Oxidation of toluene

Oxidizing toluene in the liquid phase over a cohalt acetate catalyst produces henzoic acid. The reaction occurs at about 165°C and 10 atmospheres. The yield is over 90%  

Benzoic acid (benzene carboxylic acid) is a white crystalline solid with a characteristic odor. It is slightly soluble in water and soluble in most common organic solvents. 

Though much benzoic acid gets used as a mordant in calico printing, it also serves to season tobacco, preserve food, make dentifrices, and kill fungus. Furthermore it is a precursor for caprolactam, phenol, and tereph-thalic acid. 

Caprolactam, a white solid that melts at 69°C, can be obtained either in a fused or flaked form. It is soluble in water, ligroin, and chlorinated hydrocarbons. Caprolactam s main use is to produce nylon 6. Other minor uses are as a crosslinking agent for polyurethanes, in the plasticizer industry, and in the synthesis of lysine. 

The first step in producing caprolactam from benzoic acid is its hydrogenation to cyclohexane carboxylic acid at approximately 170°C and 16 atmospheres over a palladium catalyst  

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It was first described in 1608 when it was sublimed out of gum benzoin. It also occurs in many other natural resins. Benzoic acid is manufactured by the air oxidation of toluene in the liquid phase at 150°C and 4-6 atm. in the presence of a cobalt catalyst by the partial decarboxylation of phthalic anhydride in either the liquid or vapour phase in the presence of water by the hydrolysis of benzotrichloride (from the chlorination of toluene) in the presence of zinc chloride at 100°C. 

Obtained synthetically by one of the following processes fusion of sodium ben-zenesulphonate with NaOH to give sodium phenate hydrolysis of chlorobenzene by dilute NaOH at 400 C and 300atm. to give sodium phenate (Dow process) catalytic vapour-phase reaction of steam and chlorobenzene at 500°C (Raschig process) direct oxidation of cumene (isopropylbenzene) to the hydroperoxide, followed by acid cleavage lo propanone and phenol catalytic liquid-phase oxidation of toluene to benzoic acid and then phenol. Where the phenate is formed, phenol is liberated by acidification. 

C (decomp.) It is made by the oxidation of toluene-o-sulphonamide with alkaline permanganate. Saccharin has about 550 times the sweetening power of sucrose, and is used extensively as a sweetening agent, usually in the form of the sodium salt. The use of saccharin is restricted in the U.S. 

The three chemical reactions in the toluene—benzoic acid process are oxidation of toluene to form benzoic acid, oxidation of benzoic acid to form phenyl benzoate, and hydrolysis of phenyl benzoate to form phenol. A typical process consists of two continuous steps (13,14). In the first step, the oxidation of toluene to benzoic acid is achieved with air and cobalt salt catalyst at a temperature between 121 and 177°C. The reactor is operated at 206 kPa gauge (2.1 kg/cm g uge) and the catalyst concentration is between 0.1 and 0.3%. The reactor effluent is distilled and the purified benzoic acid is collected. The overall yield of this process is beheved to be about 68 mol % of toluene. 

Chem Systems Inc. proposed a process in which ben2yl alcohol obtained by an undisclosed direct oxidation of toluene is homologated with synthesis gas to yield 2-phen5iethyl alcohol, which is then readily dehydrated to styrene (57). This process eliminates the intermediate formation of methanol from synthesis gas but does require the independent production of ben2yl alcohol. 

Benzyl Chloride. Benzyl chloride is manufactured by high temperature free-radical chlorination of toluene. The yield of benzyl chloride is maximized by use of excess toluene in the feed. More than half of the benzyl chloride produced is converted by butyl benzyl phthalate by reaction with monosodium butyl phthalate. The remainder is hydrolyzed to benzyl alcohol, which is converted to ahphatic esters for use in soaps, perfume, and davors. Benzyl salicylate is used as a sunscreen in lotions and creams. By-product benzal chloride can be converted to benzaldehyde, which is also produced directiy by oxidation of toluene and as a by-product during formation of benzoic acid. By-product ben zotrichl oride is not hydrolyzed to make benzoic acid but is allowed to react with benzoic acid to yield benzoyl chloride. 

Benzaldehyde. Annual production of ben2aldehyde requires ca 6,500—10,000 t (2-3 x 10 gal) of toluene. It is produced mainly as by-product during oxidation of toluene to benzoic acid, but some is produced by hydrolysis of ben2al chloride. The main use of ben2aldehyde is as a chemical intermediate for production of fine chemicals used for food flavoring, pharmaceuticals, herbicides, and dyestuffs. 

The only industrially important processes for the manufacturing of synthetic benzaldehyde involve the hydrolysis of benzal chloride [98-87-3] and the air oxidation of toluene. The hydrolysis of benzal chloride, which is produced by the side-chain chlorination of toluene, is the older of the two processes. It is no longer utilized ia the United States. Other processes, including the oxidation of benzyl alcohol, the reduction of benzoyl chloride, and the reaction of carbon monoxide and benzene, have been utilized ia the past, but they no longer have any iadustrial appHcation. 

Today, the air oxidation of toluene is the source of most of the world s synthetic benzaldehyde. Both vapor- and Hquid-phase air oxidation processes have been used. In the vapor-phase process, a mixture of air and toluene vapor is passed over a catalyst consisting of the oxides of uranium, molybdenum, or related metals. High temperatures and short contact times are essential to maximize yields. Small amounts of copper oxide maybe added to the catalyst mixture to reduce formation of by-product maleic anhydride. 

Conversion per pass is reported to be low, 10—20%, with equally low yields, 30—50% (5). The vapor-phase oxidation of toluene was the dominant toluene oxidation process ia the 1950s and early 1960s, but is no longer of iadustrial importance. The Hquid-phase process now dominates. 

In the Hquid-phase process, both benzaldehyde and benzoic acid are recovered. This process was iatroduced and developed ia the late 1950s by the Dow Chemical Company, as a part of their toluene-to-phenol process, and by Snia Viscosa for their toluene-to-caprolactam process. The benzaldehyde recovered from the Hquid-phase air oxidation of toluene may be purified by either batch or continuous distillation. Liquid-phase air oxidation of toluene is covered more fully (see Benzoic acid). 

The subject of natural benzaldehyde came to the forefront in 1984 when it was found that a natural benzaldehyde product, labeled "oil of benzaldehyde," was actually made synthetically by the air oxidation of toluene followed by careful fractionation to remove trace impurities. This finding was accomphshed by the Center for AppHed Isotopic Studies, University of Georgia, and involved measuring the amounts of and in that material. 

Benzoic acid [65-85-0] C H COOH, the simplest member of the aromatic carboxyHc acid family, was first described in 1618 by a French physician, but it was not until 1832 that its stmcture was deterrnined by Wn b1er and Liebig. In the nineteenth century benzoic acid was used extensively as a medicinal substance and was prepared from gum benzoin. Benzoic acid was first produced synthetically by the hydrolysis of benzotrichloride. Various other processes such as the nitric acid oxidation of toluene were used until the 1930s when the decarboxylation of phthaUc acid became the dominant commercial process. During World War II in Germany the batchwise Hquid-phase air oxidation of toluene became an important process. 

In the United States all other processes have been completely phased out and virtually all benzoic acid is manufactured by the continuous hquid-phase air oxidation of toluene. In the late 1950s and the early 1960s both Dow Chemical and Snia Viscosa constmcted faciUties for Hquid-phase toluene oxidation because of large requirements for benzoic acid in the production of phenol and caprolactam. Benzoic acid, its salts, and esters are very useful and find appHcation in medicinals, food and industrial preservatives, cosmetics, resins, plasticizers, dyestuffs, and fibers. 

Ben2oic acid is almost exclusively manufactured by the cobalt cataly2ed Hquid-phase air oxidation of toluene [108-88-3]. Large-scale plants have been built for ben2oic acid to be used as an intermediate in the production of phenol (by Dow Chemical) and in the production of caprolactam (by Snia Viscosa) (6-11). 

A number of different cobalt salts have been used in the oxidation of toluene, the most common being cobalt acetate [71-48-7] cobalt naphthenate, and cobalt octoate [1588-79-0],... 

Vapor-phase oxidation of toluene to produce benzoic acid and benzaldehyde has been tried utilizing several different catalysts, but yields are low and the process cannot compete with the Hquid-phase process (see Benzaldehyde). Other processes for the production of benzoic acid are presendy of htde commercial importance. 

Snia Viscosa. Catalytic air oxidation of toluene gives benzoic acid (qv) in ca 90% yield. The benzoic acid is hydrogenated over a palladium catalyst to cyclohexanecarboxyhc acid [98-89-5]. This is converted directiy to cmde caprolactam by nitrosation with nitrosylsulfuric acid, which is produced by conventional absorption of NO in oleum. Normally, the reaction mass is neutralized with ammonia to form 4 kg ammonium sulfate per kilogram of caprolactam (16). In a no-sulfate version of the process, the reaction mass is diluted with water and is extracted with an alkylphenol solvent. The aqueous phase is decomposed by thermal means for recovery of sulfur dioxide, which is recycled (17). The basic process chemistry is as follows ... 

Phenol was the first commercial antiseptic its introduction into hospitals in the 1870s led to a dramatic decrease in deaths from postoperative infections. Its use for this purpose has long since been abandoned because phenol burns exposed tissue, but many modern antiseptics are phenol derivatives. Toluene has largely replaced benzene as a solvent because it is much less toxic. Oxidation of toluene in the body gives benzoic acid, which is readily eliminated and has none of the toxic properties of the oxidation products of benzene. Indeed, benzoic acid or its sodium salt (Na+, C6H5COO ions) is widely used as a preservative in foods and beverages, including fruit juices and soft drinks. 

Nitration vs side-chain oxidation of toluene in dilute MA was investigated by Namba et al (Ref 69). They found that addition of sulfuric acid accelerated both reactions but nitration more than oxidation. Addition of water to the MA favors oxidation as does an increase in reaction temp... 

The oxidations of toluene, diphenylmethane and triphenylmethane are second order , relative rates at 22 °C being... 

The effect of solvent upon k2 has been reported , and it was concluded that the activated complex is not sufficiently polar to be called ionic . The oxidations of toluene and triphenylmethane exhibit primary kinetic deuterium isotope effects of 2.4 and ca. 4 respectively. No isotopic mixing occurred during formation of the Etard complex from a mixture of normal and deuterated o-nitrotoluene . The chromyl chloride oxidation of a series of substituted diphenylmethanes revealed that electron-withdrawing substituents slow reaction while electronreleasing groups have the opposite effect, the values ofp andp being —2.28 + 0.08 and —2.20 + 0.07 respectively . ... 

The transition state does not involve a large degree of charge separation and hence the relative rates of oxidation of toluene, diphenylmethane and triphenylmethane may be explained in terms of an ionic mechanism. 

The permanganate oxidation of phenols is complicated by the intervention of lower oxidation states of manganese, (c/. the oxidation of toluene, p. 298). For example, the oxidation of 2,6-dinitrophenol in weakly acidic solution displays an induction period, following second-order kinetics thereafter. However, addition of potassium fluoride inhibits reaction almost completely, but manganous ions strongly accelerate it. 

Such a pre-equilibrium closely parallels that suggested by Dewar et for the manganic acetate oxidations of several aromatic ethers and amines (p. 405). Other features of the reaction are a p value of —0.7 and identical activation energies of 25.3 kcal.mole for oxidation of toluene, ethylbenzene, cumene, diphenylmethane and triphenylmethane. 

CgHsCl as in the absence of aromatic, suggesting auto-decomposition of the oxidant as the slow step (p. 386). The oxidation of toluene was somewhat faster, implying an additional electron-transfer pathway (c/. the oxidation of aromatic ethers and amines, p. 405). 

Klotz B et al. (1998) Atmospheric oxidation of toluene in a large-volume outdoor photoreactor in situ determination of ring-retaining product yields. J Phys Chem A 102 10289-10299. 

Lovley DR, DJ Lonergan (1990) Anaerobic oxidation of toluene, phenol, and -cresol by the dissimilatory iron-reducing organism, GS-15. Appl Environ Microbiol 56 1858-1864. 

Rabus R, R Nordhaus, W Ludwig, F Widdel (1993) Complete oxidation of toluene under strictly anoxic conditions by a new sulfate-reducing bacterium. Appl Environ Microbiol 59 1444-1451. 

Biegert T, G Fuchs (1995) Anaerobic oxidation of toluene (analogues) to benzoate (analogues) by whole cells and by cell extracts of a denitrifying Thauera sp. Arch Microbiol 163 407-417. 

Biegert T, G Fuchs, J Heider (1996) Evidence that anaerobic oxidation of toluene in the denitrifying bacterium Thauera aromatica is initiated by formation of benzylsuccinate from toluene and fumarate. Eur J Biochem 238 661-668. 

Whited GM, DT Gibson (1991b) Toluene-4-monooxygenase, a three-component enzyme system that catalyzes the oxidation of toluene to p-cresol in Pseudomonas mendocina KRl. J Bacteriol 173 3010-3016. 

Seyfried B, G Glod, R Schocher, A Tschech, J Zeyer (1994) Initial reactions in the anaerobic oxidation of toluene and m-xylene by denitrifying bacteria. Appl Environ Microbiol 60 4047-4052. 

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