Dinitrogen mechanism

The catalysed reaction was considered to arise from the heterolysis of dinitrogen pentoxide induced by aggregates of molecules of nitric acid, to yield nitronium ions and nitrate ions. The reaction is autocatalytic because water produced in the nitration reacts with the pentoxide to form nitric acid. This explanation of the mechanism is supported by the fact that carbon tetrachloride is not a polar solvent, and in it molecules of nitric acid may form clusters rather than be solvated by the solvent ( 2.2). The observation that increasing the temperature, which will tend to break up the clusters, diminishes the importance of the catalysed reaction relative to that of the uncatalysed one is also consistent with this explanation. The effect of temperature is reminiscent of the corresponding effect on nitration in solutions of nitric acid in carbon tetrachloride ( 3.2) in which, for the same reason, an increase in the temperature decreases the rate. 

Another reason for discussing the mechanism of nitration in these media separately from that in inert organic solvents is that, as indicated above, the nature of the electrophile is not established, and has been the subject of controversy. The cases for the involvement of acetyl nitrate, protonated acetyl nitrate, dinitrogen pentoxide and the nitronium ion have been advocated. 

Because of the chemical similarity between benzoyl nitrate and the acetyl nitrate which is formed in solutions of nitric acid in acetic anhydride, it is tempting to draw analogies between the mechanisms of nitration in such solutions and in solutions of benzoyl nitrate in carbon tetrachloride. Similarities do exist, such as the production by these reagents of higher proportions of o-substituted products from some substrates than are produced by nitronium ions, as already mentioned and further discussed below. Further, in solutions in carbon tetrachloride of acetyl nitrate or benzoyl nitrate, the addition of acetic anhydride and benzoic anhydride respectively reduces the rate of reaction, implying that dinitrogen pentoxide may also be involved in nitration in acetic anhydride. However, for solutions in which acetic anhydride is also the solvent, the analogy should be drawn with caution, for in many ways the conditions are not comparable. Thus, carbon tetrachloride is a non-polar solvent, in which, as has been shown above,... 

In conclusion, it is very likely that the influence of solvents on the change from the heterolytic mechanism of dissociation of the C —N bond in aromatic diazonium ions to homolytic dissociation can be accounted for by a mechanism in which a solvent molecule acts as a nucleophile or an electron donor to the P-nitrogen atom. This process is followed by a one- or a two-step homolytic dissociation to an aryl radical, a solvent radical, and a nitrogen molecule. In this way the unfavorable formation of a dinitrogen radical cation 8.3 as mentioned in Section 8.2, is eliminated. 

Further evidence for this mechanism comes from the fact that a solution 0.305 M in dinitrogen pentoxide and 0.0346 M in anhydride, i.e. equivalent to the conditions above, gave almost the same rate, much slower than that obtained in the absence of the anhydride addition of further amounts of anhydride produced a regular decrease in the rate. 

Recently, nitration of organolithiums and Grignards with N204 has been developed for the preparation of certain kinds of nitro compounds (Eqs. 2.14 and 2.15).31 The success of this process depends on the reaction conditions (low temperature) and the structure of substrates. For example, 3-nitrothiophene can be obtained in 70% overall yield from 3-bromothiophene this is far superior to the older method. 3-Nitroveratrole cannot be prepared usefully by classical electrophilic nitration of veratrole, but it can now be prepared by direct o>7/ o-lithiation followed by low-temperature N204 nitration. The mechanism is believed to proceed by dinitrogen tetroxide oxidation of the anion to a radical, followed by the radical s combination. 

The readsorption and incorporation of reaction products such as 1-alkenes, alcohols, and aldehydes followed by subsequent chain growth is a remarkable property of Fischer-Tropsch (FT) synthesis. Therefore, a large number of co-feeding experiments are discussed in detail in order to contribute to the elucidation of the reaction mechanism. Great interest was focused on co-feeding CH2N2, which on the catalyst surface dissociates to CH2 and dinitrogen. Furthermore, interest was focused on the selectivity of branched hydrocarbons and on the promoter effect of alkali on product distribution. All these effects are discussed in detail on the basis... 

The alternative mechanistic scenario for the protonation and reduction of end-on terminally coordinated N2 through the Schrock cycle is represented by the Chatt cycle which has been developed many years earlier (5). This system is based on Mo(0) and W(0) dinitrogen complexes with phosphine coligands (Fig. 3). As expected, the intermediates of the dinitrogen reduction scheme are very similar to those of the Schrock cycle. Moreover, a cyclic generation of NH3 from N2 has been demonstrated on the basis of this system, however, with very small yields (3,4a). In order to obtain general insight into the mechanism of the Chatt cycle we have studied most of the intermediates of Fig. 3 with... 

The mechanism and sequence of events that control delivery of protons and electrons to the FeMo cofactor during substrate reduction is not well understood in its particulars.8 It is believed that conformational change in MoFe-protein is necessary for electron transfer from the P-cluster to the M center (FeMoco) and that ATP hydrolysis and P release occurring on the Fe-protein drive the process. Hypothetically, P-clusters provide a reservoir of reducing equivalents that are transferred to substrate bound at FeMoco. Electrons are transferred one at a time from Fe-protein but the P-cluster and M center have electron buffering capacity, allowing successive two-electron transfers to, and protonations of, bound substrates.8 Neither component protein will reduce any substrate in the absence of its catalytic partner. Also, apoprotein (with any or all metal-sulfur clusters removed) will not reduce dinitrogen. 

Pell and Armor found entirely different products in alkaline solution. Above pH 8.3, the sole ruthenium product of the reaction of Ru(NH3)g+ with NO was the dinitrogen complex Ru(NH3)5(N2)2+. Under these conditions the rate law proved to be first-order in [Ru(NH3)g+], [NO] and [OH-]. A likely mechanism is the reversible reaction of Ru(NH3)3+ with OH- to give the intermediate Ru(NH3)5(NH2)2+, followed by electrophilic NO attack at the amide ligand and release of water. However, the kinetic evidence does not exclude other sequences. 

Investigations into the mechanism of this reaction revealed several interesting facts (61). Compelling evidence was presented that a discreet Cu nitrenoid was involved in the catalytic cycle. Photolysis of a solution of tosyl azide and styrene in the presence of the catalyst afforded aziridine with the same enantioselectivity as obtained from the PhI=NTs reaction, Eq. 69. Since photolysis of tosyl azide is known to extrude dinitrogen and form the free nitrene, the authors argue that this is indicative of a common Cu-nitrenoid intermediate in this reaction. 

Basically, diazonium ions can lose dinitrogen by three mechanisms ... 

By heterolytic dissociation into carbocations and dinitrogen followed by the addition of a nucleophile (called Dn + An mechanism in the new IUPAC nomenclature103, Sjyl in the former Ingold nomenclature). 

Upon bubbling dinitrogen, two new peak-systems add to such a reduction step a reduction process at the peak-system E/F E° = -2.40 V) and an oxidation process at the peak-system I/G ( °/ = —1.84 V), respectively, Figure 41b. These results have been interpreted as due to the following electrode mechanism ... 

In reality, nitration of naphthalene with dinitrogen tetroxide in an aprotic medium is a complex process. The leading role belongs to nitrosyl cation. This species is a strong oxidant acting according to the outer-sphere mechanism (compare with Section 1.7.10) ... 

Dinitrogen pentoxide reacts with alkanes in carbon tetrachloride at 0 °C via a radical mechanism to give nitration products which can include nitrate esters.Reactions of alkanes with dinitrogen pentoxide in nitric acid are complex and of little synthetic value. 1-Adamantyl nitrate is one of the products obtained from the photochemical irradiation of a solution of adamantane and dinitrogen pentoxide in methylene chloride. ... 

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