Nitration in concentrated solutions of sulphuric acid

1 The state of nitric acid in g8-ioo% sulphuric acid In this section the pioneering work of Hantzsch will several times he mentioned. That later techniques made it necessary to modify his conclusions should not be allowed to obscure the great originality of his approach since investigations using these media provided the most compelling evidence for the existence of the nitronium ion. 

The two absorption bands, at 1050 and 1400 cm , which appear in the Raman spectra of solutions of nitric acid in concentrated sulphuric acid are not attributable to either of the acid molecules. In oleum the lower band appears at 1075-1095 cm. That these bands seemed to correspond to those in the spectra of anhydrous nitric acid and solid dinitrogen pentoxide caused some confusion in the assignment of the spectrum. The situation was resolved by examining the Raman spectra of solutions of nitric acid in perchloric or selenic acids , in which the strong absorption at 1400 cm is not accompanied by absorption at about 1050 cm . Thus, the band at 1400 cm arises from the nitronium ion, and the band at about 1050 cm can be attributed in the cases of nitric acid and solid dinitrogen pentoxide to the nitrate ion formed according to the following schemes  

The most recent work indicates that in anhydrous sulphuric acid the above conversion is complete. The slightly modified absorption band in oleum arises from the hydrogen pyrosulphate ion formed in the following way  

Raman spectroscopy provides the easiest way of estimating the concentration of nitronium ions in different media ( 2.4.1). The concentration, determined by infra-red spectroscopy, of nitronium ions in nitric acid was increased markedly by the addition of sulphuric acid.  

The conversion of nitric acid into another species in concentrated sulphuric acid was shown by the fact that, whereas the ultraviolet 

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Nitration in concentrated solutions of sulphuric acid Concentrated solutions are here considered to be those containing > c. 89% by weight of sulphuric acid. In these solutions nitric acid is completely ionised to the nitronium ion. This fact, and the notion that the nitronium ion is the most powerful electrophilic nitrating species,29 makes operation of this species in these media seem probable. Evidence on this point comes from the effect on the rate of added water ( 2.4.2)... 

NITRATION IN AQUEOUS SOLUTIONS OF MINERAL ACIDS 2.4.1 The state of nitric acid in aqueous sulphuric acid Nitric acid is completely converted into nitronium ions in concentrated sulphuric acid ( 2.3.1) ... 

The more strongly acidic a solution of nitric acid at a given concentration is in a particular organic solvent, the more readily should that solvent support zeroth-order nitration. The values of for solutions of sulphuric acid in nitromethane, sulpholan, and acetic acid show clearly the superiority of nitromethane in this respect. ... 

Nitration in the presence of strong acids or Lewis acids Solutions of dinitrogen pentoxide in sulphuric acid nitrate 1,3-dimethyl-benzene-4,6-disulphonic acid twice as fast as a solution of the same molar concentration of nitric acid. This is consistent with Raman spectroscopic and cryoscopic data, which establish the following ionisation ... 

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 ). 

Another circumstance which could change the most commonly observed characteristics of the two-stage process of substitution has already been mentioned it is that in which the step in which the proton is lost is retarded because of a low concentration of base. Such an effect has not been observed in aromatic nitration ( 6.2.2), but it is interesting to note that it occurs in A -nitration. The A -nitration of A -methyl-2,4,6-trinitroaniline does not show a deuterium isotope effect in dilute sulphuric acid but does so in more concentrated solutions (> 60 % sulphuric acid kjj/kjj = 4 8). ... 

Arsenic pentasulphide may also be prepared from aqueous arsenic acid or a solution of an arsenate by the action of hydrogen sulphide, but the nature of the product depends upon conditions. Berzelius reported 2 the formation of the pentasulphide when the gas acted on a moderately concentrated solution of arsenic acid, but Wackenroder stated 3 that the arsenic acid was first reduced by hydrogen sulphide to arsenious acid, even in the presence of hydrochloric acid, and that a mixture of arsenic trisulphide and sulphur was then precipitated. Rose,4 after passing hydrogen sulphide into a solution of arsenic acid, heated the solution and filtered off the precipitate, and then by the addition of silver nitrate showed that both arsenious and arsenic acids were present in the filtrate. It was therefore accepted that reduction takes place and the reaction was represented thus ... 

Cupric nitrite.—The nitrite is only known in solution, prepared by addition of lead nitrite to cupric-sulphate solution.5 Exposure of its dilute solution to air causes slow formation of nitrate. On evaporation of a concentrated solution over sulphuric acid, there is partial decomposition in accordance with the equation 6... 

The Rev. N. J. Callan, professor of natural philosophy at Maynooth College, Ireland, replaced platinum by platinised lead in a solution of nitric acid, potassium nitrate, and sulphuric acid, or by iron (which becomes passive) in concentrated nitric acid. Callan set up a battery of 800 cells and constructed a large horse-shoe electromagnet some of this apparatus is still in Maynooth College. 

The state of aqueous solutions of nitric acid In strongly acidic solutions water is a weaker base than its behaviour in dilute solutions would predict, for it is almost unprotonated in concentrated nitric acid, and only partially protonated in concentrated sulphuric acid. The addition of water to nitric acid affects the equilibrium leading to the formation of the nitronium and nitrate ions ( 2.2.1). The intensity of the peak in the Raman spectrum associated with the nitronium ion decreases with the progressive addition of water, and the peak is absent from the spectrum of solutions containing more than about 5% of water a similar effect has been observed in the infra-red spectrum. ... 

Potassium nitrate anticatalysed nitration in nitric acid (the solutions used also contained 2-5 mol 1 of water) but the effect was small in comparison with the corresponding effect in nitration in organic solvents ( 3.2.3 4), for the rate was only halved by the addition of 0-31 mol 1 of the salt. As in the case of the addition of sulphuric acid, the effect was not linear in the concentration of the additive, and the variation of k j with [KNOgj/mol 1 " was similar to that of with [H2SO4]/ mol 1. ... 

The ultraviolet spectra of solutions of potassium nitrate in various concentrations of sulphuric acid have been studied, and absorptions... 

Addition of water to solutions of nitric acid in 90% sulphuric acid reduces rates of nitration. Between 90% and 85% sulphuric acid the decrease in rate parallels the accompanying fall in the concentration of nitronium ions. This is good evidence for the operation of the nitronium ion as the nitrating agent, both in solutions more acidic than 90% and in weakly diluted solutions in which nitronium ion is still spectroscopically detectable. 

About 2 grams of benzaldehyde are heated in a retort with 40 c.c. of concentrated sulphuric acid, the fumes collected in a solution of silver nitrate, and the heating continued until no further precipitate insoluble in hot dilute nitric acid is obtained in the silver nitrate solution. This takes about three hours. 

The same reaction can be applied, not only to the aromatic parent substances, the hydrocarbons, but also to all their derivatives, such as phenols, amines, aldehydes, acids, and so on. The nitration does not, however, always proceed with the same ease, and therefore the most favourable experimental conditions must be determined for each substance. If a substance is very easily nitrated it may be done with nitric acid sufficiently diluted with water, or else the substance to be nitrated is dissolved in a resistant solvent and is then treated with nitric acid. Glacial acetic acid is frequently used as the solvent. Substances which are less easily nitrated are dissolved in concentrated or fuming nitric acid. If the nitration proceeds with difficulty the elimination of water is facilitated by the addition of concentrated sulphuric acid to ordinary or fuming nitric acid. When nitration is carried out in sulphuric acid solution, potassium or sodium nitrate is sometimes used instead of nitric acid. The methods of nitration described may be still further modified in two ways 1, the temperature or, 2, the amount of nitric acid used, may be varied. Thus nitration can be carried out at the temperature of a freezing mixture, at that of ice, at that of cold water, at a gentle heat, or, finally, at the boiling point. Moreover, we can either employ an excess of nitric acid or the theoretical amount. Small scale preliminary experiments will indicate which of these numerous modifications may be expected to yield the best results. Since nitro-compounds are usually insoluble or sparingly soluble in water they can be precipitated from the nitration mixture by dilution with water. 

Crystallisation was one of the earliest methods used for separation of radioactive microcomponents from a mass of inert material. Uranium X, a thorium isotope, is readily concentrated in good yield in the mother liquors of crystallisation of uranyl nitrate (11), (33), (108). A similar method has been used to separate sulphur-35 [produced by the (n, p) reaction on chlorine-35] from pile irradiated sodium ot potassium chloride (54), (133). Advantage is taken of the low solubility of the target materials in concentrated ice-cold hydrochloric acid, when the sulphur-35 as sulphate remains in the mother-liquors. Subsequent purification of the sulphur-35 from small amounts of phosphorus-32 produced by the (n, a) reaction on the chlorine is, of course, required. Other examples are the precipitation of barium chloride containing barium-1 from concentrated hydrochloric acid solution, leaving the daughter product, carrier-free caesium-131, in solution (21) and a similar separation of calcium-45 from added barium carrier has been used (60). 

Smith carried out his experiments by means of the following technique nitrocotton removed from the nitrating add was washed in running water until the water was no longer acid. The total acid content (by titrating its acetone solution), and the concentration of sulphuric acid esters were determined in the products. The results are shown in Table 63. 

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