Nitronium Ion NO

Acid—Base Chemistry. Acetic acid dissociates in water, pK = 4.76 at 25°C. It is a mild acid which can be used for analysis of bases too weak to detect in water (26). It readily neutralizes the ordinary hydroxides of the alkaU metals and the alkaline earths to form the corresponding acetates. When the cmde material pyroligneous acid is neutralized with limestone or magnesia the commercial acetate of lime or acetate of magnesia is obtained (7). Acetic acid accepts protons only from the strongest acids such as nitric acid and sulfuric acid. Other acids exhibit very powerful, superacid properties in acetic acid solutions and are thus useful catalysts for esterifications of olefins and alcohols (27). Nitrations conducted in acetic acid solvent are effected because of the formation of the nitronium ion, NO Hexamethylenetetramine [100-97-0] may be nitrated in acetic acid solvent to yield the explosive cycl o trim ethyl en etrin itram in e [121 -82-4] also known as cyclonit or RDX. 

Acid mixtures containing nitric acid and a strong acid, eg, sulfuric acid, perchloric acid, selenic acid, hydrofluoric acid, boron trifluoride, or an ion-exchange resin containing sulfonic acid groups, can be used as the nitrating feedstock for ionic nitrations. These strong acids are catalysts that result in the formation of nitronium ions, NO" 2- Sulfuric acid is almost always used industrially since it is both effective and relatively inexpensive. 

Similar to the alkylation and the chlorination of benzene, the nitration reaction is an electrophilic substitution of a benzene hydrogen (a proton) with a nitronium ion (NO ). The liquid-phase reaction occurs in presence of both concentrated nitric and sulfuric acids at approximately 50°C. Concentrated sulfuric acid has two functions it reacts with nitric acid to form the nitronium ion, and it absorbs the water formed during the reaction, which shifts the equilibrium to the formation of nitrobenzene ... 

In halogenation, halogen atoms are substituted for hydrogen atoms. The process of nitration produces a number of nitro compounds, for example, nitroglycerin. Nitration involves the reaction between benzene and nitric acid in the presence of sulfuric acid. During the reaction, the nitronium ion, NO +, splits off from nitric acid and substitutes on to the benzene ring producing nitrobenzene ... 

This scheme, however, has the disadvantage of ignoring the influence of the nitronium ion (NO ) on the reaction, whereas, as expounded in the chapter on nitration theories (Vol. I), the nitronium ion is of enormous importance for such a reaction. In this connection Lamberton [43] suggests alternative schemes which appear more probable. Scheme (8) leads to the formation of a nitramine and scheme (9) to a salt of nitric acid ... 

The simplest and most commonly used method of preparing nitric esters consists in the O-nitration reaction of alcohols with nitric acid, usually in the presence of sulphuric acid. The reaction is accompanied by reversible hydrolysis, which is typical of esterification reactions. It is very likely that the main nitrating agent is the nitronium ion NO (nitryl cation). It acts through electrophilic substitution. The presence of sulphuric and perchloric acids in the esterifying mixed acid favours the esterification, as it increases the concentration of NO ion. 

The electrophilic nitronium ion (NO ) can be generated from concentrated nitric acid by the action of concentrated sulfuric acid (the classical mixed acid Scheme 4.2) or with acetic acid and/or acetic anhydride. The latter may involve the formation of acetyl nitrate. Some metal nitrates, such as copper(II) nitrate or vanadium(V) oxytrinitrate, have been employed in nitrations, whilst the combination of ozone and nitrogen dioxide has provided a relatively non-acidic methodology, based on the formation of N2O5 and its fission to give NOj NOj. ... 

Mixtures of nitric and sulphuric acids contain nitronium sulphates which have been described by Ingold and associates, Woolf and Emeleus(Vo). I,p. 19). Reval-licr and co-workers [10] have found by Raman spectroscopy and vapour pressure measurements, that compounds made by acting with SOj on nitric acid are salts of nitronium ion (NO ) and sulphate anions. Vitse [11] established the structure of the compound N2 0s,4S03 as nitronium ion salt by X-ray crystallography. The salts of pyrosulphuric acid (Vol. 1, p. 12) can be present only in a mixture of nitric acid with oleum or SO3. They are described in the paragraph on nitronium salts (p. 27). The presence of NO in various solutions was discussed in Vol. I, pp. 14- 49. 

A reference should be made to the paper by Nagakura and Tanaka [11] on the calculated reactivity of the nitronium ion NO " and aromatic hydrocarbons. They suggested the formation ... 

The nitrogen resonance spectra have been reported for the nitronium ion (NO,+) [S6J. 

Thus Surfleet and Wyatt [14] studied the nitration of benzenesulphonic acid in sulphuric acid and found that the addition of hydrogen sulphates of various metals increases the nitration rate. The most marked effect occurred with calcium. and barium hydrogen sulphates. An explanation of the effect was sought in terms of the Bronsted salt-effect theory. It was suggested that the main influence of ionic solutes is in the activity coefficient of the nitrated substance (fg) since the activity coefficients of the other two species, the nitronium ion (/no ) similarly charged transition complex 0 )as a ratio in the Bron-... 

The yield of nitration depends upon the concentrations of the acids, temperature and the length of the nitration process. In the nitration process, a maximum of three nitronium ions (NO -) can be attached to the benzene ring since the nitro group is a meta director. However attaching the third nitro group is very difficult. The existence of the -CH3 or -OH group on a benzene ring makes nitration easier. 

Taking into consideration the results of experiments reported by different authors, fieri, Andress and E. Escales [31] have constructed a chart resembling Sapozhnikov s graph, to depict the relationship between the nitrogen content of nitrocellulose and the ratio of spent acid (Fig. 126). In principle the shape of the curves does not differ from those outlined in Sapozhnikov s work and a resemblance exists between those curves and the curves of nitronium ions (NO ) concentration (Fig. 125). 

Notice that the nitronium ion (NO 2) is linear with an sp hybridized nitrogen at the centre. It is isoelectjronic with CO2. It is also very reactive and combines with benzene in the way we have just describe. Benzene attacks the positively charged nitrogen atom but one of the N=0 bonds must be broken at the same time to avoid five-valent nitrogen. 

The function of the sulphuric acid is to furnish a strongly acid medium and to convert the nitric acid into the highly reactive nitronium ion NO,, which is the real nitrating agent ... 

The N(V) species display a behavior similar to that of N(III), but shifted toward higher acidities. Thus, molecular nitric acid is the predominant species from 60% to about 75% sulfuric acid. At that point nitronium ion (NO g) begins to become important, and at 90% acid, the conversion is essentially complete. (Note that since nitric acid is a considerably stronger acid than nitrous acid, nitrate is seen in 20% sulfuric acid, whereas nitrous acid is present down to below -Hp = 0.)... 

It is now generally accepted that the nitronium ion NO is the main nitrating agent. Although most industrial nitrations are carried out by nitric acid--sulphuric acid mixtures, some compounds can be nitrated with nitric acid alone (production of tetryl, Vol. ill, p, 42). Some products, such as PETN (Vol. II, p. 185) are usually obtained by nitration with nitric acid alone and Cyclonite (if made by nitration) is produced exclusively with nitric acid free of N2O4 (Vol. Ill, p. 87). 

The reduction potential for the nitrate(V)/nitrate(III) couple in acid solution of -1-0.94 V indicates from the limited data in Table 6.12 that nitrate(V) ion in acidic solution is a reasonably good oxidizing agent. However, nitric acid as an oxidant usually functions in a different manner, with the production of brown fumes with a metal e.g. copper) or a metal sulfide (e.g. FeS2). The brown fumes consist of N2O4 (brown gas) and its monomer NOs (colourless gas). Concentrated nitric acid consists of about 70% of the acid in an aqueous solution. In such a solution there is some dissociation of the nitric acid molecules to give the nitronium ion, NO , which represents the primary oxidizing species ... 

In nitration, sulphuric acid acts as an acid catalyst for the formation of a nitronium ion (NO " ) that is generated from nitric acid by a similar mechanism to that used in the generation of sulphur trioxide from sulphuric acid ... 

A d.t.a. study of HNO3-BF3 mixtures revealed the existence of a single compound HNOa,2BF3 (m.p. 42°C). This was confirmed by Raman spectra of the liquid state which indicate the presence of the nitronium ion NO+, i.e. the complex may be formulated as NO [(BF3)aOHI . The structure of the anion was not discussed in this paper. 

Raman spectroscopic [46J and cryoscopic [47 investigations of solutions of N2O4 in sulfuric acid gave proof that the effective nitrating agent in the solutions is the nitronium ion (NO ) and also explained the formation of an equimolar amount ofnitrosyl sulfuric acid (NO HSOT). 

One of the early applications of a superacid medium was the preparation and identification of the nitronium ion NO from nitric acid dissolved in 100% H2SO4. 

In nitration of benzene, the species which brings about nitration giving nitrobenzene is the electrophilic nitronium ion, NO. ... 

The next stage of the reaction can be viewed as a further oxidation to yield a diketone. This stage is initiated by nucleophilic attack on a nitronium ion (NO ) derived from either the nitric or nitrous add. The nudeophile is the enol tautomer of the ketone, and the reaction forms an a-nitrosoketone, w hich is in tautomeric equilibrium with a mono-oxime. This spedes rapidly hydrolyzes under acidic conditions to yield an a-diketone intermediate. This sequence is shown here ... 

In addition to the mechanisms of electrophile formation, another critical consideration relates to electrophile strength. There has been a vast amount of work done to characterize electrophile strengths [19]. Although much of the work relates to chemistry with n-type nucleophiles and nonaromatic tc-nucleophiles, some studies have sought to estimate electrophile strengths in SgAr reactions. Relative electrophile strengths became apparent as the synthetic S Ar reactions were developed. While the nitronium ion (NO ) salts react with benzene under mild conditions, carboxonium ions such as protonated formaldehyde (CH2=OH+) are weaker electrophiles and consequently do not react with benzene. 

Complete the following table by matching the species hydroxylamine (H2NOH), nitrosonixim ion (NO" "), nitronium ion (NO ), nitrite ion (NO ), and nitrate ion (NO ) with the corresponding N-O bond distances in the last column. 

Nitration introduces a nitro group (— NO ) onto an aromatic ring. Electrophilic aromatic substitution requires nitric acid (HNOj), with sulfuric acid as a catalyst. Nitronium ion, (NO ), is the electrophile. It forms in two steps by the reaction of nitric acid with sulfuric acid. 

Similarly, reacting sulfuric acid with potassium nitrate can be used to produce nitric acid and a precipitate of potassium bisulfate. When combined with nitric acid, sulfuric acid acts both as an acid and a dehydrating agent, forming the nitronium ion NO+... 

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