Nitration with nitronium ions

NITRATIONS WITH NITRONIUM IONS THE GENERAL CASE 6.2.1 Evidence from solvent effects... 

NITRATIONS WITH NITRONIUM IONS SPECIAL CASES... 

The a-basicity of alkanes obviously is lower than the n-basi-city of lone-pair donor molecules used as solvent systems. This represents a major difficulty in achieving practical aliphatic nitrations with nitronium ions. 

While studying the 0-nitration of alcohols, glycols, and glycerin with excess nitric acid in nitromethane solution, Ingold et al. [102] found the reactions to be of zeroth order and identical in absolute rate with one another. For the nitration of methyl alcohol, low concentration of sulfuric acid increased, whereas nitrate ion decreased the rates. When sufficient water was added, the kinetics changed to first order. Clearly the forraation of the nitronium ion is rate limiting in nitration in the absence of significant amounts of water. O" (and also studied N ) nitrations thus show close similarity to electrophilic aromatic C-nitrations with nitronium ion. 

Parker and Bethell expressed doubt concerning the conclusions that the Wheland intermediates are not involved in aromatic brominations which differ principally from, say nitration with nitronium ion, by the fact that a strong nucleophile (bromide ion) is present under the former conditions and not the latter. It seemed reasonable that computations would not reveal the presence of the positive sigma complex since its formation might be expected to be very slow compared to the very rapid subsequent reactions. It is commonly observed that it is difficult to observe an intermediate of the latter nature by computation if the transition state for its formation is not found. Several transition states were found in the previous study but all of them were depicted as complex processes (see Table 2 in Ref 157) involving the simultaneous making and breaking of more than one bond. 

The Raman spectrum of nitric acid shows two weak bands at 1050 and 1400 cm. By comparison with the spectra of isolated nitronium salts ( 2.3.1), these bonds were attributed to the nitrate and nitronium ion respectively. Solutions of dinitrogen pentoxide in nitric acid show these bands , but not those characteristic of the covalent anhydride , indicating that the self-dehydration of nitric acid does not lead to molecular dinitrogen pentoxide. Later work on the Raman spectrum indicates that at —15 °C the concentrations of nitrate and nitronium ion are 0-37 mol 1 and 0 34 mol 1 , respectively. The infra-red spectrum of nitric acid shows absorption bands characteristic of the nitronium ion. The equivalence of the concentrations of nitronium and nitrate ions argues against the importance of the following equilibrium ... 

Solutions of dinitrogen pentoxide in nitric acid or sulphuric acid exhibit absorptions in the Raman spectrum at 1050 and 1400 cm with intensities proportional to the stoichiometric concentration of dinitrogen pentoxide, showing that in these media the ionization of dinitrogen pentoxide is complete. Concentrated solutions in water (mole fraction of NgOg > 0-5) show some ionization to nitrate and nitronium ion. Dinitrogen pentoxide is not ionized in solutions in carbon tetrachloride, chloroform or nitromethane. ... 

Under the same conditions the even more reactive compounds 1,6-dimethylnaphthalene, phenol, and wt-cresol were nitrated very rapidly by an autocatalytic process [nitrous acid being generated in the way already discussed ( 4.3.3)]. However, by adding urea to the solutions the autocatalytic reaction could be suppressed, and 1,6-dimethyl-naphthalene and phenol were found to be nitrated about 700 times faster than benzene. Again, the barrier of the encounter rate of reaction with nitronium ions was broken, and the occurrence of nitration by the special mechanism, via nitrosation, demonstrated. 

It has already been noted that, as well as alkylbenzenes, a wide range of other aromatic compounds has been nitrated with nitronium salts. In particular the case of nitrobenzene has been examined kinetically. Results are collected in table 4.4. The reaction was kinetically of the first order in the concentration of the aromatic and of the nitronium salt. There is agreement between the results for those cases in which the solvent induces the ionization of nitric acid to nitronium ion, and the corresponding results for solutions of preformed nitronium salts in the same solvent. 

The nitronium ion is the electrophile in nitrations with nitronium salts in organic solvents. 

The identification of a specific nitrating species can be approached by comparing selectivity with that of nitration under conditions known to involve the nitronium ion. Examination of part B of Table 10.7 shows that the position selectivity exhibited by acetyl nitrate toward toluene and ethylbenzene is not dramatically different from that observed with nitronium ion. The data for i-propylbenzene suggest a lower ortho para ratio for acetyl nitrate nitrations. This could indicate a larger steric factor for nitration by acetyl nitrate. 

The nature of the electrophile in this nitrating mixture is still not wholly agreed upon whereas kinetic evidence can be interpreted as consistent with nitration by nitronium ion, the fact that substituents with lone pairs of electrons or it-electrons give markedly different ortho para ratios from other nitrating mixtures is usually conceded to be consistent with the electrophile being something other than the nitronium ion. The balance of evidence at present is in favour of pro-tonated acetyl nitrate being the electrophile. 

Kinetic studies with benzene in acetic anhydride containing 0.4-2 M nitric acid at 25 °C show the reaction to be first-order in benzene and approximately second-order in nitric acid this falls to first-order in nitric acid on addition of sulphuric acid, which also increases the first-order rate coefficient (first-order in benzene) from 4.5 x 10-4 to 6.1 x 10 4. By contrast the addition of as little as 0.001 M sodium nitrate reduced the rate to 0.9 x 10-4 without affecting the kinetic order70. These results were, therefore, interpreted as nitration by nitronium ion via equilibria (21a) and (22). 

Figure 12.3 adapts the general mechanism of electrophilic aromatic substitution to the nitration of benzene. The first step is rate-determining in it benzene reacts with nitronium ion to give the cyclohexadienyl cation intermediate. In the second step, the aromaticity of the ring is restored by loss of a proton from the cyclohexadienyl cation. 

I lG. 5. Simplified energy diagram of the nitration of aromatics with nitronium ion. 

An example of a sequential-reaction extractive reaction is the manufacture of 2,4-dinitrotoluene, an important precursor to 2,4-diaminotoluene and toluene diisocyanate (TDl) polyurethanes. The reaction involves nitration of toluene by using concentrated nitric and sulfuric acids which form a separate phase. Toluene transfers into the acid phase where it reacts with nitronium ion, and the reaction product transfers back into the organic phase. Careful control of liquid-liquid contacting conditions is required to obtain high yield of the desired product and minimize formation of unwanted reaction products. A similar reaction involves nitration of benzene to mononitrobenzene, a precursor to aniline used in the manufacture of many products including methylenediphenylisocyanate (MDI) for polyurethanes [Quadros, Reis, and Baptista, Ind. Eng. Chem. Res., 44(26), pp. 9414-9421 (2005)]. 

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