Para isomerization

The corresponding para-isomeres are less reactive. That this lack of reactivity is not due to a shorter excited state Hfetime, is demonstrated with 4-nitrovera-trole, in which specifically the m-methoxy group is replaced upon irradiation in the presence of the respective nucleophiles ... 

Im Fall monosubstituierter Benzole entstehen das ortho- und das para-Isomere in ver-gleichbaren Mengen neben einer geringen Menge an meta-Isomeren z.B. ... 

Previously, the most widely used method for preparation of 4-alkyl-benzoyl chlorides on a laboratory scale has been from the benzoic acids obtained by oxidation of aromatic ketones, usually 4-alkylacetophe-nones. 14 The latter are usually prepared by acylating alkylbenzenes. Although this sequence gives high yields, it is lengthy (three completely separate steps) and the scale is restricted in the second step because of the large volumes required. The submitters state that they were unable to repeat the reported alkylation of toluic acid.15 Methods that lead to formation of ortho and para isomeric intermediates are inconvenient since they require that the isomers be separated.16"19... 

Xylene A compound of the general type C6H4(CH3)2, which may exist in the ortho, meta or para isomeric forms. 

The solubility of PNI based on l,3-bis(l,8-dicarboxynaphthoyl)benzene dianhydride is almost identical to that of PNI prepared from 4,4 -bis(l,8-dicarboxynaphthoxy-4)benzophenone dianhydride. Contrary to the DNTA-based PNI [14], para isomeric... 

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. 

As in the case of hydrogen and tritium, deuterium exhibits nuclear spin isomerism (see Magnetic spin resonance) (14). However, the spin of the deuteron [12597-73-8] is 1 instead of S as in the case of hydrogen and tritium. As a consequence, and in contrast to hydrogen, the ortho form of deuterium is more stable than the para form at low temperatures, and at normal temperatures the ratio of ortho- to para-deuterium is 2 1 in contrast to the 3 1 ratio for hydrogen. 

Ortho-Para Tritium. As in the case of molecular hydrogen, molecular tritium exhibits nuclear spin isomerism. The spin of the tritium nucleus is S, the same as that for the hydrogen nucleus, and therefore H2 and T2 obey the same nuclear isomeric statistics (16). Below 5 K, molecular tritium is... 

Examples of first-order reversible reaetions are gas phase eis-trans isomerization, isomerizations in various types of hydroearbon systems, and the raeemization of a and (3 glueoses. An example of a eatalytie reaetion is the ortho-para hydrogen eonversion on a niekel eatalyst. 

A comparison of ortho vs. para direct deactivation by a methoxy group has been made by Karmas and Spoerri in 2,3-dibromo-5,6-dimethyl- and 2,5-dibromo-3,6-dimethyl-pyrazine. The former gives monomethoxy-debromination with one equivalent of methanolic methoxide (65°, 6 hr) and disubstitution via 198 with excess reagent for a longer time (10 hr). In contrast, the isomeric 2,5-dibromo compound gave only monosubstitution, forming 199, under the latter conditions. 

If the para position is not already occupied, this isomerization generally leads to the para isomer. The desired C-azo product can be obtained in one laboratory step. ... 

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. 

Like the isomeric 5,6- and 5,8-quinazolinequinones, ° both ortho (5,6)- and para (5,8)-quinoxalinequinones are stable, but reactive entities that may be made by oxidation of a variety of precursors. 

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