Terephthalic acid

Terephthalic Acid. The oxidation of p-xylene to terephthalic acid is a more difficult reaction.866 Once one of the methyl groups is oxidized, the second methyl group in p-toluic acid formed is deactivated by the electron-withdrawing effect of the carboxylic group. Bromine-promoted catalytic processes and cooxidation in the presence of suitable additives are usually practiced.970,978,990 

The product mixture, consisting mainly of p-toluic acid, monomethyl terephtha-late, and some terephthalic acid, is esterified and methyl p-toluate is recycled after separation from the end-product dimethyl terephthalate. The special advantage of cooxidation is that the hydroperoxy radical (111) formed in the autoxidation of p-xylene participates in initiation through hydrogen abstraction from methyl p-toluate, thus promoting oxidation of the latter  

In the manufacture of terephthalic acid by the oxidation of p-xylene, separation of the xylene from its isomeric mixture is necessary (see Section 2.5.2). An alternative process introduced in Japan uses the oxidation of p-tolualdehyde, which is obtained in good regioselectivity by the HF—BF3 catalyzed carbonylation of toluene without the necessity of separation of the isomers. 

Maleic Anhydride. Gas-phase catalytic oxidation of benzene or n-butane is the principal process for the industrial production of maleic anhydride.973 996-999 Until the 1970s commercial production was based predominantly on benzene. Because of its more favorable economics, a switch to butane as an alternative feedstock has taken place since then.966,999-1002 At present almost all new facilities use n-butane as the starting material. Smaller quantities of maleic anhydride may be recovered as a byproduct of phthalic anhydride manufacture (about 5-6%).1003,1004 

More than 70% of benzene is converted to maleic anhydride. Maleic anhydride is partially condensed, and the remaining quantity is absorbed in water to form maleic 

Terephthalic acid is almost insoluble in hot and cold water. It sublimes without melting. It can be identified most readily by converting it into its dimethyl ester as described below. 

Terephthalic acid, also called paraphthalic acid or 1,4-benzene dicarboxylic acid, 

Dicarboxylic aromatic acids form polymers by reacting with dialcohols such as glycol. 

Polyethylene terephthalate can be obtained by a direct acid - catalyzed esterification. It is also known as Dacron, Terylene or Mylar. 

What is the function of H2S04 used in the nitration of benzene Explain. 

When nitrobenzene is nitrated with nitric acid and sulfuric acid, to which position will the nitro group attach Explain why  

At first, the /7-toluic acid was esterified with methanol in a separate step before final oxidation to monomethyl terephthalic acid. Alternatively, if methanol was used as a solvent for the /7-xylene, both methyl groups were esterified in a single step, if the residence time was longer than 20 h. The reaction mechanism involved the conversion of the methyl group to a benzyl radical by reaction with trivalent cobalt ions. The benzyl radical then reacts with oxygen and forms a peroxy species that decomposes to the oxidation product. Trivalent cobalt ions are regenerated as aldehydes from the peroxy or hydroperoxy species. 

The purification of terephthalic acid is complicated because it does not melt and, as it was not soluble in either water or other solvents, it could not be crystallized. On the other hand, the dimethyl ester of terephthalic acid could be easily crystallized from methanol or xylene. When the Mid Century Process was introduced by Scientific Design and Amoco in 1956, it became possible to produce and purify terephthalic acid directly. This process used air oxidation conditions similar to those for previous processes, with a ttuxed trivalent cobalt and manganese acetate catalyst in glacial acetic acid, but introduced an ammonium bromide cocatalyst in conjunction with tetrabromomethane. Cobalt or molyb-demun bromides or hydrobromic acid have also been used, and following reaction with the trivalent cobalt, provided a source of brottune free radicals. The free radicals activated the methyl groups of the /7-xylene and led to the for- 

A relatively low-molecular-weight polymer is produced and generally used to make fibers and other products. The higher-molecular-weight polymer needed for bottles or packaging is formed by further polymerization in the solid state.  

In line with the o- and m-xylene isomers, the industrial importance of p-xylene is based virtually exclusively on the production of the respective dicarboxylic add, namely terephthalic acid. Terephthalic add and its esters are used in the production of polyester fibers, which, alongside polyamide and acrylic fibers, are the major commerdal fibers. 

Initially, the development of fibers based on terephthalic acid met with extraordinary difficulties. Terephthalic add is a white powder, which is virtually insoluble in almost all solvents, does not melt and cannot be distilled. These properties render refining of crude terephthalic acid very intricate. Since high purity of the monomer feedstock material is an absolute necessity for the production of synthetic fibers, an alternative purification route via the dimethyl ester was developed. Dimethyl terephthalate (DMT) is a crystallizable substance which can also be distilled it is therefore relatively easy to produce in pure form. 

Initially, oxidation of p-xylene was carried out with dilute nitric acid (30 to 40%) at 165 °C and a pressure of 10 bar the NO formed was recirculated. The terephthalic add so produced contained nitrogen compounds and therefore war-rented an improvement in its purity. 

The Eastman-Kodak process is operated in a similar way to the Amoco process, adding acetaldehyde as an oxidation accelerator. A further variant is the Toray process, which works with paraldehyde as an oxidation accelerator. 

When bromine compounds are used as catalysts, only expensive reactor materials such as Hastelloy or titanium can be used, since the bromine compounds are strongly corrosive. 

As mentioned earlier, xylene chemistry is primarily related to the methyl substituents, which are amenable to oxidation. 

Approximately 65% of the isolated xylenes are used to make chemicals. The rest are either used as solvents or blended with gasolines. The 1998 U.S. production of mixed xylenes for chemical use was approximately 9.5 million pounds. p-Xylene alone was about 7.7 million pounds that year. 

The catalyzed oxidation of p-xylene produces terephthalic acid (TPA). Cobalt acetate promoted with either NaBr or HBr is used as a catalyst in an acetic acid medium. Reaction conditions are approximately 200°C and 15 atmospheres. The yield is about 95%  

Thermodynamic equilibrium composition of Cs aromatics at three temperatures  

Special precautions must be taken so that the reaction does not stop at the p-toluic acid stage. One approach is to esterify toluic acid as it is formed with methanol. This facilitates the oxidation of the second methyl group. The resulting dimethyl terephthalate (DMT) may be hydrolyzed to terephthalic acid. 

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Condensation polymerization differs from addition polymerization in that the polymer is formed by reaction of monomers, each step in the process resulting in the elimination of some easily removed molecule (often water). E.g. the polyester polyethylene terephthalate (Terylene) is formed by the condensation polymerization (polycondensation) of ethylene glycol with terephthalic acid ... 

Decarbonylation of aromatic aldehydes proceeds smoothly[71], Terephthalic acid (86), commercially produced by the oxidation of p-.xylene (85), contains p-formylbenzoic acid (87) as an impurity, which is removed as benzoic acid (88) by Pd-catalyzed decarbonylation at a high temperature. The benzoic acid produced by the decarbonylation can be separated from terephthalic acid (86) based on the solubility difference in water[72]. 

Poly(ethylene Terephthalate). Poly(ethylene terephthalate) is prepared by the reaction of either terephthalic acid or dimethyl terephthalate with ethylene glycol, and its repeating unit has the general structure. 

PCTA Copolyester. Poly(l,4-cyclohexanedimethylene terephthalic acid) (PCTA) copolyester is a polymer of cyclohexanedimethanol and terephthalic acid, with another acid substituted for a portion of the terephthalic acid otherwise required. It has the following formula ... 

Those polymers which are the condensation product of two different monomers are named by applying the preceding rules to the repeat unit. For example, the polyester formed by the condensation of ethylene glycol and terephthalic acid is called poly(oxyethylene oxyterphthaloyl) according to the lUPAC system, as well as poly (ethylene terephthalate) or polyethylene terephthalate. 

See also Terephthalic acid.) pHTHALIC ACID AND OTHERBENZENEPOLYCARBOXYLIC ACIDS] (Vol 18)... 

Purified terephthalic acid and dimethyl terephthalate are used as raw materials for the production of saturated polyesters. During 1993, the combined worldwide production of purified terephthafic acid plus dimethyl terephthalate exceeded 14 x 10 t (42), which is 80% of the total benzenepolycarboxyfic acid production. Terephthafic acid is also produced ia technical or cmde grades which are not pure enough for manufacture of poly(ethylene terephthalate). In almost all cases, the technical-grade material is immediately converted to purified terephthafic acid or dimethyl terephthalate, which together are the articles of commerce. 

Table 13. Physical Constants of Terephthalic Acid and Dimethyl Terephthalate...

Manufacture and Processing. Terephthalic acid and dimethyl terephthalate did not become large-volume industrial chemicals until after World War II. Imperial Chemical Industries in the United Kingdom in 1949 and Du Pont in the United States in 1953 commercialized fibers made from poly(ethylene terephthalate). Dimethyl terephthalate and ethylene glycol were the comonomers used by both companies (see Fibers, polyester). 

Technical-Grade Terephthalic Acid. All technical-grade terephthahc acid is produced by catalytic, hquid-phase air oxidation of xylene. Several processes have been developed, but they all use acetic acid as a solvent and a multivalent heavy metal or metals as catalysts. Cobalt is always used. In the most popular process, cobalt and manganese are the multivalent heavy-metal catalysts and bromine is the renewable source for free radicals (51,52). 

Fig. 2. Terephthalic acid production by catalytic, liquid-phase air oxidation of -xylene.

Fig. 4. The Amoco purification process for polymer-grade terephthalic acid.

Table 18. World Capacities for Terephthalic Acid/Dimethyl Terephthalate, 10 t...

Table 20. Specifications for Polymer-Grade Terephthalic Acid...

Table 23. Properties of terephthalic acid ester and Terephthaloyl Chloride...

Like terephthalic acid, isophthalic acid is used as a raw material in the production of polyesters. Much of the isophthaUc acid is used for unsaturated polyesters, whereas terephthaUc acid is used almost exclusively in saturated (thermoplastic) polyesters. However, a considerable amount of isophthaUc acid is used as a minor comonomer in saturated polyesters, where the principal diacid is terephthaUc acid. The production volume of isophthaUc acid is less than 2% that of terephthahc. IsophthaUc acid was formerly produced in technical or cmde grades and only a small amount was purified. Now, however, it is all purified to a standard similar to that of terephthahc acid. 

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