Nitration reaction mechanism

Nitrations are highly exothermic, ie, ca 126 kj/mol (30 kcal/mol). However, the heat of reaction varies with the hydrocarbon that is nitrated. The mechanism of a nitration depends on the reactants and the operating conditions. The reactions usually are either ionic or free-radical. Ionic nitrations are commonly used for aromatics many heterocycHcs hydroxyl compounds, eg, simple alcohols, glycols, glycerol, and cellulose and amines. Nitration of paraffins, cycloparaffins, and olefins frequentiy involves a free-radical reaction. Aromatic compounds and other hydrocarbons sometimes can be nitrated by free-radical reactions, but generally such reactions are less successful. 

Write a plausible reaction mechanism, including appropriate rate expressions, for the toluene nitration example in Section 1.3. 

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. 

Fig. 9.1. Simplified reaction mechanisms in the hydrolytic decomposition of organic nitrates. Pathway a Solvolytic reaction (Reaction a) with formation of a carbonium ion, which subsequently undergoes SN1 addition of a nucleophile (e.g., HO ) (Reaction b) or proton E1 elimination to form an olefin (Reaction c). Pathway b HO -catalyzed hydrolysis (,SN2). Pathway c The bimolecular carbonyl-elimination reaction, as catalyzed by a strong base (e.g., HO or RO ), which forms a carbonyl derivative and nitrite.

There are a number of reaction mechanisms by which nitrates can undergo hydrolytic decomposition, as summarized in simplified form in Fig. 9.1 [23], Pathway a is the unimolecular solvolytic route, which forms nitrate and a carbonium ion (Pathway a, Reaction a). The latter can add a nucleophile,... 

The reactivity of organic nitrites toward hydrolysis differs markedly from that of organic nitrates. The reaction mechanisms are summarized in Fig. 9.2. The base-catalyzed hydrolysis of organic nitrites (Fig. 9.2,a) is quite slow, in... 

As a result of their very low reactivity with the HO anion, the organic nitrates show a practically pH-independent (and very slow) hydrolysis at pH 7 and above [27], In contrast, they will react rapidly with alcohols in nonpolar media by nitrosyl exchange (Fig. 9.2,b) [28]. Formally, H+ and NO+ (nitrosyl cation) are exchanged, but the actual reaction mechanism is quite complex. The reaction leads to an equilibrium, which depends on the partners and presumably also on conditions. However, the biological relevance of this reaction remains to be understood. 

The kinetics and mechanism of iV-nitration reactions have been reviewed and the nitration of alkanes with nitronium hexafluorophosphate in CH2CI2 or EtN02 has been shown to involve direct electrophilic insertion of NO2+ into C-H and C-C a-bonds. ... 

In comparison with gas-phase conditions, the calculated IP of N02 in a solvent with a dielectric strength of 78.5 (as that in water) is reduced significantly. The diminntion is stronger than that of either benzene or tolnene (Feng et al. 1986). This is a consequence of the fact that N02 ion is preferentially stabilized in dielectric media. The nature of the solvent is, therefore, quite important for nitration reaction in the sense of which mechanism is operative. 

The ion-radical mechanism is characteristic in cases of substrates, which are ready for one-electron oxidation and capable to give stable cation-radicals in appropriate solvents. As the cited examples show, such a mechanism can really be revealed. However, very rapid transformations of aromatic cation-radicals can mask the ion-radical nature of many other reactions and create an illusion of their nonradical character. At the same time, the ion-radical mechanism demands its own approaches for farther optimization of commercially important cases of nitration. This mechanism deserves onr continned attention. 

Recently, Kochi et al. described a novel photochemical synthesis for a-nitration of ketones via enol silyl ethers. Despite the already well-known classical methods, this one uses the photochemical excitation of the intermolecular electron-donor-acceptor complexes between enol silyl ethers and tetranitrometh-ane. In addition to high yields of nitration products, the authors also provided new insights into the mechanism on this nitration reaction via time-resolved spectroscopy, thus providing, for instance, an explanation of the disparate behavior of a- and (3-tetralone enol silyl ethers [75], In contrast to the more reactive cross-conjugated a-isomer, the radical cation of (3-tetralone enol silyl ether is stabilized owing to extensive Tr-delocalization (Scheme 50). 

The direct action of nitric acid and its mixtures on the parent alcohol is by far the most important method for the production of nitrate esters on both an industrial and laboratory scale." While such reactions are essentially esterifications they are commonly referred to as 6>-nitrations because the reaction mechanism, involving substitution of hydrogen for a nitro group, is not dissimilar to other nitrations and frequently involves the same nitrating species. 

Blom was the first to demonstrate, in 1928, the formation of HA by an unknown mixture of bacteria which utilized nitrate as their sole nitrogen source to produce ammonia , an observation substantiated by Lindsey and Rhines who generalized this reaction to a diverse set of microorganisms capable of producing NH3 by reduction of both nitrites and nitrates. The mechanism of the 6-electron reduction of nitrite to ammonia (i.e. conversion of the [N + 02] species to by bacterial cytochrome c nitrite reductase... 

A number of studies have highlighted the possibility of the use of monometallic Pd catalyst for effective nitrate/nitrite reduction if a reducible support is selected. Epron et proposed a reaction mechanism for Pd/Ce02, (Fig. 1) in which oxygen vacancies at the Pd-support interface were active sites for the reduction of nitrate. Although the selectivity to nitrogen was poor, the... 

A stepwise reaction mechanism which involves adsorption of nitrate at a bimetallic site, reduction to nitrite, desorption in to the aqueous phase and re-adsorption at a monometallic e.g. Pd) site has been proposed and is supported by theoretical prediction. A reaction scheme based on the use of a bimetallic catalyst is illustrated in Fig. 2. 

In contrast to the facile reduction of aqueous V(III) (—0.26 V versus NHE) [23, 24], coordination of anionic polydentate ligands decreases the reduction potential dramatically. The reduction of the seven-coordinate capped-octahedral [23] [V(EDTA)(H20)] complex = —1.440 V versus Cp2Fe/H20) has been studied extensively [25,26]. The redox reaction shows moderately slow electron-transfer kinetics, but is independent of pH in the range from 5.0 to 9.0, with no follow-up reactions, a feature that reflects the substitutional inertness of both oxidation states. In the presence of nitrate ion, reduction of [V(EDTA) (H20)] results in electrocatalytic regeneration of this V(III) complex. The mechanism was found to consist of two second-order pathways - a major pathway due to oxidation of V(II) by nitrate, and a minor pathway which is second order in nitrate. This mechanism is different from the comproportionation observed during... 

The successful nitration of purines by trifluoroacetyl nitrate was originally presumed to proceed through hetero-lytic dissociation of the reagent into nitroso and trifluoroacetyl radicals, followed by radical attack on the purine <1995TL7875>. However, detailed investigation has shown the reaction mechanism to be a combination of electrophilic and radical steps (see Section 10.11.5.2.3). 

A. S. Morkovnik, the Oxidation-Reduction Stage in the Nitration Reaction, Russ. Chem. Rev. 57,144 (1988). L. Eberson u. F. Radner, Electron-Transfer Mechanisms in Electrophilic Aromatic Nitration, Acc. Chem. 

His work on nitration reactions did much to elucidate the mechanism of these reactions, particularly the nitration of aromatic compounds. He became a fellow of the Royal Society in 1924 was knighted in 1958. He was the recipient of many scientific honors medals Refs.-1) World Who s Who in Science, (A G. Debus, Ed), Marquis, Chicago (1968) 2) C ... 

The intermediate formation of chloramine explains the catalytic action of hydrochloric acid in the nitration of amines, as mentioned above. The following reaction mechanism was drawn up by Wright [41]... 

The transient formation of hexamine in method 4 was confirmed by Winkler, Gillies and Williams [60] (see equation (45)) who examined the reaction mechanism of cyclonite preparation by method 4. They found that hexamine dinitrate is formed at 35°C as an intermediate product. At the same time nitric acid is evolved hence nitrolysis of hexamine dinitrate may occur, in other words the mechanism of cyclo-nite formation would be similar to that of direct nitration of hexamine with nitric acid. 

In certain cases the hydrolysis process does not result in the formation of the initial alcohol at all, but a series of other compounds is obtained, by a reaction mechanism which is often unknown. As an example, the hydrolytic reaction of nitroglycerine performed in an aqueous or alcoholic solution of sodium or potassium hydroxide can be quoted. Oxidation and reduction processes take place simultaneously to form organic acids as well as nitrates and nitrites. After Hay [32] the course of the reaction can be summarized by the eqn. (2) ... 

In the investigation with n-butyl nitrate by Merrow, van Dolah and Cristol [41] already cited, neither the presence of the nitrate ion nor of mercaptan was detected. For this reason the following reaction mechanism was proposed for the hydrolysis of nitric acid alcohol esters by means of hydrosulphide ... 

However, the experiments by Miles [8] do not confirm these thermodynamic calculations, due to our incomplete and faulty knowledge of the mechanism of the nitration reaction. 

Nitrations are highly exothermic, i.e., ca 126 kJ/mol (30 keal/raol) However, the heat of reaction varies with the hydrocarbon that is nitrated. The mechanism of a nitration depends on the reactants and [he... 

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