Terephthalic acid
Terephthalic acid is an organic compound with formula CH(COH). This white solid is a commodity chemical, used principally as a precursor to the polyester PET, used to make clothing and plastic bottles. Several million tonnes are produced annually. The common name is derived from the turpentine-producing tree "Pistacia terebinthus" and phthalic acid. History. Terephthalic acid was first isolated (from turpentine) by the French chemist Amédée Cailliot (1805–1884) in 1846. Terephthalic acid became industrially important after World War II. Terephthalic acid was produced by oxidation of "p"-xylene with dilute nitric acid. Air oxidation of "p"-xylene gives "p"-toluic acid , which resists further air-oxidation. Conversion of "p"-toluic acid to methyl p-toluate (CHCHCOCH) opens the way for further oxidation to monomethyl terephthalate, which is further esterified to dimethyl terephthalate. In 1955, Mid-Century Corporation and ICI announced the bromide-promoted oxidation of "p"-toluic acid to teraphthalic acid. This innovation enabled the conversion of "p"-xylene to terephthalic acid without the need to isolate intermediates. Amoco (as Standard Oil of Indiana) purchased the Mid-Century/ICI technology. Synthesis. Amoco process. In the Amoco process, which is widely adopted worldwide, terephthalic acid is produced by catalytic oxidation of "p"-xylene: The process uses  a cobalt–manganese–bromide catalyst. The bromide source can be sodium bromide, hydrogen bromide or tetrabromoethane. Bromine functions as a regenerative source of free radicals. Acetic acid is the solvent and compressed air serves as the oxidant. The combination of bromine and acetic acid is highly corrosive, requiring specialized reactors, such as those lined with titanium. A mixture of "p"-xylene, acetic acid, the catalyst system, and compressed air is fed to a reactor. Mechanism. The oxidation of "p"-xylene proceed by a free radical process. Bromine radicals decompose cobalt and manganese hydroperoxides. The resulting oxygen-based radicals abstract hydrogen from a  methyl group, which have weaker C–H bonds than does the aromatic ring. Many intermediates have been isolated. "p"-xylene is converted to "p"-toluic acid, which is less reactive than the p-xylene owing to the influence of the electron-withdrawing carboxylic acid group. Incomplete oxidation produces 4-carboxybenzaldehyde (4-CBA), which is often a problematic impurity. Challenges. Approximately 5% of the acetic acid solvent is lost by decomposition or "burning". Product loss by decarboxylation to benzoic acid is common. The high temperature diminishes oxygen solubility in an already oxygen-starved system. Pure oxygen cannot be used in the traditional  system due to hazards of flammable organic–O mixtures. Atmospheric air can be used in its place, but once reacted needs to be purified of toxins and ozone depleters such as methylbromide before being released. Additionally, the corrosive nature of bromides at high temperatures requires the reaction be run in expensive titanium reactors. Alternative reaction media. The use of carbon dioxide overcomes many of the problems with the original industrial process. Because CO is a better flame inhibitor than N, a CO environment allows for the use of pure oxygen directly, instead of air, with reduced flammability hazards. The solubility  of molecular oxygen in solution is also enhanced in the CO environment. Because more oxygen is available to the system, supercritical carbon dioxide ("T" = 31 °C) has more complete oxidation with fewer byproducts, lower carbon monoxide production, less decarboxylation and higher purity than the commercial process. In supercritical water medium, the oxidation can be effectively catalyzed by MnBr with pure O in a medium-high temperature. Use of supercritical water instead of acetic acid as a solvent diminishes environmental impact and offers a cost advantage. However, the scope of such reaction systems is limited by the even harsher  conditions than the industrial process (300−400 °C, >200 bar). Promotors and additives. As with any large-scale process, many additives have been investigated for potential beneficial effects. Promising results have been reported with the following. Alternative routes. Terephthalic acid can be prepared in the laboratory by oxidizing many "para"-disubstituted derivatives of benzene, including caraway oil or a mixture of cymene and cuminol with chromic acid. Although not commercially significant is the so-called "Henkel process" or "Raecke process", named after the company and patent holder, respectively. This process involves the transfer of carboxylate groups. For example potassium benzoate  disproportionates to potassium terephthalate and potassium phthalate rearranges to potassium terephthalate. Lummus (now a subsidiary of McDermott International) has reported a route from the dinitrile, which can be obtained by ammoxidation of "p"-xylene. Applications. Virtually the entire world's supply of terephthalic acid and dimethyl terephthalate are consumed as precursors to polyethylene terephthalate (PET). World production in 1970 was around 1.75 million tonnes. By 2006, global purified terephthalic acid (PTA) demand had exceeded 30 million tonnes. A smaller, but nevertheless significant, demand for terephthalic acid exists in the production of polybutylene terephthalate and several other engineering polymers. Solubility. Terephthalic acid  is poorly soluble in water and alcohols; consequently, until about 1970 terephthalic acid was purified as its dimethyl ester. It sublimes when heated. Toxicity. Terephthalic acid and its dimethyl ester have very low toxicity, with s over 1 g/kg (oral, mouse).