Ortho, meta, para isomers

Dow Chemical. 1978. Acute toxicological properties and industrial handlinghazards of cresol (ortho, meta, para isomers). Unpublished data submitted toEPA/OTS. Fiche no. OTS206146. 

The classical argument concerting the equivalence of the positions on the benzene ring is based on the existence of three (ortho, meta. para) isomers of xylene (dimethylbenzene). How many isomers ore there of dimelhylhorazine ... 

Isomerization of arene oxides to phenols proceeds by complex and often multiple pathways660 668 that show a marked dependence on pH. Arene oxides as key intermediates provide the basis for explaining the ortho/meta/para isomer ratios observed in enzymatic hydroxylations.655a,b For example, the absence of m-cresol in the metabolites of toluene can be attributed to the fact that none of the three possible isomers of toluene oxide rearranges to this product. 

The examples of C6o dissolution in benzene derivatives considered in the present work evidence the clear dependence of C6o solubility on the electron density distribution in the benzene ring. We have identified a priori the electron density with the distribution of ortho-, meta-, para-isomers which form in the reactions of electrophilic substitution of the benzene derivative considered. This identification is evaluated but in some cases, such as in a series of homologs for alkyl derivatives of benzene, the total agreement between the C6o solubility and the amount of ort/io-isomers is observed (Table 2 and Fig. 7). 

AI3-24178-X (USDA) Aminodimethylbenzene Dimethylaminobenzene Dimethylaniline Dimethylphenylamine EINECS 215-091-4 HSDB 6464 UN1711 Xilidine Xylidine Xylidine (mixed isomers) Xylidine isomers Xylidine mixed ortho-meta-para isomers Xylidinen Xylidines. Liquid d = 0.97-0.99 bp = 213-226 slightly soluble In H2O, soluble in EtOH, acids. 

Xanthophyll, all-trans-(+)- 4109 Xylidine mixed ortho-meta-para isomers 4120... 

CD-containing mobile phases in HPLC have been successfully used for the separation of various isomers such as structural isomers, diastereomers and enantiomers. For example, ortho, meta, para isomers of cresol, xylene and aU six isomers of nitrocinnamic acid were separated on the Lichrosorb RP-C18 column with jS-CD solution as mobile phase [41]. Similar results were also obtained for ortho, meta and para isomers of nitrophenol, nitroaniline, fluoronitrobenzene. 

The methylsulfonylation of toluene by reaction with methylsulfonyl chloride in the presence of aluminium chloride catalyst affords methyl tolyl sulfone (52% yield) with an ortho meta para isomer ratio of 53 14 33. Attempts have been made to obtain greater selectivity in sulfonylations a supported Lewis acid catalyst has been recommended for selective sulfonylation, but it did not give good results in the methylsulfonylation of toluene. The best pura-selectivity... 

Compound Reagent Isomers (%) f ortho meta para Ref. 

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. 

Simultaneous inter- and intramolecular reaction have been suggested on the basis153 of the varying yields of the ortho and para isomers and also of the acetyl-ated product of added diphenyl ether as the dilution was changed 5-fold in the reaction of meta-tolyl acetate in the presence of diphenyl ether. In particular it was noted that as the dilution increased the amounts of acetylated diphenyl ether and the para product both decreased whilst that of the ortho product rose very slightly. 

The orbital phase theory can be applied to the thermodynamic stability of the disubstituted benzene isomers. The cyclic orbital interaction in the benzene substituted with two EDGs is shown in Scheme 21. The orbital phase is continuous in the meta isomer and discontinuous in the ortho and para isomers (Scheme 22, cf. Scheme 4). 

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. 

The startling observation made in this work was that meta derivatives were more reactive toward photosubstitution than ortho and para isomers, as shown below for the photoreaction of dimethoxynitrobenzene and methyl amine, in contradistinction to the rules of classical ground state nucleophilic substitution ... 

Poro-xylene is an industrially important petrochemical. It is the precursor chemical for polyester and polyethylene terephthalate. It usually is found in mixtures containing all three isomers of xylene (ortho-, meta-, para-) as well as ethylbenzene. The isomers are very difficult to separate from each other by conventional distillation because the boiling points are very close. Certain zeoHtes or mol sieves can be used to preferentially adsorb one isomer from a mixture. Suitable desorbents exist which have boiling points much higher or lower than the xylene and displace the adsorbed species. The boihng point difference then allows easy recovery of the xylene isomer from the desorbent by distillation. Because of the basic electronic structure of the benzene ring, adsorptive separations can be used to separate the isomers of famihes of substituted aromatics as weU as substituted naphthalenes. 

To elevate p-selectivity in nitration of toluene is another important task. Commercial production of p-nitrotoluene up to now leads with twofold amount to the unwanted o-isomer. This stems from the statistical percentage of o m p nitration (63 3 34). Delaude et al. (1993) enumerate such a relative distribution of the unpaired electron densities in the toluene cation-radical—ipso 1/3, ortho 1/12, meta 1/12, and para 1/3. As seen, the para position is the one favored for nitration by the attack of NO (or NO2 ) radical. A procednre was described (Delande et al. 1993) that used montmorillonite clay supported copper (cupric) nitrate (claycop) in the presence of acetic anhydride (to remove excess humidity) and with carbon tetrachloride as a medinm, at room temperature. Nitrotoluene was isolated almost quantitatively with 23 1 76 ratio of ortho/meta/para mononitrotoluene. 

Another peculiar feature that sparked further study was that in these seemingly nucleophilic photosubstitutions it was the meta-nitro-compounds rather than the ortho- and para-isomers, that showed the most efficient and clean reaction. This behaviour strongly contrasted with what might have been expected on the basis of the classical orientation rules for thermal reactions. 

It was recognized at an early stage that the fact that m-nitrophenyl phosphate and m-nitroanisole give a much cleaner and more efficient reaction than their ortho- and para-isomers, in itself, is not sufficient to conclude that the rate of reactions of the excited meta-compound with the nucleophile is higher. Such a more efficient photoreaction might also be explained by a longer lifetime of the excited metacompound, by a different intersystem crossing efficiency, etc. 

Physical Form. Ortho- and para isomers are solids the meta isomer and isomer mixtures are yellowish liquids... 

Rats survived 8 hours of inhaling air saturated with the vapor. Irritation of the nose and eyes and some deaths were observed in mice exposed to saturated concentrations 1 hour/day for 10 days. Animal experiments have produced varying results with regard to concentrations necessary to produce death. In general, the ortho and para isomers are considered equal in toxicity, with the meta isomer regarded as the least toxic. ... 

Substrate T (°C) Yield (%) Isomer distribution (%) ortho meta para ... 

Hydroxybenzenediazonium tetrafluoroborates constitute a special case the meta-isomer has the structure of a true diazonium salt, while the ortho- and para-isomers are in ready equilibrium with the corresponding quinone-diazide dimers which have been isolated as com-... 

However, there are some contradictory reports on the composition of the products of toluene alkylation or benzene dialkylation at high conversions. In some cases, compositions corresponding to the thermodynamic equilibrium between ortho, meta and para isomers were found, and in other cases, kinetic control of orientation, giving mostly the ortho + para substitution, prevailed. Consecutive isomerisation of the ortho and para isomers to the more stable meta isomer seems to be the cause of the disagreement. More active catalysts gave more meta derivatives than the less active ones [343] and increasing the temperature has the same effect [351]. 

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