Nitrosonium ions nitration

Ridd - has reinterpreted the results concerning the anticatalysis of the first-order nitration of nitrobenzene in pure and in partly aqueous nitric acid brought about by the addition of dinitrogen tetroxide. In these media this solute is almost fully ionised to nitrosonium ion and nitrate ion. The latter is responsible for the anticatalysis, because it reduces the concentration of nitronium ion formed in the following equilibrium ... 

Nitrous acid or nitrite salts may be used to catalyze the nitration of easily nitratable aromatic hydrocarbons, eg, phenol or phenoHc ethers. It has been suggested that a nitrosonium ion (NO + ) attacks the aromatic, resulting initially in the formation of a nitro so aromatic compound (13). Oxidation of the nitro so aromatic then occurs ... 

The kinetics of aromatic nitrosation at ring carbon have received little attention. The first attempt to determine the nature of the electrophile was made by Ingold et a/.117, who measured the rates of the nitrous acid-catalysed nitration of 4-chloroanisole by nitric acid in acetic acid which proceeds via initial nitrosation of the aromatic ring. Assuming that the electrophiles are the nitrosonium ion and... 

A further point of preparative significance still requires explanation, however. Highly reactive aromatic compounds, such as phenol, are found to undergo ready nitration even in dilute nitric acid, and at a far more rapid rate than can be explained on the basis of the concentration of N02 that is present in the mixture. This has been shown to be due to the presence of nitrous acid in the system which nitrosates the reactive nucleus via the nitrosonium ion, NO (or other species capable of effecting nitrosation, cf. p. 120) ... 

A facile method for the oxidation of alcohols to carbonyl compounds has been reported by Varma et al. using montmorillonite K 10 clay-supported iron(III) nitrate (clayfen) under solvent-free conditions [100], This MW-expedited reaction presumably proceeds via the intermediacy of nitrosonium ions. Interestingly, no carboxylic acids are formed in the oxidation of primary alcohols. The simple solvent-free experimental procedure involves mixing of neat substrates with clayfen and a brief exposure of the reaction mixture to irradiation in a MW oven for 15-60 s. This rapid, ma-nipulatively simple, inexpensive and selective procedure avoids the use of excess solvents and toxic oxidants (Scheme 6.30) [100]. Solid state use of clayfen has afforded higher yields and the amounts used are half of that used by Laszlo et al. [17,19]. 

Nitrous acid catalysis also takes place in the nitration of such compounds (naphthalene) that are unable to undergo nitrosation on the given conditions or whose nitrosation proceeds slower than nitration. As accepted, the nitrosonium ion is formed from HNOj in acid media. The nitrosonium ion oxidizes an aromatic substrate into a cation-radical and transforms into nitric oxide. The latter reduces nitronium cation to nitrogen dioxide that gives a a-complex with the aromatic cation-radical ... 

A solution of dinitrogen tetroxide in sulfuric acid is also a powerful nitrating agent. In this medium dinitrogen tetroxide is ionized to nitronium and nitrosonium ions. Titov reported using a solution of dinitrogen tetroxide in oleum for the nitration of nitrotoluene to dinitrotoluene and then to trinitrotoluene, the two separate steps proceeding in 98 % and 85 % yields respectively. 

The unique hydride abstraction property has been gainfully employed in developing novel synthetic reactions.530 Reactive hydrocarbons such as triphenylmethane, adamantane, and diamantane are readily fluorinated in the presence of nitrosonium ion in HF-pyridine media.537 In the presence of a suitable oxygen donor such as dimethyl sulfoxide, the nitrosonium ion can act as a nitrating agent538 [Eq. (4.152)]. The initially formed nitrito onium ion 223 transfer nitrates aromatics rather readily.245 The NO+-induced reactions are further reviewed in Chapter 5. 

Similarly, NO+ is also capable of halogen abstraction from alkyl halides.513 514 In the presence of a suitable oxygen donor such as dimethylsulfoxide, nitrosonium ion can act as a nitrating agent493,494 [Eq. (5.194)]. 

Phenol may be converted into a mixture of o- and p-nitrophenols (Expt 6.102) by reaction with dilute nitric acid the yield of p-nitrophenol is increased if a mixture of sodium nitrate and dilute sulphuric acid is employed. Upon steam distillation of the mixture of nitrophenols, the ortho isomer passes over in a substantially pure form the para isomer remains in the distillation flask, and can be readily isolated by extraction with hot 2 per cent hydrochloric acid. The mechanism of the substitution probably involves an electrophilic attack (cf. Section 6.2.1, p. 851) by a nitrosonium ion at a position either ortho or para to the activating hydroxyl group, to yield a mixture of o- and p-nitrosophenols, which are then oxidised by the nitric acid to the corresponding nitrophenols. The reaction depends upon the presence in the nitric acid of traces of nitrous acid which serve as the source of the nitrosonium ion. 

According to more recent views, nitrogen dioxide in sulphuric acid solution gives the nitrosonium ion NO+ Mid a nitric acid molecule. The latter, treated with an excess of sulphuric acid, gives a nitronium ion, which is the actual nitrating agent ... 

Ingold and his co-workers [152] when investigating the nitration of phenols and their ethers, came to the conclusion that it was the nitrosonium ion, NO+, formed as a result of the hypothetical reactions (a) and (b) ... 

Moreover, the facile bimolecular reactions of the cationic donor RH and/or the anicmic acceptor A, especially with additives diat are present during oxidation, can accomplish the same displacement of the redox equilibria in measure with the competition from back electron transfer. For example, the arene activation with nitrosonium ion merely reaches a low steady-state concentration of the radical pair, which persists indefinitely in equation (13). However, oxygen r idly tnq>s even small amounts of nitric oxide to render back electron transfer ineffective, and successfully effects aromatic nitration (Scheme 3). ... 

Alternatively, the reaction may proceed through nitrosation by the nitrosonium ion, NO", and subsequent oxidation of the nitroso compound by nitric acid (Scheme 7.3). There is only a small concentration of NO" in dilute nitric acid and so catalytic amounts of sodium nitrite are sometimes added to increase the quantity. This technique is a useful means of effecting smooth, low-temperature nitrations. 

More recently the possible interaction of peroxynitrite with excess NO may cause the production of the nitrosonium ion, as peroxynitrite can cause one electron oxidations, which may then go on to form nitrosothiols by a nitrosation reaction [85]. Peroxynitrite itself is apparently not capable of this nitrosation reaction but in the presence of metal catalysts it can act as a nitrating agent [86]. 

H2O—NO or NOCl) that can easily transfer to the ring. The nitrosonium ion is very weakly electrophilic compared with the reagents involved in nitration, sulfonation, halogenation, and the Friedel-Crafts reaction nitrosation ordinarily occurs only in rings bearing the powerfully activating dialkylamino (—NR2) or hydroxy (—OH) group. 

The scope of the Ritter reaction can also be extended by avoiding strong acid as the carbenium ion initiator. In at least one case it is possible to omit this reagent entirely because of the facility of 5 1 reaction. Thus, reflux of r-butyl bromide in acetonitrile or propionitrile for 24 h led to formation of A -f-butylamide products. Less commonly used initiators have included cation exchange resins and clayfen. In this latter case, the clay-supported iron(III) nitrate is believed to give rise to nitrosonium ions, which react with the alkyl halide substrate to produce a carbenium ion. Yields of amide from this technique are generally modest (21-35%). 

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