Methyl aromatic contributions

The purpose of the present paper is to offer a contribute to the understanding of the mechanisms of these reactions by using an IR spectroscopic method and well-characterized "monolayer" type vanadia-titania (anatase) as the catalyst. We will focus our paper in particular on the following subjects i) the nature of the activation step of the methyl-aromatic hydrocarbon ii) the mechanism of formation of maleic anhydride as a by-product of o-xylene synthesis iii) the main routes of formation of carbon oxides upon methyl-aromatic oxidation and ammoxidation iv) the nature of the first N-containing intermediates in the ammoxidation routes. 

The second overtone region (1150-1210 nm or 8264-8696 cm ) has also been used for quantitative measurements, in particular to measure methyl, methylene, methine, and aromatic contributions. The methyl groups of long-chain paraffinic hydrocarbons appear between 8365 and 8375 cm (1194-1195nm). In pentane and hexane, the methyl group absorbs at 8396 cm (1191 nm), in heptane it absorbs at 8388 cm (1192 nm), and in decane it is at 8378 cm (1194 nm). See Figure 2.4. 

Table 7 also indicates that the rate enhancements for a 3- and 5-methyl group vary significantly among 1,2-azoles. The difference between the increments in log units for a 3-and 5-methyl group, which should vary directly with bond fixation in the ground state, is larger for isoxazole (1.4) than for pyrazole (0.7) and for isothiazole (0.2). This indicates that the aromaticity increases in the same order and contributes the first quantitative evidence that the 1,2-azoles follow the same aromaticity order as furan < pyrrole < thiophene. 

X-ray analysis of 2-methoxy-4-hydroxy-5//-l-benzazepin-5-one (a benzazatropolone), prepared by methylation of the corresponding 4-hydroxy-l-benzazepin-2,5-dione with Meerwein s reagent, demonstrates the presence of a planar seven-membered ring but, in contrast to tropolone, little 71-electron delocalization.17 Likewise, ll//-dibenz[f>,e]azepin-ll-ones display no significant aromatic character.18 In contrast, 7-chloro-8//-thieno[3,2-c]azepin-8-one (12) has azepine ring hydrogen resonances at 8.7 and 9.02 ppm that indicate a substantial contribution from the polar zwitterionic mesomer 13.19... 

In the production of TNT from the reaction between toluene and mixed acids (nitric/sulfuric), TeNMe forms in amounts between 0.2—0.4% of the total wt of TNT. This TeNMe has been held responsible for several expins which have occurred in TNT plants, causing fatal injuries to personnel and severe damage to facilities. These expins were attributed to the presence of TeNMe in the acid fume lines and the acid storage tanks. Mixts of TeNMe and readily oxidizable materials are known to form very powerful and sensitive expl mixts. Since TeNMe is also isolated from the nitration of Nitrobenzene (NB), the TeNMe formed in the nitration of toluene may arise from the oxidation of the aromatic ring and/or methyl group. In an effort to gain more informa-. tion on the origin of TeNMe from TNT production, radioactive carbon-14 (14C) was used as a tracer to determine the extent to which each of the carbon atoms in the toluene skeleton of the various nitro-substituted isomers contributes to... 

The largest single contribution (54.2%) is from ring carbon-1 attached to the methyl group, and the bulk (94%) of the TeNMe formed is via oxidation of the aromatic ring... 

Electronically related to such borirenes are the salts of the dianion of 1,2-di-tert-butyl-3-[bis(trimethylsilyl)]methyl]-l, 2-diborirane.85 An XRD analysis of the dipotassium salt (86a,b) uncovered shortened B—B (1.58 A) and B—C (1.50 A) bonds, as suggested by resonance contributions 86a and 86b. Salt 86 can be considered to have largely a 2- r-electron Hiickel aromatic ring. 

Heterocyclic systems have played an important role in this historical development. In addition to pyridine and thiophene mentioned earlier, a third heterocyclic system with one heteroatom played a crucial part protonation and methylation of 4//-pyrone were found by J. N. Collie and T. Tickle in 1899 to occur at the exocyclic oxygen atom and not at the oxygen heteroatom, giving a first hint for the jr-electron sextet theory based on the these arguments.36 Therefore, F. Arndt, who proposed in 1924 a mesomeric structure for 4//-pyrone, should also be considered among the pioneers who contributed to the theory of the aromatic sextet.37 These ideas were later refined by Linus Pauling, whose valence bond theory (and the electronegativity, resonance and hybridization concepts) led to results similar to Hiickel s molecular orbital theory.38... 

The aromatic rings contribute to the polycarbonate s high glass transition temperature and stiffness (Table 4.4). The aliphatic groups temper this tendency giving polycarbonate a decent solubility. The two methyl groups also contribute to the stiffness because they take up... 

Table 11 summarizes the main results on the tautomerism of mono-hydroxy-, -mercapto-, -amino- and -methyl-azines and their benzo derivatives, in water. At first sight the equilibrium between 2-hydroxypyridine (71) and pyridin-2-one (72) is one between a benzenoid and a non-benzenoid molecule respectively (71a 72a). However, the pyridinone evidently has a continuous cyclic p- orbital system, containing six it- electrons, the usual aromatic count, if the carbonyl group contributes none. This assumption implies the formula (72b), from which by redistribution of electrons we arrive at (72c), which has the same benzenoid system as (71a). Further canonical forms (71b, 71c) can be drawn of (71) which correspond to the non-benzenoid forms of (72). The elusive property of aromaticity is therefore possessed by both tautomers, although not necessarily by both equally. When the carbonyl oxygen of (72) is replaced by less electronegative atoms, as in the imine tautomers of amino heterocycles, or the methylene tautomers of methyl derivatives, the tendency towards polarization in forms corresponding to (72b) and (72c) is considerably less, and the amino and methyl tautomers are therefore favoured in most instances. 

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