Nitration reaction types

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. 

As already mentioned (Section 1), the,prepn of most of the commonly used high expl compds involves one or more nitration reactions. Indeed, except for ammonium nitrate (AN), primary expls, and BkPdr, it is difficult to bring to mind any expl in common use (or even a laboratory curiosity) that was not prepared by nitration. In Table 1, we list the most important military and commercial high expl compds produced by nitration. We have grouped these compds by nitration type, ie C-nitration, O-nitration, and N-nitration. Note that either nitric acid or mixed acid are the nitrating agents principally employed in industry. This will be discussed further in the next section. The Table also gives Encyclopedia references for those compounds already described in previous Encyclopedia volumes... 

The simplest intermediate of the nitrogen cation type is the nitronium ion, the active species in most aromatic nitration reactions. There is both cryoscopic and spectroscopic (Raman and infrared) evidence for its existence.802 On the other hand, it has a structure with quaternary rather than electron deficient nitrogen, a structure compatible with the centrosymmetric geometry demanded by the spectra. The Raman line at 1400 cm.-1 has been assigned to the totally symmetric vibration of the linear triatomic molecule. 

Nitration reactions are mostly substitution type, forming a wide variety of products including nitrobenzene, nitrotoluenes, nitroglycerine, nitrocellulose, trinitrotoluene, nitrophenols and nitroparaffins, many of which are known chemical explosives. Some examples are ... 

The branching ratios shown are estimates based on reference 65. The further oxidation of each of the isomers is similar, so only the first isomer will be examined in detail. In continental areas with at least tens of pptrv s of NO, the peroxy radical produced in the first step will react with NO. This reaction can result in either oxidation of NO to N02 or production of an organic nitrate (bold type indicates organic nitrates that are assumed to be stable) ... 

Reaction intermediates formed in the nitration of a series of a,/i-unsaturatcd esters, such as (47), with N02BF4 have been reported to exhibit the expected behaviour of a-carbonyl cations. Three diagnostic reaction types were observed (1) Ritter reaction (2) cyclopropane formation from propyl cations (3) Wagner-Meerwein migration of alkyl groups. Semi-empirical calculations of the relative gas-phase stabilities of the proposed intermediate cations have also been performed.53... 

With respect to the chemical structure of compounds resulting from nitration processes, three types of nitration reactions are distinguished which may be referred to as ... 

The reaction between different types of radical cation and nitrite ion (E° = 0.7 V in acetonitrile) has been extensively studied (Ristagno and Shine, 1971 Johnson and Dolphin, 1976 Shine et al., 1979 Smith et al., 1979 Eberson and Radner, 1980). Generally, one obtains nitration products from this reaction, and in view of its exergonic nature the mechanism most probably consists of an initial, very fast electron-transfer step, followed by a slower nitration reaction, caused by N02 (N204) from the first step. 

Zeolites offer possibilities for both catalysis and selectivity enhancement in aromatic substitution reactions, where such effects are much needed to overcome the many problems caused by the traditional methods. Two reaction types where superior methodologies are particularly needed, namely selective nitration of simple benzenoids and 2,6-dialkylation of naphthalene, have been addressed in the present study. 

Here according to expectation bromination and nitration will mainly lead to the ortho and para derivatives and, in fact, will be associated with a general activation. The reaction type is indeed correct but there is in fact deactivation because, besides the effect of the resonance (R effect), there is also an electrostatic induction effect (I effect). Actually the strongly electronegative bromine atom will withdraw electrons from the phenyl nucleus. This induction effect decreases with the distance from the key atom and will thus influence the ortho-positions most. 

As we have seen in Table 1.2, practically all formulations used by the military contain TNT, as well as RDX and/or HMX. Additionally, PETN and nitroglycerine (in dynamite) play an important role. All these substances are obtained by nitration reactions. An overview of different types of nitration is given in Table 10.2. 

In numerous chemical reactions, ionic species are formed as intermediates which are subsequently converted into more stable, neutral, reaction products. Examples are provided by numerous well-known substitution reactions such as Friedel-Crafts acylations, nitration reactions, or chlorination reactions. The initial steps of these reactions are completely analogous to reactions of type (3) or (4). 

Possibly one of the earliest accounts in which an ionic liquid structure-induced effect was reported is the reaction of toluene and nitric acid reported by Earle et al. [127], who not only observed an improvement in conversion rate or selectivity, but demonstrated that the reaction type (nitration, halogenation, oxidation) is governed by the choice of the ionic liquid. Thus, the nitration of toluene using 67% nitric acid... 

Predict whether or not a solid is formed when we mix the following identify any solid product by name and identify the reaction type (a) copper(II) nitrate solution and... 

The use of reaction sequences is also common in the analysis of pharmaceuticals. For example, for the purity test for hydrochlorothiazide (HCT) the DAB specifies a nitration reaction followed by reaction with naphthylethylenediamine dihydrochloride, in which the reaction sequence is performed as in the example given below. HCT is a diuretic which is often used in combination with beta-blockers for the treatment of hypertension (high blood pressure). Diuretics of this type are also abused in sport (doping) and appear on the list of banned substances prepared by the IOC (International Olympic Committee). In the quantitative determination of HCT in urine, the red azo dyes formed in the derivatization (parent substance and metabohtes) is regarded as a doping-positive result [111]. The complete specification for this analytical method, known as Application A-43.2, can be obtained from CAM AG. 

The aTctual nitration reactions in a continuous process are carried out in the same type of vessels as those used for batch nitrations, with the exceptions that an overflow pipe or weir arrangement is provided for the continuous withdrawal of products and that continuous feed of all reactants is provided. Schematic diagrams of two typical nitrators for continuous operation are shown in Figs. 4-9 and 4-10. Figure 4-9 shows the kind of nitrator designed for the German Schmid-Meissner system. In this apparatus the material to be nitrated is fed into the top of the nitrator and is... 

The nitration of aromatic compounds with nitric acid in an ionic liquid was shown by Earle et al. [96,97]. It was found that triflate and triflimide ionic liquids catalyze nitration reactions with nitric acid. This methodology has the advantage that water is the only by-product (Scheme 5.2-44). This process could also be carried out in phosphonium ionic liquids [46]. Acidic ionic liquids such as [EMIM][HS04] or the Davis-type ionic liquids [41] could also be used but gave lower reaction rates and selectivities [98]. The effect of metal triflates such as Yb(OTf)3 or Cu(OTf)2 dissolved in [N-butyl-N-methylpyrrolidium][triflimide] was investigated by Handy and Egrie [99]. These gave similar yields and selectivities to ionic liquids systems... 

Sulfonations are a further important type of electrophilic substitution reaction. However, only very few examples can be found in the literature describing the use of microstructured reactors for the strongly exothermic liquid-phase sulfonation of aromatics (sulfonation of toluene wdth gaseous SO3 was described by Jaehnisch et al. [34]). Burns and Ramshaw [25, 35] claimed that their concept of performing liquid/liquid nitration reactions in a slug-flow capillary-microreactor can be also... 

Friedel-Crafts reaction catalysts like anhydrous aluminum chloride are readily soluble in the nitroalkanes. Solutions containing up to 50% aluminum chloride are easily prepared in nitroalkane solvents. These catalytically active complexes, AICI3-RNO2, can be isolated and used in solvents other than the nitroalkane. The reactants in the Friedel-Crafts reaction are often soluble in the nitroalkane reaction medium. Other catalysts like boron trifluoride (BF3), titanium tetrachloride (TiCl4), and stannic tetrachloride (SnCl4) are also soluble in the nitroalkane solvents. Reaction types which use nitroparaffins as solvents include alkylation of aromatics, acetylation of aromatics, halogenations, nitrations, and the reaction of olefins and hydrogen sulfide to yield mercaptans. 

Reaction indices have been frequently used to analyze and predict positional selectivity of S Ar reactions. The average local ionization energy, 7(r), seems to have the best predictive power for this reaction type among the commonly used descriptors. Local minima in /(r) reflect both the positional selectivity for S Ar and the relative reactivity in aromatic and heteroaromatic systems. The 7(r) approach generally has quantitative predictive capacity for nitrations and halogenation, but it has problems for systems where steric hindrance is of key importance for the regioselectivity. 

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