Toluene and the xylenes

Should this be a separate chapter The chemistry and hardware involved in making toluene and xylenes are for the most part the same as their sibling, benzene. While that may be true, there are a few chemical principles that can be demonstrated better using toluene. The separation processes for purifying toluene and xylenes are different also. There s enough, then, for.a healthy bite without tagging on to the last chapter. 

The manufacture routes to toluene, like benzene, include cat reforming, olefin plant production, recovery of the small amounts naturally occurring in crude oil, and coke production. More than two-thirds of toluene comes from cat reforming. The volume of coal-derived toluene, which evolves in the same manner that was described in the benzene chapter, almost rounds off to zero now. 

In the cat reforming process, two important variables control toluene make the composition of the feed and the operating conditions in the reactors. As to the first, some compounds are more suitable for reforming into 

When the naphtha feed to a cat reformer has a naturally high content of these precursors, the yields of toluene are high. Other than this fortuitous circumstance, theres generally not too much attention paid to toluene in the reforming operation for several reasons  

Separation of toluene from the other components can be by solvent extraction or extractive distillation, just as described in the benzene chapter. The boiling points of benzene and toluene are far enough apart that the feed to separation unit of choice can be split (fractionated) rather easily into benzene concentrate and a toluene concentrate. Alternatively, the separation unit can be thought of as aromatics recovery unit. Then an aromatics concentrate stream is fed to the solvent extraction unit, and, the aromatics outturn can be split into benzene and toluene streams by fractionation. Both schemes are popular. 

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Originally the chemical industry was inorganic in the 1960s organic chemicals (means they contain carbon) came into prominence with the compounds benzene, phenol, ethylene, and vinyl chloride. The organic chemicals benzene, phenol, toluene, and the xylenes compose the aromatic group. 

Assinder SJ, PA Williams (1990) The TOL plasmids determinants of the catabolism of toluene and the xylenes. Adv Microb Physiol 31 1-69. 

A number of important pollutant gases and vapors have signatures in the UV region of the electromagnetic spectrum. Among these are NO , SO CO, O3, benzene, toluene, and the xylenes. UV-vis spectroscopy of a host of environmentally relevant gases and vapors is reported by Bosch Ojeda and Sanchez Rojas." In this chapter, two cases are presented toluene (see Section 4.7) and ozone. 

The olefins—ethylene, propylene, and the butylenes—are derived from natural gas and petroleum. Methane is the major constituent in natural gas. The aromatics— benzene, toluene, and the xylenes— are derived from petroleum. About 90% by weight of the organic chemicals in the world comes from natural gas and petroleum. But actually only 3% of this crude oil and 6% of refinery output in the U.S. is processed into chemicals, with the rest going as various fuels. Although we are a small user of the petroleum industry, this 3-6% going to petrochemical feedstock is important to us ... 

Benzene, naphthalene, toluene, and the xylenes are naturally occurring compounds obtained from coal tar. Industrial synthetic methods, called catalytic reforming, utilize alkanes and cycloalkanes isolated from petroleum. Thus, cyclohexane is dehydrogenated (aromatization), and n-hexane(cycli> zation) and methylcyclopentane(isomerization) are converted to benzene. Aromatization is the reverse of catalytic hydrogenation and, in the laboratory, the same catalysts—Pt, Pd, and Ni—can be used. The stability of the aromatic ring favors dehydrogenation. 

Arsenic triiodide is soluble in carbon disulphide, alcohol, ether, chloroform, benzene, toluene and the xylenes.5 The solution in carbon disulphide gradually darkens owing to absorption of oxygen and liberation of iodine.6 With alcohol at 150° C. ethyl iodide is formed. In methylene iodide 5 the triiodide dissolves to the extent of 17-4 parts of AsI3 in 100 parts of solvent at 12° C. The dipole moment in various solvents has been determined.7... 

A further deviation from additivity which should be taken account of is the substitution effect, or increase of stability on substitution in hydrocarbons, mentioned in Section 10,6.6. This, for example, causes the observed resonance energy to be greater in toluene and the xylenes than in benzene. 

Carbonium ions can be generated at a variety of oxidation levels. The alkyl carbocation can be generated from alkyl halides by reaction with a Lewis acid (RCl + AICI3) or by protonation of alcohols or alkenes. The reaction of an alkyl halide and aluminium trichloride with an aromatic ring is known as the Friedel-Crafts alkylation. The order of stability of a carbocation is tertiary > secondary > primary. Since many alkylation processes are slower than rearrangements, a secondary or tertiary carbocation may be formed before aromatic substitution occurs. Alkylation of benzene with 1-chloropropane in the presence of aluminium trichloride at 35 °C for 5 hours gave a 2 3 mixture of n- and isopropylbenzene (Scheme 4.5). Since the alkylbenzenes such as toluene and the xylenes (dimethylbenzenes) are more electron rich than benzene itself, it is difficult to prevent polysubsiitution and consequently mixtures of polyalkylated benzenes may be obtained. On the other hand, nitro compounds are sufficiently deactivated for the reaction to be unsuccessful. 

In a more recent membrane model study [11] a large (twinned) silicalite-1 crystal (100 100 300 p.m) was embedded in an epoxy matrix, using an aluminium gasket as a support. Polishing improved the crystal surface exposure. Mcropore diffiisitivities were measured for benzene, toluene and the xylenes. 

Today, petroleum is the chief source of the enormous quantities of benzene, toluene, and the xylenes required for chemicals and fuels. Half of the toluene and xylenes are utilized in high-test gasoline where, in a sense, they replace the aliphatic compounds—inferior as fuels—from which they were made. (A considerable fraction even of naphthalene, the major component of coal tar distillate, is now being produced from petroleum hydrocarbons.)... 

Due to the relatively high water solubility, monoaromatic hydrocarbons and phenolic compounds are among the most frequently identified water pollutants. Monoaromatics like benzene, toluene and the xylenes, are the main constituents of the water soluble fraction of gasoline and other oil products, and they are widely used as solvents. Phenohcs often occur in coimection with creosote and tar pollution in groimd water and else in mai types of industrial process water discharge (Cooper and Wheatstone, 1973). Fmthermoie, they are often identified as intermediary metabolites in the degradation of other aromatic compoimds. 

Phenol, the cresols, and the dimethylphenols are formed from the atmospheric degradation of benzene, toluene, and the xylenes, respectively, and data are available concerning the atmospheric reactions of these compounds. The potential atmospheric loss processes of phenolic compounds are reaction with N03, OH radicals, and 03, together with wet and dry deposition (these compounds are readily incorporated into rain- and cloud-water and fog). The OH radical reactions proceed by OH radical addition to the aromatic ring and by H-atom abstraction from the substituent -OH and -CH3 groups (Atkinson, 1989) ... 

More recently gas-phase nitration was treated theoretically with MNDO (modified neglect of diatomic differential overlap) and INDO (intermediate neglect of differential overlap) self-consistent field calculations (34). Electron transfer and radical-pair recombination were favored for the nitration of toluene and the xylenes but not for nitrobenzene, for which a classical nitration route via a tt complex was favored. The calculations could not make a distinction between the two routes in the nitration of benzene. More information is needed about these coupling reactions and how they differ in the gas and heterogeneous-solution phases. 

Homologues of Pyridine.—Many homologues of pyridine have been obtained from bone-oil and coal-tar or by syntheses from other compounds. The methyl derivatives of pyridine are called picoUneSy the dimethyl derivatives are called lutidines, and the trimethyl derivatives are called collidines. As in the case of toluene and the xylenes, the side-chains are converted into carboxyl groups by oxidation. The a, /S, and 7 mono-carboxylic acids of pyridine are called picolinic acid, nicotinic acid, and isonicotinic acid, respectively. 

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