Toluene transfer nitration

It has been mentioned ( 4.4.2) that nitronium tetraffuoroborate reaets with pyridine to give i-nitropyridinium tetraffuoroborate. This compound and several of its derivatives have been used to effect what is called the transfer nitration ofbenzeneandtoluene. i-Nitropyridinium tetraffuoroborate is only sparingly soluble in acetonitrile, but its homologues are quite soluble and ean be used without isolation from the solution in which they are prepared. i-Nitropyridinium tetra-fluoroborate did nitrate toluene in boiling aeetonitrile slowly, but not at 25 In eontrast, i-nitro-2-pieolinium tetraffuoroborate readily... 

The ambiphilic reactivity of aromatic cation radicals, as described in Schemes 12 and 13, is particularly subtle in the charge-transfer nitration of toluene and anisole, which afford uniformly high (>95%) yields of only isomeric nitrotoluenes and nitroanisoles, respectively, without the admixture of other types of aromatic byproducts. Accordingly, let us consider how the variations in the isomeric (ortho meta para) product distributions with... 

Finally, we ask, if the reactive triads in Schemes 1 and 19 are common to both electrophilic and charge-transfer nitration, why is the nucleophilic pathway (k 2) apparently not pertinent to the electrophilic activation of toluene and anisole One obvious answer is that the electrophilic nitration of these less reactive [class (ii)] arenes proceeds via a different mechanism, in which N02 is directly transferred from V-nitropyridinium ion in a single step, without the intermediacy of the reactive triad, since such an activation process relates to the more conventional view of electrophilic aromatic substitution. However, the concerted mechanism for toluene, anisole, mesitylene, t-butylbenzene, etc., does not readily accommodate the three unique facets that relate charge-transfer directly to electrophilic nitration, viz., the lutidine syndrome, the added N02 effect, and the TFA neutralization (of Py). Accordingly, let us return to Schemes 10 and 19, and inquire into the nature of thermal (adiabatic) electron transfer in (87) vis-a-vis the (vertical) charge-transfer in (62). 

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)]. 

Whether toluene metabolism was coupled to nitrate reduction was determined in several ways. Activity on toluene by the enrichment culture could be transferred and sustained in the transferred cultures. As shown in Table III, active enrichment cultures were provided with substrate toluene, with nitrate or with both. In the absence of the electron-acceptor nitrate with... 

Manufacture and Processing. Mononitrotoluenes are produced by the nitration of toluene in a manner similar to that described for nitrobenzene. The presence of the methyl group on the aromatic ring faciUtates the nitration of toluene, as compared to that of benzene, and increases the ease of oxidation which results in undesirable by-products. Thus the nitration of toluene generally is carried out at lower temperatures than the nitration of benzene to minimize oxidative side reactions. Because toluene nitrates at a faster rate than benzene, the milder conditions also reduce the formation of dinitrotoluenes. Toluene is less soluble than benzene in the acid phase, thus vigorous agitation of the reaction mixture is necessary to maximize the interfacial area of the two phases and the mass transfer of the reactants. The rate of a typical industrial nitration can be modeled in terms of a fast reaction taking place in a zone in the aqueous phase adjacent to the interface where the reaction is diffusion controlled. 

One example of normal-phase liquid chromatography coupled to gas chromatography is the determination of alkylated, oxygenated and nitrated polycyclic aromatic compounds (PACs) in urban air particulate extracts (97). Since such extracts are very complex, LC-GC is the best possible separation technique. A quartz microfibre filter retains the particulate material and supercritical fluid extraction (SPE) with CO2 and a toluene modifier extracts the organic components from the dust particles. The final extract is then dissolved in -hexane and analysed by NPLC. The transfer at 100 p.1 min of different fractions to the GC system by an on-column interface enabled many PACs to be detected by an ion-trap detector. A flame ionization detector (PID) and a 350 p.1 loop interface was used to quantify the identified compounds. The experimental conditions employed are shown in Table 13.2. 

Toluene is continuously nitrated to mononitrotoluene in a cast-iron vessel, 1 m diameter, fitted with a propeller agitator 0.3 m diameter rotating at 2.5 Hz. The temperature is maintained at 310 K by circulating 0.5 kg/s cooling water through a stainless steel coil 25 mm o.d. and 22 mm i.d. wound in the form of a helix, 0.80 m in diameter. The conditions are such that the reacting material may be considered to have the same physical properties as 75 per cent sulphuric acid. If the mean water temperatute is 290 K, what is the overall coefficient of heat transfer ... 

Nitration can be catalyzed by lanthanide salts. For example, the nitration of benzene, toluene, and naphthalene by aqueous nitric acid proceeds in good yield in the presence of Yb(03SCF3)3.5 The catalysis presumably results from an oxyphilic interaction of nitrate ion with the cation, which generates or transfers the N02+ ion.6 This catalytic procedure uses a stoichiometric amount of nitric acid and avoids the excess strong acidity associated with conventional nitration conditions. 

Moreover, the thermal nitration of various aromatic substrates with different X-PyNO cations shows the strong rate dependence on the acceptor strength of X-PyNO and the aromatic donor strength. This identifies the influence of the HOMO-LUMO gap in the EDA complexes (see Chart 3), and thus provides electron-transfer activation as the viable mechanistic basis for the aromatic nitration. Indeed, the graphic summary in Fig. 18 for toluene nitration depicts the isomeric composition of o-, m- and p-nitrotoluene to be singularly invariant over a wide range of substrate selectivities (k/kQ based on the benzene... 

Isomeric product distributions. Isomeric product distributions obtained from toluene and anisole have been the subject of considerable mechanistic discussion in electrophilic aromatic nitration (Schofield, 1980 Olah et al., 1989). As applied to nitrations with iV-nitropyridinium ion, the yellow colour of the EDA complex immediately attendant upon the mixing of toluene and PyN02 in acetonitrile persists for about a day (in the dark), whereas the charge-transfer colour of toluene and Me2PyNOj is discharged within 10 min at 25°C. Both bleached solutions afford an identical product mixture (81), consisting of o- (62%), m- (4%) and p-nitrotoluenes (34%)... 

Since the latter conditions pertain to aromatic nitration solely via the homolytic annihilation of the cation radical in Scheme 16, it follows from the isomeric distributions in (81) that the electrophilic nitrations of the less reactive aromatic donors (toluene, mesitylene, anisole, etc.) also proceed via Scheme 19. If so, why do the electrophilic and charge-transfer pathways diverge when the less reactive aromatic donors are treated with other /V-nitropyridinium reagents, particularly those derived from the electron-rich MeOPy and MePy The conundrum is cleanly resolved in Fig. 17, which shows the rate of homolytic annihilation of aromatic cation radicals by NO, (k2) to be singularly insensitive to cation-radical stability, as evaluated by x. By contrast, the rate of nucleophilic annihilation of ArH+- by pyridine (k2) shows a distinctive downward trend decreasing monotonically from toluene cation radical to anthracene cation radical. Indeed, the... 

Feng et al. (1986) performed quantum-chemical calculations of aromatic nitration. The resnlts they obtained were in good accordance with the IPs of N02 and benzene and its derivatives. The radical-pair recombination mechanism is favored for nitration whenever the IP of an aromatic molecule is much less than that of N02. According to calculations, nitration of toluene and xylene with N02 most probably proceeds according to ion-radical mechanism. Nitration of nitrobenzene and benzene derivatives with electron-acceptor substituents can proceed through the classical polar mechanism only. As for benzene, both mechanisms (ion-radical and polar) are possible. Substituents that raise the IP of an aromatic molecule to a value higher than that of N02 prevent the formation of this radical pair (one-electron transfer appears to be forbidden). This forces the classical mechanism to take place. It shonld be nnderlined that a solvent plays the decisive role in nitration. 

Conversion of toluenes to the benzoic acid is also accomplished by anodic oxidation in acetic acid containing some nitric acid. It is not clear if this reaction involves the aromatic radical-cation or if the oxidising agents are nitrogen oxide radicals generated by electron transfer from nitrate ions [66, 67]. Oxidation of 4-fluorotoluene at a lead dioxide anode in dilute sulphuric acid gives 4-fluorobenzoic acid in a reaction which involves loss of a proton from the aromatic radical-cation and them in further oxidation of the benzyl radical formed [68]. 

Potassium stock solution, 1000 pg K+ mh - weigh 1.293 g potassium nitrate (previously dried for 1 h at 105°C and cooled in a desiccator) into a 100-ml beaker. Dissolve in water, add 1 ml hydrochloric acid (approximately 36% m/m HCI) and 1 drop of toluene, then transfer with washings to a 500-ml volumetric flask, make up to the mark and mix well by shaking. 

MO studies of aromatic nitration cast doubt on the existence of jt-complexes and electron-transfer complexes in liquid-phase nitrations.14 The enthalpy of protonation of aromatic substrates provides a very good index of substrate reactivity to nitration. Coulomb interaction between electrophile and substituent can be a special factor influencing regioselectivity. A detailed DFT study of the reaction of toluene with the nitronium ion has been reported.15 Calculated IR spectra for the Wheland intermediates suggest a classical SE2 mechanism. MO calculations of cationic localization energies for the interaction of monosubstituted benzenes with the nitronium ion correlate with observed product yields.16... 

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