Para-meta isomerization

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. 

PCBs are industrial compounds used as industrial, dielectric and heat transfer fluids, organic solvents, flame retardants, plasticizers, sealant and surface coatings. They may also be released to the atmosphere by waste incineration (Fig. 3). The worldwide production of this compound has been 1.3 million tonnes, of which 97% in the northern hemisphere [23]. The amount of chlorine atoms in the biphenyl mixtures is related to the duration and temperature of the chlorination process. The commercial mixtures were distributed under names such as Aroclor (Monsanto, USA) or Clophen (Bayer, EU). The chlorine atoms can substitute the para, meta and/or ortho positions of the biphenyls. There are 209 possible congeners. PCBs can be divided into nine isomeric groups and one decachlorobiphenyl, all with an empirical formula of C Hjo-uCln (n = 1-10) (Fig. 4). 

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. 

Kamijo and Yamamoto developed a palladium-catalyzed three-component synthesis of N-cyanoindoles (131) (Scheme 5.39) [81]. The reaction of a 2-alkynyliso-cyanobenzene (132), allyl methyl carbonate, and trimethylsilylazide in the presence of Pd2(dba)3 (2.5 mol%) and tri(2-furyl)phosphine (10 mol%) at 100 °C afforded the N-cyanoindoles 131 in good yield. Key steps of this reaction involved the formation of jt-allylpalladium complex 133, its Curtius-like rearrangement to intermediate 134, and subsequent isomerization to the Jt-allylpalladium cyanamide complex 135. A wide range of functional groups is tolerated at the para, meta, and even ortho positions of the aromatic ring. 

The reaction of toluene with 1 could conceivably lead to four different trans-isomeric products by attack at the ortho, meta, para, or alpha positions. The carbanion chemistry of toluene would predict attack at the methyl group but, as in other transition metal arene activation studies (8,9,10), this is not found. The NMR spectrum of the toluene adducts displays two singlets attributable to trans-tolyl adducts in an intensity ratio of 1 1.6 in addition to resonances of cis-tolyl adducts. Selective mono-deuteration experiments show that the weaker downfield resonance is attributable to the para isomer while the stronger one is attributable to the meta isomer no ortho isomer is observed. If one corrects for statistical effects, the preference for para, meta, and ortho isomers is 1 0.8 0. We attribute this distribution of isomers primarily to... 

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... 

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. 

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. 

Table 2.4 shows a comparison of the experimental and PPP-MO calculated electronic spectral data for azobenzene and the three isomeric monoamino derivatives. It is noteworthy that the ortho isomer is observed to be most bathochromic, while the para isomer is least bathoch-romic. From a consideration of the principles of the application of the valence-bond approach to colour described in the previous section, it might have been expected that the ortho and para isomers would be most bathochromic with the meta isomer least bathochromic. In contrast, the data contained in Table 2.4 demonstrate that the PPP-MO method is capable of correctly accounting for the relative bathochromicities of the amino isomers. It is clear, at least in this case, that the valence-bond method is inferior to the molecular orbital approach. An explanation for the failure of the valence-bond method to predict the order of bathochromicities of the o-, m- and p-aminoazobenzenes emerges from a consideration of the changes in 7r-electron charge densities on excitation calculated by the PPP-MO method, as illustrated in Figure 2.14. 

A highly substantive bluish red bis(sulphatoethylsulphone) structure (7.73) has been patented recently [29]. In a kinetic study using the t.l.c. double-scanning method, the condensation reactions between the bis(aminochlorotriazine) dye Cl Reactive Red 120 (7.48 X = Cl) and the two isomeric sulphatoethylsulphone anilines 7.38 and 7.39 to yield the two corresponding bis(sulphatoethylsulphone) isomeric dyes were compared. The rate constant of the reaction between Red 120 and the meta isomer 7.39 was about ten times as large as that for the para isomer 7.38 [64]. 

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