Aromatic nitrations

Benzene was converted to nitrobenzene in 80% selectivity at 40% conversion by passing benzene and concentrated nitric acid through a reactor containing an H-Y catalyst at 170°C. 3 The liquid phase nitration of benzene has been accomplished by heating a mixture of benzene and n-butyl nitrate at 80°C in the presence of 

Nitration of chlorobenzene with n-butyl nitrate and a Nafion-H catalyst gave only a 15% yield of the chloronitrobenzenes but with acetone cyanohydrin nitrate a 49% yield was obtained with the para isomer produced with 70% selectivity.65 Using an iron exchanged montmorillonite to promote the nitration of chlorobenzene with nitric acid and acetic anhydride gave a 90% yield of the nitro chlorobenzenes in 15 minutes at 80°C. The para isomer was produced in 92% selectivity (Eqn. 22.28).68 

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Certain features of the addition of acetyl nitrate to olefins in acetic anhydride may be relevant to the mechanism of aromatic nitration by this reagent. The rapid reaction results in predominantly cw-addition to yield a mixture of the y -nitro-acetate and y5-nitro-nitrate. The reaction was facilitated by the addition of sulphuric acid, in which case the 3rield of / -nitro-nitrate was reduced, whereas the addition of sodium nitrate favoured the formation of this compound over that of the acetate. As already mentioned ( 5.3. i), a solution of nitric acid (c. i 6 mol 1 ) in acetic anhydride prepared at — 10 °C would yield 95-97 % of the nitric acid by precipitation with urea, whereas from a similar solution prepared at 20-25 °C and cooled rapidly to —10 °C only 30% of the acid could be recovered. The difference between these values was attributed to the formation of acetyl nitrate. A solution prepared at room... 

The authors of this work were concerned chiefly with additions to alkenes, and evidence about the mechanism of aromatic nitration arises by analogy. Certain aspects of their work have been repeated to investigate whether the nitration of aromatic compounds shows the same phenomena ( 5-3-6). It was shown that solutions of acetyl nitrate in acetic anhydride were more powerful nitrating media for anisole and biphenyl than the corresponding solutions of nitric acid in which acetyl nitrate had not been formed furthermore, it appeared that the formation of acetyl nitrate was faster when 95-98% nitric acid was used than when 70 % nitric acid was used. 

Melander first sought for a kinetic isotope effect in aromatic nitration he nitrated tritiobenzene, and several other compounds, in mixed acid and found the tritium to be replaced at the same rate as protium (table 6.1). Whilst the result shows only that the hydrogen is not appreciably loosened in the transition state of the rate-determining step, it is most easily understood in terms of the S 2 mechanism with... 

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). ... 

It is the purpose of this and the following chapter to report the quantitative data concerning the relationship of structure to orientation and reactivity in aromatic nitration. Where data obtained by modern analytical methods are available they are usually quoted in preference to the results of older work. Many of the papers containing the latter are, however, noted in the brief discussion which is given of interpretations of the results. 

Nitration by electrophilic aiomatic substitution is not limited to benzene alone but IS a general reaction of compounds that contain a benzene ring It would be a good idea to write out the answer to the following problem to ensure that you understand the rela tionship of starting materials to products m aromatic nitration before continuing to the next section... 

Nitrobenzene was first synthesized in 1834 by treating benzene with fuming nitric acid (1), and was first produced commercially in England in 1856 (2). The relative ease of aromatic nitration has contributed significantly to the large and varied industrial appHcations of nitrobenzene, other aromatic nitro compounds, and their derivatives. 

K. IhtMoIiPtA, Aromatic nitration, Cambridge University Press, Cambridge, UK, 1980. 

The normal course of a kinetic investigation involves postulating likely mechanisms and comparing the observed rate law with those expected for the various mechanisms. Those mechanisms that are incompatible with the observed kinetics can be eliminated as possibilities. Let us consider aromatic nitration by nitric acid in an inert solvent as a typical example. We will restrict the mechanisms being considered to the three shown below. In an actual case, such arbitrary restriction would not be imposed, but instead all mechanisms compatible with existing information would be considered. 

J. H. Ridd, Acc. Chem. Res. 4 248 (1971) J. H. Ridd, in Studies on Chemical Structure and Reactivity, J. H. Ridd, ed., John Wiley Sons, New Vbik, 1966, Chapter 7 J. G. Hoggett, R. B. Moodie, J. R. Penton, and K. Schofield, Nitration and Aromatic Reactivity, Cambridge University Press, Cambridge, 1971 K. Schofield, Aromatic Nitration, Cambridge University Press, Cambridge, 1980 G. A. Olah, R. Malhotra, and S. C. Narang, Nitration, Methods and Mechanisms, VCH Publishers, New i)rk, 1989. 

A study of the aromatic nitration reaction in aqueous nitric acid revealed that when no aromatic substrate was present, an incorporation of 0 from labeled water into nitric acid occurred. 

TWo types of rate expressions have been found to describe the kinetics of most aromatic nitration reactions. With relatively unreactive substrates, second-order kinetics, first-order in the nitrating reagent and first-order in the aromatic, are observed. This second-order relationship corresponds to rate-limiting attack of the electrophile on the aromatic reactant. With more reactive aromatics, this step can be faster than formation of the active electrq)hile. When formation of the active electrophile is the rate-determining step, the concentration of the aromatic reactant no longer appears in the observed rate expression. Under these conditions, different aromatic substrates undergo nitration at the same rate, corresponding to the rate of formation of the active electrophile. 

A substantial body of data, including reaction kinetics, isotope effects, and structure-reactivity relationships, has permitted a thorough understanding of the steps in aromatic nitration. As anticipated from the general mechanism for electrophilic substitution, there are three distinct steps ... 

One aspect of aromatic nitration that has received attention is the role of charge-transfer and electron-transfer intermediates on the path to the ff-complex intermediate. For... 

Such solutions are extensively used in aromatic nitration reactions in the heavy organic chemicals industry. See also pp. 457-8. 

Scheme 5.1-37 Aromatic nitration reactions in ionic liquids.

R Substituents affect the reactivity of the aromatic ring. Some substituents activate the ring, making it more reactive than benzene, and some deactivate the ring, making it less reactive than benzene. In aromatic nitration, for instance, an -OH substituent makes the ring 1000 times more reactive than benzene, while an —N02 substituent makes the ring more than 10 million times less reactive. 

The appropriate mixed acid compns for the nitration processes that produce militarily and industrially important expls will be described in Sections V VI. Typical MA compns for aromatic nitrations contain 110 to 200% nitric acid over the stoichiometric requirement. For the nitration of toluene to MNT DNT, a typical MA compn in round figures is 30% HN03,60% H2S04, 10% H20. For the nitration of DNT to TNT the MA contains no water and is approx 20% HN03, 80% H2S04... 

Nitrous acid can have both a catalytic and an anticatalytic effect on aromatic nitration, the former being appropriate in dilute (ca. 6 M) nitric acid solutions and the latter appropriate to more concentrated solutions. 

For a review of radical processes in aromatic nitration, see Ridd, J.H. Chem. Soc. Rev., 1991,20,149. For a review of aromatic substitutions involving radical cations, see Kochi, J.K. Adv. Free Radical Chem. (Greenwich, Conn.), 1990, 1, 53. 

Since many dyes contain aromatic amines one of the most important reactions carried out in the synthesis of dye intermediates is aromatic nitration, involving use of stoichiometric amounts of nitric and sulfuric acid as discussed previously. Many cleaner nitration processes have been proposed, but here discussion will be limited to the use of lanthanide... 

Drivers for Performing Aromatic Nitrations in Micro Reactors... 

Beneficial Micro Reactor Properties for Aromatic Nitrations... 

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