Meta compound

I. B. Bersuker, Electronic Structure and Propertie. of Tramition Meta Compound. John Wiley Sons, New York (1996). 

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. 

The fully side-chaia chloriaated products, 1,3-his(trichioromethy1)henzene [881-99-1] and 1,4-his(trichioromethy1)henzene [68-36-0] are manufactured by exhaustive chlorination of meta and para xylenes. For the meta compounds, ring chlorination cannot be completely eliminated ia the early stages of the reactioa. The xyleae hexachlorides are iatermediates ia the manufacture of the xylene hexafluorides and of iso- and terephthaloyl chloride [100-20-9] (see Phthalic acids). 

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. 

The secular equation for the meta compound is obtained from this by omitting the fourth row and column. That for the para compound is ... 

The ordering of the two values is hard to rationalize because there are two opposing influences. Because of its shape, p-dinitrobenzene may be expected to pack better than the m-isomer, thereby increasing its relative enthalpy of fusion. However, the latter has a nonzero dipole moment while the former does not. Intermolecular, electrostatic attraction is expected to be stronger for the crystalline meta compound. 

The radical anions of various phenyldiphosphaalkenes (Scheme 11) were studied by EPR. Their reduction is easier than that of monophosphaalkenes and is dependent on the nature of the isomer. Both EPR spectra and DFT calculations showed that in the radical anion the unpaired electron belongs to a Jt orbital and that its delocalization is dependent on the relative position of the two phos-phaalkene moieties and on the nature of the bridging group. The spin density on the phosphaalkene carbon is higher for the meta compound than for the ortho and para compounds. 

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. 

When the dramatic improvement in activity was found with the 4,6-dimethylpyrimidine heterocycle, sulfonylureas of this system were prepared from the three isomeric tolyl sulfonyl isocyanates. The para compound, which was a literature compound (Ref. 9), was devoid of activity whereas, the meta compound was found to have substantial activity. When the ortho tolyl compound was prepared, another major boost in activity was observed (Figure 10). 

On diluting the methyl alcoholic filtrate with water, 10-20 cc. of oil separates more of this oil is formed by nitrating at higher temperatures, amounting to 57 cc. at 50° and 100 cc. at 70°. The oil consists not merely of methyl o-nitrobenzoate with some of the meta compound but contains also traces of dinitrobenzoic ester and nitrophenolic compounds. Unchanged methyl benzoate does not appear to be present. 

The para-salt obtained here may be converted to the meta-compound by acidifying with nitric acid. 

The mixture consists of about 65—70% ortho-, and about 30% pwa-, with a small percentage of the meta-compound. 

The process is similar to that used for making the meta-compound. In this case, however, p-nitroaniline is added to the mixture of iron powder, water, and acid. The following quantities are used —... 

The meta-orientation of the two methoxyl groups does, however, greatly increase the susceptibility of the aromatic ring to electrophilic attack. This is one of the three possible meta-dimethoxy substituted amphetamines, and it is the best studied one in the pursuit of potential radio-halogen substituted brain blood-flow agents. This strategy is discussed under 1DNNA the other two meta-compounds are discussed under 3,4-DMA. 

Vangronsveld, J., Clijsters, H. (1992), in Meta Compounds in Environment and Life. Interrelation between Chemistry and Biology Merian, E., Haerdi, W. (eds.) Northwood Science and Technology Letters pp. 117—125. 

On the other hand, if the methoxy group is in the meta position, the benzyl alcohol reacts at only about one-third the rate at which benzyl alcohol itself reacts. The tn-methoxybenzyl carbonium ion is not stabilized by resonance forms analogous to those of the para compound. The ease of attaining the carbonium ion is thus greatly diminished, and if the ion is formed in the transition state, the energy of activation should be greater in the meta compound than it is in the para compound. The order of reactivity of the various substituted benzyl alcohols is... 

With substituted benzene rings, an alternative way of identifying the positions of the substituents is to use the terms ortho, meta, and para. Ortho compounds are 1,2-disubstituted, meta compounds are 1,3-disubstituted, and para compounds are 1,4-disubstituted. Some examples should make this clear. 

This is a useful way to make amino ethers with a meta relationship as both groups are ortho, para-directing and so the meta compounds cannot be made by electrophilic substitution. The alternative is the long-winded approach using a diazonium salt that was described in the previous section. 

The orienting power of the meta directing substituents is much smaller throughout than that of the Class 1 substituents. This is the reason that, as a rule, ortho and para isomers are formed as byproducts along with the meta compound., ... 

Blair et al. have also carried out Langmuir film balance measurements on monolayers of the photochromic polyamides mentioned above, namely the 3,3 - and 4,4 -poly-(azodibenzoyl-trans-2,5-monomer unit was observed and interpreted as resulting from azo trans to cis isomerization. The differences between the pressure-area curves of the meta(3,3 -) and para (4,4 -)polyamides were interpreted on the assumption that the meta compound has a helical structure and that the para compound is linear. 

By comparing the above summaries, it is seen that the ortho compound of the earlier chemists is identical with the present meta nitrotoluene, and vice versa. That the early view was incorrect may be further proved by investigating the properies of the meta compounds. In all reactions the meta compounds react differently from the ortho and para compounds. For instance, in the nitration of toluene, the ortho and para nitrotoluenes are formed in large amounts, and the meta in small amounts. Further, when the mono-nitrotoluenes are nitrated to dinitrotoluene, the ortho and para mononitrotoluenes are very easily nitrated further, and the meta difficultly so. In other words, the ortho and para compounds form one part of the class, while the meta stands alone as the other part. With the early nomenclature, the meta and para would be placed together, while the ortho would stand alone. If, now, this view of the matter is accepted. Von Richter s statement becomes entirely correct. 

Ortho-nitrobenzyl chloride was prepared in 1883 by Abelli, together with the meta-compound, by the reaction of concentrated nitric acid on benzyl chloride. It may also be obtained, according to Haussermann and Beek, by the action of chlorine at 130° to 140° C. on a mixture of 2 parts of o-nitrotoluene and i part sulphur. 

The reaction may be carried out in 50 per cent, alcohol in the presence of sodium iodide, except in the case of more reactive chloracetyl derivatives, where condensation takes place in boihng aqueous solution. Of the three arsanilic acids the meta compound condenses the most readily. The derivatives generally have weakly basic and acidic properties. The free arsinie acids do not, as a rule, possess sharp melting or decomposition points, the values varying with the rate of heating. Taken on the whole the acids are not very soluble in the usual solvents. The sodium salts are fairly soluble in water. A list of these compounds is given in the Appendix, Table VII. 

Of the xylenes, the meta compound yields a trinitro derivative more readily than toluene does, but trinitro-m-xylene (TNX) melts somewhat higher than is desirable and is not quite powerful enough when used alone. It has been used in shells in mixtures with TNT and with ammonium nitrate. The other xylenes yield only dinitro derivatives by direct nitration. A mixture of o- and p-xylene may be converted into an explosive—an oily mixture of a large number of isomers, which has been used in the composition of non-freezing dynamites—by chlorinating at an elevated CH,... 

Meta.—In the same way positions 1-3, 1-5, 2-4, 2-6, 3-5, 4-6 are all alike and only one di-substitution product is possible where the substitution is in carbon groups separated from each other by one intervening carbon. A compound of this type is known as a meta compound. 

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