennecke Kobe (Ref 46), and in particular those of Titov (Refs 28 65). These studies suggest the necessity for a non-nitronium nitration mechanism and such a mechanism will be examined in the next subsection Free Radical Nitration... 

The concentrations of all these depend on local conditions, the time of day, and both altitude and latitude. Values of ca. 10 molecules/cm for OH, 10 -10 ° molecules/cm for NO3, and ca. 10 molecules/cm for ozone have been reported. Not all of these reactants are equally important, and the rates of reaction with a substrate vary considerably. Reactions with hydroxyl radicals are generally the most important, and some illustrative values are given for the rates of reaction (cmVs/molecule) with hydroxyl radicals, nitrate radicals, and ozone (Atkinson 1990 summary of PAHs by Arey 1998)... 

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... 

Indole will mainly exist in the vapor phase and is degraded in the atmosphere by reaction with photo-chemically produced hydroxyl radicals, nitrate radicals, and ozone with estimated half-lives of 2-3 h, < 1 min, and 6 h, respectively. Indole is expected to readily biodegrade under both aerobic and anaerobic conditions in soil and water. Bioconcentration in aquatic organisms should be low, given an estimated bioconcentration factor value of 25. 

Challenger and Masters (7) presented excellent evidence to support their viewpoint that the increase in G(Cem) induced by nitrate ion was because of the oxidation of nitrate ion by OH radical nitrate ion simultaneously reduced the G-value for isotopic exchange between radioactive cerium (III) and inactive cerium (IV) induced by oxidation of cerium-(III) by OH radical. Although we previously presented (15, 23) excellent evidence that any reaction of OH with nitrate ion could not be the cause of the increase in G(Cem), no alternative mechanism was previously proposed for the isotopic exchange results of Challenger and Masters (7). We now propose that the lower G-value for cerium (III)-cerium(IV) exchange observed in 1.0M nitric acid is caused by the reaction of a significant fraction of the OH and N03 radicals with nitrous acid. 

Radical nitration of a hydrocarbon with cerium ammonium nitrate begins... 

That, indeed, with highly reactive nitrating agents statistical substitution can take place was shown when carrying out free-radical nitration of benzene and toluene, for example, with... 

Table XTV. Free Radical Nitration of Toluene % Niirotoluenes Ortho Meta Para...

Reactions with nitrate ion are similar to those of nitrite ion. That is, 6 " " gave 2-nitrodibenzodioxin (12) and I " gave thianthrene 5-oxide (O. The last reaction was interpreted analogously to eq. 44, but the mechanism of the ring nitration is entirely unknown. Anodic nitrations which appear to be cation radical-nitrate ion reactions can be found in the literature, and their mechanisms are also unsolved. 

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