Para isomer

The high boiling point residue contains p- (b.p. 173°, m.p. 53°) and o-dichloro-benzene (b.p. 179°), which may be separated, upon cooling in ice, the moderately pure solid para isomer separate out. 

Phenol may be nitrated with dilute nitric acid to 3deld a mixture of o- and nitrophenols the 3deld of p-nitrophenol is increased if a mixture of sodium nitiute and dilute sulphuric acid is employed. Upon steam distilling the mixture, the ortho isomer passes over in a substantially pure form the para isomer remains in the distillation flask, and can be readily isolated by extraction with hot 2 per cent, hydrochloric acid. The preparation of m-nitrophenol from wt-nitroaniline by means of the diazo reaction is described in Section IV,70. 

The preparation of a number of miscellaneous acids is described. m-Nitrobenzoic acid. Although m-nitrobenzoic acid is the main product of the direct nitration of benzoic acid with potassium nitrate and concentrated sulphuric acid, the complete separation of the small quantity of the attendant para isomer is a laborious process. It is preferable to nitrate methyl benzoate and hydrolyse the resulting methyl w-nitrobenzoate, which is easily obtained in a pure condition ... 

Some ortho substituted phenols such as o mtrophenol have significantly lower boiling points than those of the meta and para isomers This is because the intramolec ular hydrogen bond that forms between the hydroxyl group and the substituent partially compensates for the energy required to go from the liquid state to the vapor... 

The Gattermann-Koch synthesis is suitable for the preparation of simple aromatic aldehydes from ben2ene and its substituted derivatives, as well as from polycychc aromatics. The para isomers are produced preferentially. Aromatics with meta-directing substituents cannot be formylated (108). 

Although 4-hydroxybenzaldehyde can be made by the saligenin route, it has been made historically by the Reimer-Tiemann process, which also produces sahcylaldehyde (64). Treatment of phenol with chloroform and aqueous sodium hydroxide results in the formation of benzal chlorides, which are rapidly hydrolyzed by the alkaline medium into aldehydes. Acidification of the phenoxides results in the formation of the final products, sahcylaldehyde and 4-hydroxybenzaldehyde. The ratio of ortho and para isomers is flexible and can be controlled within certain limits. The overall reaction scheme is shown in Figure 1. Product separation is accomphshed by distillation, but this process leads to environmental problems because of the quantities of sodium chloride produced. 

OOH maybe attached to any of the three positions indicated. Para isomer shown. 

Properties. The physical properties of the ortho, meta, and para isomers of nitrochlorobenzene are summarized in Table 7. 

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. 

Health nd Safety Factors. The mononitrochlorobenzenes are toxic substances which may be absorbed through the skin and lungs giving rise to methemoglobin. Their toxicity is about the same as or greater than that of nitrobenzene. The para isomer is less toxic than the ortho isomer, and the maximum allowable concentration that has been adopted for -nitrochlorobenzene is 1 mg/m (0.1 ppm) (6). The mononitrochlorobenzenes are moderate fire hazards when exposed to heat or flame. They ate classified by the ICC as Class-B poisons. The same handling precautions should be used for these compounds as are used for nitrobenzene. 

Uses. (9-Nitrochlorobenzene is used in the synthesis of azo dye intermediates such as o-chloroaniline (Fast YeUow G Base), i9-nitroani1ine (Fast Orange GR Base), o-anisidine (Fast Red BB Base), o-phenetidine, and (9-aminophenol (see Azo dyes). It also is used in corrosion inhibitors, pigments, and agriculture chemicals. -Nitrochlorobenzene is used principally in the production of intermediates for azo and sulfur dyes. Other uses include pharmaceuticals (qv), photochemicals, mbber chemicals (qv), and insecticides (see Insectcontroltechnology). Typical intermediates manufactured from the para isomer are -lutioaruline (Fast Red GC Base), anisidine, -aminophenol, -nitrophenol, -phenylenediamine, 2-chloro-/)-anisidine (Fast Red R Base), 2,4-dinitrochlorobenzene, and l,2-dichloro-4-nitrobenzene. 

The mononitration of toluene results in the formation of a mixture of the ortho, meta, and para isomers of nitrotoluene. The presence of the methyl group on the aromatic ring facilitates the nitration, but it also increases the ease of oxidation. 

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. 

Terephthalic acid [100-21 -0] the para isomer, may be, but is rarely, used in making alkyds. The resultant resins have even better thermal stabiUty than isophthahc alkyds (5). However, it has all the disadvantages of isophthaUc acid and is more expansive. 

In the reaction of arulinium sulfate [542-16-5] with fuming sulfuric acid, the major products are m- and -aminoben2enesulfonic acid with less than 2% of the ortho isomer. With excess concentrated sulfuric acid (96.8—99.9%) at 60—100°C, the sulfate salt gives mainly the ortho and para isomers of the sulfonic acid (42). 

Until the mid-1960s, phenylenediamines were used primarily for oxidative purposes the para isomer was of major importance. Since then, the use of phenylenediamines to manufacture polymers has far exceeded their use for oxidative purposes. The y -phenylenediamines, (2,4 and 2,6)-toluenediamine, are widely used for the manufacture of polyurethanes. Phenylenediamines are dihinctional and react with other dihinctional compounds, such as dianhydrides, diacyl chlorides, dicarboxyHc acids, and disulfonyl chlorides to give polyamides. Phenylenediamines also give polymers with epoxides, diols, diacetals. 

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. 

The ortho and para isomers are obtained by catalyticaHy hydrogenating the corresponding nitroarulines, made by heating the chloronitroben2ene with aqueous ammonia at about 450°C under pressure. These isomers can also be prepared by heating the corresponding dichloroben2enes with aqueous ammonia at 210°C under pressure ia the presence of a copper salt (23). 

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. 

Vinyltoluene, comprising a mixture of ca 33% para- and 67% y /i7-methylstyrene, has been marketed for ca 45 yr by Dow Chemical Company and also by Cosden. However, the performance properties of the polymers prepared from the para isomer are not only superior to those of the polymer prepared from the typical mixed isomers, but are generally superior to those of polystyrene (60). This advantage, coupled with a raw material cost advantage over styrene, suggests that i ra-methylstyrene may displace significant amounts of styrene, currendy a 3.2 x 10 t/yr domestic market. 

The para isomer is used for preparation of specialty chemicals. Annual toluene requirements are ca 6500 t (2 x 10 gal). 

The ortho para isomer ratio depends on several factors, including the nature of the diazo component, the nature of the solvent, the pH of the medium, the temperature of coupling, the presence of catalysts, and the position of substituents. Simple benzenediazonium compounds couple to... 

Diazophenols, ie, o-hydroxyaryldiazonium salts, couple to 1-naphthol in weaMy basic solution primarily in the para position, but as the hydroxyl ion concentration is increased, formation of the ortho isomer is favored and is frequentiy the sole product. Pyridine and pyridine derivatives, urea, and acetate, etc, used as buffers can also catalyze azo coupling reactions (28). l-amino-2-naphthol-4-sulfonic acid [116-63-2] (1,2,4-acid) and 1-naphthol yield the important Eriochrome Black A [3564-14-5] (18a, R = H) (Cl Mordant Black 3 Cl 14640) which is reportedly (20) a mixture of ortho and para isomers. 

Resonance effects are the primary influence on orientation and reactivity in electrophilic substitution. The common activating groups in electrophilic aromatic substitution, in approximate order of decreasing effectiveness, are —NR2, —NHR, —NH2, —OH, —OR, —NO, —NHCOR, —OCOR, alkyls, —F, —Cl, —Br, —1, aryls, —CH2COOH, and —CH=CH—COOH. Activating groups are ortho- and para-directing. Mixtures of ortho- and para-isomers are frequently produced the exact proportions are usually a function of steric effects and reaction conditions. 

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