Meta isomers

The sulphonation of toluene at 100-120° results in the formation of p-toluene-sulphonic acid as the chief product, accompanied by small amounts of the ortho and meta isomers these are easily removed by crystallisation in the presence of sodium chloride. Sulphonation of naphthalene at about 160° 3uelds largely the p-sulphonic acid at lower temperatures (0-60°) the a-siil-phonic acid is produced almost exclusively. 

I5 C. The figure reported for PhCH2NMea was actually 88% of the meta-isomer. ... 

Give reagents suitable for carrying out each of the following reactions and wnte the major organic products If an ortho para mixture is expected show both If the meta isomer is the expected major product wnte only that isomer... 

Mixtures of HNO, H2SO4, and SO also result in high concentrations of NO/, and toluene can be readily nitrated at —40 to — 10°C as a result (6). At these low temperatures, the formation of the meta-isomer of mononitrotoluene (MNT) is greatiy reduced. Such a reduction is highly desired in the production both of dinitrotoluenes (DNTs) employed to produce intermediates for polyurethane production and of trinitrotoluene (TNT), which is a high explosive. > -MNT results in the production of undesired DNT and TNT isomers (see Nitrobenzene and nitrotoluenes). 

Economic Aspects. U.S. production of chloronitrobenzenes in 1993 was 54,431 metric tons per year of which 19,099 metric tons were the ortho isomer and 35,332 metric tons the para isomer. The meta isomer is not isolated in U.S. production. The bulk, fob prices of o- and / -chloronitrobenzene were 1.72/kg and 2.01/kg, respectively. Chloronitrobenzenes are manufactured by Du Pont and Monsanto Co. 

The separation of the isomers is carried out by a combination of fractional distillation and crystallization. In a fractional vacuum distillation step, the distillate, obtained at a head temperature of 96—97°C at 1.6 kPa (12 mm Hg), is fairly pure o-nitrotoluene and can be purified further by crystallization. The meta isomer is distilled from a mixture of m- and -nitrotoluene and can be purified further by additional distillation and crystallization steps. The bottoms product from the distillation steps is cooled in a crystallizer to obtain nitrotoluene. 

Economic Aspects. Annual 1993 U.S. production of the mononitrotoluenes is 26,000 metric tons, with about 16,120 metric tons of the ortho isomer, 780 metric tons of the meta isomer, and 9,100 metric tons of the para isomer. The prices of (9-, m-., and -nitrotoluene in bulk fob are 1.15/kg, 2.54/kg, and 3.64/kg, respectively. The mononitrotoluenes are manufactured by Du Pont and First Chemical Corp. 

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. 

Polymers from the meta isomer are useful for their heat and flame resistance polymers from the para isomer are noted for high tensile strength and high modulus. 

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. 

Separation of a chemical species from a mixture of similar compounds can also be achieved by melt crystallization, which is, for example, an important means of separatingpara- s.yXen.e (p-xylene) from the ortho and meta isomers. -Xylene is crystallized at the top of a vertical column and crystals are moved downward countercurrentiy to Hquid. The Hquid flowing upward is generated by adding heat to melt the crystals at the bottom of the column a portion of the melt is removed as product and the remainder flows up the column to contact the downward-flowing crystals. Effluent mother Hquor, consisting almost entirely of the ortho and meta isomers of xylene, is removed from the top of the column. 

In the case of nitrobenzene, the electron-withdrawing nitro group is not able to stabilize the positive charge in the complex intermediate. In fact, it strongly destabilizes die intermediate. This destabilization is greatest in the o- and />-intermediates, which place positive charge on the nitrosubstituted caibon. The meta transition state is also destabilized relative to that for benzene, but not as much as the ortho and para transition states. As a result, nitrobenzene is less reactive than benzene, and the product is mainly the meta isomer. 

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. ... 

Crafts alkylations are, of course, well known from the benzene series, where large proportions of the meta isomers are obtained upon alkylation of benzenes with ortho-para directing substituents. H. C. 

Arynes are intermediates in certain reactions of aromatic compounds, especially in some nucleophilic substitution reactions. They are generated by abstraction of atoms or atomic groups from adjacent positions in the nucleus and react as strong electrophiles and as dienophiles in fast addition reactions. An example of a reaction occurring via an aryne is the amination of o-chlorotoluene (1) with potassium amide in liquid ammonia. According to the mechanism given, the intermediate 3-methylbenzyne (2) is first formed and subsequent addition of ammonia to the triple bond yields o-amino-toluene (3) and m-aminotoluene (4). It was found that partial rearrangement of the ortho to the meta isomer actually occurs. 

Eupe and Majewski attempted to determine by experiment the influence of the relative positions of osmophores on each other in the same molecule. In the case of the three methyl tri-azo-benzoates no great difference in the type of odour exists, only a difference in the strengths, the para compound being the strongest and the ortho the weakest. Of the three methoxy-acetophenones, as another example, the meta isomer is almost odourless in comparison with the ortho and para. 

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. 

Ortho- and meta-isomers do not separate very well. Para-isomers elute last. 

The equilibrium concentrations of many disubstituted benzenes (containing alkyl and halogen substituents) show that the meta isomer is in nearly all cases the most thermodynamically stable. It is not obvious why this should be so. Shine182 had discussed this problem in terms of the relative sizes of the standard enthalpy and entropy changes between any pair of isomers. 

Halogenation of diphenyl or methyl phenyl sulphoxides by Cl2 or Br2 affords mainly para-halogeno derivatives, whereas the meta-isomers are formed in low percentages or not at all606,607. In contrast, nitration in concentrated sulphuric acid leads to metasubstitution whose extent increases with acidity of the medium (up to 100%)6°8. 

Eq. 14) [81]. Although this transformation does not appear to be a metathesis reaction, it is thought to proceed via the formation of ruthenium carbene species and not via classical [2+2+2]-cycloaddition pathways. A rationale for the strong preference of the meta isomer 99 was provided on the basis of a metathesis-type mechanism. 

It had been known many years earlier that aromatic nucleophilic substitution occasionally results in substitution at a different position. This is called cine substitution and can be illustrated by the conversion of o-bromoanisole to m-aminoanisole. In this particular case, only the meta isomer is formed. The... 

The thermodynamic product distribution in the Friedel-Crafts methylation (Scheme 20) is in contrast to the kinetic distribution. The reaction kinetically shows the ortho and para orientations. Thermodynamic stabilities of the products prefer the meta isomer as a major product. 

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). 

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