Meta isomerization

The stable anion-radical in Scheme 3.63 contains two perchlorotriphenylmethyl radical units linked by an all-trani-p-divinylbenzene bridge. At 200 K, the unpaired electron of the anion-radical is localized (within the ESR timescale) on one stilbenelike moiety only. At 300 K, thermal activation forces the nnpaired electron at one strong electrophilic center to move to another one. Such an electron transfer takes place between two eqnivalent redox sites (Bonvoisin et al. 1994). In contrast to this situation, no electron transfer was observed for the anion-radical that contains two perchlorotriphenylmethyl radical units linked by an all-trani -m-divinylbenzene bridge (Rovira et al. 2001). Such results can be ascribed to the localization of frontier orbitals in the meta-isomeric anion-radical because of the meta connectivity of this non-Kekule structure. 

The polyimides prepared with 6F dianhydride (Table II) were exceedingly soluble in the solvents studied. Solubility increased with incorporation of meta or ortho isomerism which serves to create more "kinks and dissymmetry in the polymer chains. The same trend was observed for ODPA-containing films (Table III) except to a lesser degree. Ortho isomerism appeared to have a greater effect on the solubility of ODPA films than did meta isomerism as had been noted previously. ( )... 

Novel f-butyl esters of oxydianiline/pyromellitic dianhydride polyamic acid were prepared in good yield. The polymers were prepared with either meta or para repeating units. The cure behavior of these f-butyl esters was studied by IR, MS and TG analysis and was compared to that of both the parent polyamic acid and its methyl ester. It was found that a rapid deprotection of the f-butyl group occurs at around 200°C with liberation of free polyamic acid. Consequently, the cure behavior at 200 C of the f-butyl ester approaches that of the parent polyamic acid. Furthermore, the isomerism of the repeating units does not appear to have any detectable effect on the cure behavior of the polymer, although, meta isomerism appears to enhance solubility of the polymer in organic solvents. 

On prolonged heating, the complexes decompose [43, 44d] to produce biaryls, chlorinated arenes and platinum(II) derivatives. The process of formation of the CT-tolyl complex of platinum(IV) is accompanied by its para-meta isomerization [43, 44e, 47], If in the initial instant the substitution takes place mainly (to extent of 90%) in the />ara-position in toluene, then the statistical distribution metaipara = 2 1 is gradually attained (Figure VII.4). 

Figure VIL4. The reaction of H2FtCl6 with toluene in CF3COOH-H2O to afford the o-tolyl complex of platinumflV). Accumulation and decomposition of the o-tolyl complex (the yield of a sum of para and meta isomers is given at the bottom) and para-meta isomerization of this complex (I), is the content of paru isomer in the mixture para t- meta).

The activation energies of the formation and para-meta isomerization are ca. 25 kcal mol". ... 

Para-meta isomerization in this case is impossible since the activation energy of the isomerization is ca. 25 kcal mo and its rate at room temperature is too low. 

Comparison of data presented in Table 1.5, demonstrates that the isomeric structure of the polymer unit affects the heat resistance of the polymers. For the identical chemical structures of the corresponding polymer pairs (XXII and XXV, XXni and XXVI, and XXTV and XXVII), PNI containing meta isomeric fragments (see Table 1.5, polymers XXV, XXVI, XXIV, and XXVH) have lower T. ... 

The suggestion outlined above about the way in which through-conjugation influences the nitration of p-chloronitrobenzene is relevant to the observed reactivities (ortho > meta > para) of the isomeric chloronitrobenzenes. Application of the additivity principle to the... 

When the phenyIhydra2one bears a meta-substituent, two isomeric indoles are possible ortho-substituents also frequentiy introduce compHcations. 

If pure isomers are required, the ortho and meta compounds can be prepared by indirect methods. o-Nitrotoluene can be obtained by treating 2,4-dinitrotoluene with ammonium sulfide followed by diazotization and boiling with ethanol. / -Nitrotoluene can be prepared from -toluidine by acetylation, nitration deacetylation, diazotization, and boiling with ethanol. A fairly pure -nitrotoluene, which has been isolated from the isomeric mixture, can be purified further by repeated crystallization. 

Analytical and Test Methods. o-Nitrotoluene can be analyzed for purity and isomer content by infrared spectroscopy with an accuracy of about 1%. -Nitrotoluene content can be estimated by the decomposition of the isomeric toluene diazonium chlorides because the ortho and meta isomers decompose more readily than the para isomer. A colorimetric method for determining the content of the various isomers is based on the color which forms when the mononitrotoluenes are dissolved in sulfuric acid (45). From the absorption of the sulfuric acid solution at 436 and 305 nm, the ortho and para isomer content can be deterrnined, and the meta isomer can be obtained by difference. However, this and other colorimetric methods are subject to possible interferences from other aromatic nitro compounds. A titrimetric method, based on the reduction of the nitro group with titanium(III) sulfate or chloride, can be used to determine mononitrotoluenes (32). Chromatographic methods, eg, gas chromatography or high pressure Hquid chromatography, are well suited for the deterrnination of mononitrotoluenes as well as its individual isomers. Freezing points are used commonly as indicators of purity of the various isomers. 

Ethyltoluene is manufactured by aluminum chloride-cataly2ed alkylation similar to that used for ethylbenzene production. All three isomers are formed. A typical analysis of the reactor effluent is shown in Table 9. After the unconverted toluene and light by-products are removed, the mixture of ethyltoluene isomers and polyethyltoluenes is fractionated to recover the meta and para isomers (bp 161.3 and 162.0°C, respectively) as the overhead product, which typically contains 0.2% or less ortho isomer (bp 165.1°C). This isomer separation is difficult but essential because (9-ethyltoluene undergoes ring closure to form indan and indene in the subsequent dehydrogenation process. These compounds are even more difficult to remove from vinyltoluene, and their presence in the monomer results in inferior polymers. The o-ethyltoluene and polyethyltoluenes are recovered and recycled to the reactor for isomerization and transalkylation to produce more ethyltoluenes. Fina uses a zeoHte-catalyzed vapor-phase alkylation process to produce ethyltoluenes. 

Mass transport selectivity is Ulustrated by a process for disproportionation of toluene catalyzed by HZSM-5 (86). The desired product is -xylene the other isomers are less valuable. The ortho and meta isomers are bulkier than the para isomer and diffuse less readily in the zeoHte pores. This transport restriction favors their conversion to the desired product in the catalyst pores the desired para isomer is formed in excess of the equUibrium concentration. Xylene isomerization is another reaction catalyzed by HZSM-5, and the catalyst is preferred because of restricted transition state selectivity (86). An undesired side reaction, the xylene disproportionation to give toluene and trimethylbenzenes, is suppressed because it is bimolecular and the bulky transition state caimot readily form. 

Aromatic Ring Reactions. In the presence of an iodine catalyst chlorination of benzyl chloride yields a mixture consisting mostly of the ortho and para compounds. With strong Lewis acid catalysts such as ferric chloride, chlorination is accompanied by self-condensation. Nitration of benzyl chloride with nitric acid in acetic anhydride gives an isomeric mixture containing about 33% ortho, 15% meta, and 52% para isomers (27) with benzal chloride, a mixture containing 23% ortho, 34% meta, and 43% para nitrobenzal chlorides is obtained. 

A good deal of experimental care is often required to ensure that the product mixture at the end of a Friedel-Crafts reaction is determined by kinetic control. The strong Lewis acid catalysts can catalyze the isomerization of alkylbenzenes, and if isomerization takes place, the product composition is not informative about the position selectivity of electrophilic attack. Isomerization increases the amount of the meta isomer in the case of dialkylbenzenes, because this isomer is thermodynamically the most stable. ... 

Cohn points out that position isomerism is of the greatest importance as regards the odours of isomerides, this is strikingly instanced in the case of the tri-nitro tertiary butyl xylenes since the only one possessing the powerful musk odour is that in which the nitro groups are situated each in the meta position to the two others again the ortho-amido-benzaldehyde has a strong odour but the meta and para isomerides are odourless. 

The substitution reaction of toluene with Br2 can, in principle, lead to the formation of three isomeric bromotoluene products. In practice, however, only o- and jp-bromotoluene are formed in substantial amounts. The meta isomer is not formed. Draw the structures of the three possible carbocation intermediates (Problem 15.48), and explain why ortho and para products predominate over meta. 

Aromatic hydrocarbons substituted by alkyl groups other than methyl are notorious for their tendency to disproportionate in Friedel-Crafts reactions. This tendency has previously limited the application of the isomerization of para- or ortho-) m ky -benzenes to the corresponding meta compounds. At the lower temperature of the present modification, disproportionation can be minimized. 

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