Iron and acid

Compounds containing two primary amino groups attached to a benzene ring can be prepared by the reduction of dinitro compounds and of nitroanilines, usually with tin or stannous chloride and hydrochloric acid or with iron and very dilute hydrochloric acid. / ara-diamines may also be obtained by the reduction of aromatic amino-azo compounds (e.g., p-aminodimethylanihne from methyl orange, see Section IV,78). p-Phenylenediamine may also be prepared from p-nitroacetanilide reduction with iron and acid yields p-amino-acetaniUde,.which may be hydrolysed to the diamine. 

The Zinin reduction is also usehil for the reduction of aromatic nitro compounds to amines in the laboratory. It requires no special equipment, as is the case with catalytic hydrogenations, and is milder than reductions with iron and acid. Usually ammonium or alkah sulfides, hydrosulftdes or polysulftdes are used as the reactant with methanol or ethanol as the solvent. 

A spore-forming strain of Desulfitobacterium chlororespirans was able to couple the dechlorination of 3-chloro-4-hydroxybenzoate to the oxidation of lactate to acetate, pyruvate, or formate (Sanford et al. 1996). Whereas 2,4,6-trichlorophenol and 2,4,6-tribro-mophenol supported growth with the production of 4-chlorophenol and 4-bromophenol, neither 2-bromophenol nor 2-iodophenol was able to do so. The membrane-bound dehalogenase contains cobalamin, iron, and acid-labile sulfur, and is apparently specific for ortho-substituted phenols (Krasotkina et al. 2001). 

A brief historical note on the structure of the iron-sulfur clusters in ferredoxins is relevant. After the first analytical results revealed the presence of (nearly) equimolar iron and acid-labile sulfur, it was clear that the metal center in ferredoxins did not resemble any previously characterized cofactor type. The early proposals for the Fe S center structure were based on a linear chain of iron atoms coordinated by bridging cysteines and inorganic sulfur (Blomstrom et al., 1964 Rabino-witz, 1971). While the later crystallographic analyses of HiPIP, PaFd, and model compounds (Herskovitz et al., 1972) demonstrated the cubane-type structure of the 4Fe 4S cluster, the original proposals have turned out to be somewhat prophetic. Linear chains of sulfide-linked irons are observed in 2Fe 2S ferredoxins and in the high-pH form of aconitase. Cysteines linked to several metal atoms are present in metallothionein. The chemistry of iron-sulfur clusters is rich and varied, and undoubtedly many other surprises await in the future. 

Ferredoxins with 4Fe4S clusters are small, low potential electron carriers that function in bacterial electron transfer. Like the binuclear clusters, each tetranu-clear cluster can reversibly accept a single electron. The tetranuclear prosthetic group is a cubane with iron and acid-labile inorganic sulfide groups at alternate vertices the four iron atoms and the four sulfides form interpenetrating tetrahedra of slightly different sizes. 

Pour 50 cc. of concentrated sulfuric acid slowly into about 1 liter of water in an evaporating dish and add 50 g. of iron nails or turnings. When the action becomes slow heat the dish until the acid is practically all neutralized, as indicated by the fact that evolution of hydrogen ceases. Filter from the undissolved iron, carbon, silicon, and other residues, using a folded filter, and evaporate to crystallization. If the solution oxidizes appreciably in the operation additional iron and acid must be added to effect reduction. Oxidation will be indicated by a change in color from bottle green to a yellowish shade of green, or by the formation of a rusty precipitate. 

Inosine formed by either route is then phosphorolyzed to yield hypoxanthine. Although, as we have previously seen, much of the hypoxanthine and guanine produced in the mammalian body is converted to IMP and GMP by a phosphoribosyltransferase, about 10% is catabolized. Xanthine oxidase, an enzyme present in large amounts in liver and intestinal mucosa and in traces in other tissues, oxidizes hypoxanthine to xanthine, and xanthine to uric acid (see fig. 23.20). Xanthine oxidase contains FAD, molybdenum, iron, and acid-labile sulfur in the ratio 1 1 4 4, and in addition to forming hydrogen peroxide, it is also a strong producer of the superoxide anion 02, a very reactive species. The enzyme oxidizes a wide variety of purines, aldehydes, and pteridines. 

The succinate dehydrogenase of S. acidocaldarius (DSM 639) is located in the cytoplasmic and membrane fractions when cells are disrupted either by sonication or decompressive disruption. About 10-30% of the activity is associated with the membrane fraction [30]. The purified membrane-bound succinate dehydrogenase activity (M, 141 000) consists of four subunits (Mr 66000, 31 000, 28 000, and 12,800). The enzyme contains a covalently-bound flavin as well as iron and acid-labile sulfide but no cytochrome [111]. The dehydrogenase reduces the following acceptors (listed in order of decreasing... 

The oxidation of the sulfide of the pyrite to sulfate (equation 19a) releases dissolved ferrous iron and acidity into the water. Subsequently, the dissolved ferrous iron undergoes oxygenation to ferric iron, which then hydrolyzes to form insoluble ferric hydroxide, releasing more acidity to the stream and coating the streambed. Ferric iron can also be reduced by pyrite itself, as in equation 19d, where sulfide is again oxidized and acidity is released along with additional ferrous iron, which may reenter the reaction cycle via reaction 19b. 

The traditional treatment of nitrobenzene (1) with iron and acid, called Bechamp reduction, was employed almost exclusively in the production of aniline (2) and many aromatic amines until the 1960s1,2 (Scheme 1). The reduction is straightforward, and can also be achieved by catalytic hydrogenation, sodium sulfide reduction and zinc reduction with caustic soda. Nitrotoluenes and nitroxylenes are hydrogenated under pressure over a nickel catalyst supported on kieselguhr. The sulfide reduction is useful in selective reduction, such as of m-dinitrobenzene to m-nitroaniline. 

Metal and acid. Iron and acid (B champ method) is the major example in this category, but other metals (tin, zinc) have also been employed. Generally, hydrochloric acid is preferred, but sulfuric, acetic, and formic acids have also been used. 

When the compound to be treated contains more than one nitro group, the products of reduction depend upon the agents used. Thus, m-phenyl-enediamine is obtained by the iron and acid reduction of m-dinitrobenzene, while the alkaline sulfide reduction yields m-nitroaniline ... 

Explanations of the mechanism of iron and acid (Bechamp) reductions can be made on the basis of observed chemical phenomena as well as electronic theory. Both explanations are useful in providing a better understanding of the reduction process and in providing guides for efficient industrial operations. 

Electronic Mechanism. According to Luder and Zuffanti, the use of iron and acids to reduce nitrobenzene to aniline can be explained as follows Any acid (using the word in the Lewis sense) will increase the concentration of hydrogen ions (hydrated, of course) in water solution. The salts (FeCh and RNH3CI) and cations that have appeared in the equations above are acids in this general Sense, and in the presence of iron and water a plentiful supply of both electrons and protons is available. 

According to the proponents of the electron theory, such a small amount (about 1 per cent of total requirements) can be attributed to the difficulty in maintaining a practically homogeneous reaction mass Thus, the liberated hydrogen may be due to the reaction in Eq. (2) above, because the total iron surface is not in contact with molecules of nitrobenzene. It is rather remarkable, however, that such a high efficiency in proton or hydrogen-ion utilization is obtained in the iron and acid reduction. 

The reducers for iron and acid reductions are sometimes equipped with side and bottom cast-iron lining plates, which may be reversed or replaced, in order to protect the vessel against the continuous erosive action of the iron borings. Alternatively, the reducers may be lined partially or entirely with acid-resisting brick or tile. Such a protective coating lasts almost indefinitely and performs satisfactorily in service. 

Wooden equipment is also used, particularly for the reduction of solid nitro compounds such as p-nitroaniline. These vessels are made of staves 2-3 in. in thickness, and the base is constructed of a double layer of 2-in. boards. Such tubs may range from 6-12 ft in diameter and up to a height of 12 ft. They are equipped with wooden exhaust stacks to vent the vapors and gases liberated during the reaction. The top of such a reducer contains a large opening through which the solid nitro body, iron, and acid are delivered, and it is provided with a removable cover. Bronze pipes are employed for the introduction of steam to such reducers. 

The method of sulfide reductions, although more expensive than iron and acid or catalytic reduction, has a wide field of application, particularly with respect to partial reductions and reductions in the anthraquinone series. Sodium sulfide, sodium hydrosulfide (the sulfhydrate of commerce), sodium polysulfides, ammonium sulfide, etc., are employed. Quite a number of benzene derivatives are normally reduced by this general method, and it is noteworthy that the first reduction of nitrobenzene was that by Zinin in 1842, using ammonium hydrosulfide. Where the presence of free... 

Alkaline reductions are milder than the iron and acid reductions, and for this reason, modifications of this process find extensive use in technical operations. By utilizing alkali or metal sulfides, it is possible to control better the rate and degree of reduction. Although ammonium sulfide is sometimes used, sodium sulfide or sodium disulfide is most frequently employed. The sulfides of iron and manganese have also been found of value in reduction processes but thus far have not found widespread industrial applications. 

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