Nitric acid substitution

Between nitric acid and ammonia, or between nitric acid and free nitrogen, stand the lower oxidation products of nitrogen, viz., nitrous and hyponitrous acids, or the lower oxides of nitrogen, NO2, N2O3, NO and N2O. In the same way nitric acid substitution products are reduced to ammonia substitution products, i.e., nitro compounds are reduced to amino compounds. 

The most widely used reactions are those of electrophilic substitution, and under controlled conditions a maximum of three substituting groups, e.g. -NO2 (in the 1,3,5 positions) can be introduced by a nitric acid/sul-phuric acid mixture. Hot cone, sulphuric acid gives sulphonalion whilst halogens and a Lewis acid catalyst allow, e.g., chlorination or brom-ination. Other methods are required for introducing fluorine and iodine atoms. Benzene undergoes the Friedel-Crafts reaction. ... 

A brief account of aromatic substitution may be usefully given here as it will assist the student in predicting the orientation of disubstituted benzene derivatives produced in the different substitution reactions. For the nitration of nitrobenzene the substance must be heated with a mixture of fuming nitric acid and concentrated sulphuric acid the product is largely ni-dinitrobenzene (about 90 per cent.), accompanied by a little o-dinitrobenzene (about 5 per cent.) which is eliminated in the recrystallisation process. On the other hand phenol can be easily nitrated with dilute nitric acid to yield a mixture of ortho and para nitrophenols. It may be said, therefore, that orientation is meta with the... 

Further evidence that the nitronium ion was not the electrophile in the uncatalysed reaction, and yet became effective in the catalysed reaction, came from differences in the orientation of substitution. The nitration of chlorobenzene in the uncatalysed reaction yielded only 43 % of the para compound, whereas, when the catalysed reaction was made important by adding some nitric acid, the ratio of substitution was that usually observed in nitration involving the nitronium ion ( 5.3.4). In the case of the uncatalysed reaction however, the reaction was complicated by the formation of nitrophenols. 

The catalysed nitration of phenol gives chiefly 0- and />-nitrophenol, (< 0-1% of w-nitrophenol is formed), with small quantities of dinitrated compound and condensed products. The ortho para ratio is very dependent on the conditions of reaction and the concentration of nitrous acid. Thus, in aqueous solution containing sulphuric acid (i 75 mol 1 ) and nitric acid (0-5 mol 1 ), the proportion of oriha-substitution decreases from 73 % to 9 % as the concentration of nitrous acid is varied from o-i mol l i. However, when acetic acid is the solvent the proportion of ortAo-substitution changes from 44 % to 74 % on the introduction of dinitrogen tetroxide (4-5 mol 1 ). 

Nitration using this reagent was first investigated, by Francis. He showed that benzene and some of its homologues bromobenzene, benzonitrile, benzoyl chloride, benzaldehyde and some related compounds, and phenol were mono-nitrated in solutions of benzoyl nitrate in carbon tetrachloride anilines would not react cleanly and a series of naphthols yielded dinitro compounds. Further work on the orientation of substitution associated this reagent with higher proportions of o-substitution than that brought about by nitric acid this point is discussed below ( 5.3.4). 

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,... 

Dewar and his co-workers, as mentioned above, investigated the reactivities of a number of polycyclic aromatic compounds because such compounds could provide data especially suitable for comparison with theoretical predictions ( 7.2.3). This work was extended to include some compounds related to biphenyl. The results were obtained by successively compounding pairs of results from competitive nitrations to obtain a scale of reactivities relative to that of benzene. Because the compounds studied were very reactive, the concentrations of nitric acid used were relatively small, being o-i8 mol 1 in the comparison of benzene with naphthalene, 5 x io mol 1 when naphthalene and anthanthrene were compared, and 3 x io mol 1 in the experiments with diphenylamine and carbazole. The observed partial rate factors are collected in table 5.3. Use of the competitive method in these experiments makes them of little value as sources of information about the mechanisms of the substitutions which occurred this shortcoming is important because in the experiments fuming nitric acid was used, rather than nitric acid free of nitrous acid, and with the most reactive compounds this leads to a... 

This h)rpothesis has, however, been supported. The o p-ratio in chlorobenzene was found to be lower when acetic anhydride was the solvent, than when nitric acid or mixed acids were used. The ratio was still further reduced by the introduction into the solution of an even less polar solvent such as carbon tetrachloride, and was increased by the addition of a polar solvent such as acetonitrile. The orientation of substitution in toluene in which the substituent does not posses a strong dipole was found to be independent of the conditions used. The author... 

In solutions of acetyl nitrate in acetic anhydride, prepared from purified nitric acid, the 0 -ratio increases slightly with increasing concentrations of acetyl nitrate (table 5.7, expts. 11,13,16). The use of fuming nitric acid in the preparation of the acetyl nitrate considerably accelerates the rates of reaction and also increases the proportion of o-substitution (table 5.7, expts. 12, 15, 18). These effects resemble, but are much stronger than the corresponding effects in nitrations with solutions of nitric acid in acetic acid contaimng dinitrogen tetroxide. 

Fluoronaphthalene [321-38-0] is prepared from 1-naphthylamine by the Balz-Schiemaim reaction in 52% yield or by diazotization in anhydrous hydrogen fluoride in 82% yield. Electrophilic substitution occurs at the 4-position, eg, nitration with fuming nitric acid in acetic acid gave 88% yield of l-fluoro-4-nitro-naphthalene [341 -92-4]. 

The anhydride of nitric acid, N20, reacts with 2-methyl aziridine to give the dinitrate (105). In the case of A/-substituted aziridines, the reaction stops at the stage of the nitramine nitrate prior to elimination of N2O (106). 

In the examples, a nitro group is substituted for a hydrogen atom, and water is a by-product. Nitro groups may, however, be substituted for other atoms or groups of atoms. In Victor Meyer reactions which use silver nitrite, the nitro group replaces a hahde atom, eg, I or Br. In a modification of this method, sodium nitrite dissolved in dimethyl formamide or other suitable solvent is used instead of silver nitrite (1). Nitro compounds can also be produced by addition reactions, eg, the reaction of nitric acid or nitrogen dioxide with unsaturated compounds such as olefins or acetylenes. 

In addition to the conventional mixed acids commonly used to produce DNT, a mixture of NO2 and H2SO4 (8), a mixture of NO2 and oxygen (9), and just HNO (10) can also be used. TerephthaUc acid and certain substituted aromatics are more amenable to nitrations using HNO, as compared to those using mixed acids. For compounds that are easily nitratable, acetic acid and acetic anhydride are sometimes added to nitric acid (qv). Acetyl nitrate, which is a nitrating agent, is produced as an intermediate as follows ... 

Nitration of pyrroles by the usual methods leads to extensive degradation. However, nitration can be achieved with an equimolar nitric acid—acetic anhydride mixture at low temperatures. In the case of pyrrole, the reaction leads predominandy to substitution at the -position (34), ie, in the following 51% 3-nitropyrrole [5930-94-9] (21) and 13% 2-nitropyrrole [5919-26-6] (22). 

During certain substitution reactions, the carboxyl group is often replaced by the entering group. An example is fuming nitric acid, which results in the formation of trinitrophenol. Another is the bromination of saUcyhc acid in aqueous solution to yield 2,4,6-tribromophenol [25376-38-9] (eq. 6). 

Nitration of > -hydroxyben2oic acid with filming nitric acid in the presence of sulfuric acid and acetic anhydride gives a mixture of the 2-nitro [602-00-6] and 4-nitro [619-14-7] substitution products. Bromination and iodination yield the 4-halogenated derivatives (4-bromo [14348-38-0] and 4-iodo [58123-77-6]). When > -hydroxyben2oic acid is treated with formalin in the presence of hydrochloric acid, 4-hydroxyphthahde [13161 -32-5] is obtained as shown in equation (10). 

Electrophilic substitution reactions of unsubstituted quinoxaline or phenazine are unusual however, in view of the increased resonance possibilities in the transition states leading to the products one would predict that electrophilic substitution should be more facile than with pyrazine itself (c/. the relationship between pyridine and quinoline). In the case of quinoxaline, electron localization calculations (57JCS2521) indicate the highest electron density at positions 5 and 8 and substitution would be expected to occur at these positions. Nitration is only effected under forcing conditions, e.g. with concentrated nitric acid and oleum at 90 °C for 24 hours a 1.5% yield of 5-nitroquinoxaline (19) is obtained. The major product is 5,6-dinitroquinoxaline (20), formed in 24% yield. 

Isoxazoles are stable toward peracids and are fairly stable to other acidic oxidizing agents such as chromic and nitric acids, and acidic permanganate. 3-Substituted isoxazoles are... 

Nitration of 3-phenyl-1,2-benzisoxazole with fuming nitric acid has been shown to give dinitro products of undetermined substitution pattern (67AHC(8)277, p. 290>. However, more satisfactory studies have now been described, especially on the kinetics and mechanism of nitration of 3-methyl-l,2-benzisoxazole (77JCS(P2)47). Nitration in cold, concentrated mixed acids yields the 5-nitro derivative exclusively, nitration in 80-90% sulfuric acid occurring on the free base whereas at higher acidities the conjugate acid is the species involved in the nitration. 

The chemical resistance of polyethylene is, to a large measure, that expected of an alkane. It is not chemically attacked by non-oxidising acids, alkalis and many aqueous solutions. Nitric acid oxidises the polymer, leading to a rise in power factor and to a deterioration in mechanical properties. As with the simple alkanes, halogens combine with the hydrocarbon by means of substitution mechanisms. 

Nitrates have only been prepared from saturated equatorial 3-hydroxyl groups by reaction with concentrated nitric acid and acetic anhydride at low temperature.Electrophilic substitution at C-6 precludes the satisfactory formation of nitrates from A -3j5-ols. 

The reaction of aldoses with nitric acid leads to the formation of aldaric acids by oxidation of both the aldehyde and the terminal primary alcohol function to carboxylic acid groups. Aldaric acids are also known as saccharic acids and are named by substituting -aric acid for the -ose ending of the conesponding carbohydrate. 

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