Aromatic azo

The fact that aromatic azo compounds are all diamagnetic shows that their normal state is the 2 state. The difference between O2 and RN2R is probably connected with the greater bond energy of the latter, and the resultant changing of s-p quantization for the nitrogen atoms. 

Arenediazonium ions can, of course, bring about electrophilic aromatic substitution giving aromatic azo-compounds. Using PhN=N and PhO , polarized signals have been observed in the N-spectrum (6 MHz) of the coupled product (A, A) and reactant, suggesting that the reaction proceeds, at least in part, by a mechanism involving preliminary reversible electron transfer between the reactants (Bubnov et al., 1972). 

Possibly the most significant discovery in the metabolism of aromatic azo compounds had implications that heralded the age of modem chemotherapy. It was shown that the bactericidal effect of the azo dye Prontosil in vivo was in fact due to the action of its transformation product, sulfanilamide, which is an antagonist of 4-aminobenzoate that is required for the synthesis of the vitamin folic acid. Indeed, this reduction is the typical reaction involved in the first stage of the biodegradation of aromatic azo compounds. 

Aromatic azo compounds, many of which are sulfonated, are components of many commercially important dyes, colorants, and pigments, so that attention has been directed to their degradation and transformation. These compounds are often considered recalcitrant, although then-transformation has been accomplished by reduction to amines with scission of the Ar-N=N-Ar bond to produce arylamines. The amines may then be degraded, for example, 6-aminonaphthalene-2-sulfonate by dioxygenation and ring fission to 5-aminosalicylate (Hang et al. 1991). This is then... 

Some simple azo compounds, because of restricted rotation about the ( N=N ) double bond, are capable of exhibiting geometrical isomerism. The geometrical isomerism of azobenzene, the simplest aromatic azo compound which may be considered as the parent system on which the structures of most azo colorants are based, is illustrated in Figure 3.1. The compound is only weakly coloured because it absorbs mainly in the UV region giving a 2m.lx value of 320 nm in solution in ethanol, a feature which may be attributed to the absence of auxochromes (see Chapter 2). 

Aromatic azo compounds such as azobenzene undergo cis-trans photoisomerization but do not decompose, due to the inherent instability of phenyl radicals. 

Azodicarbonyl compounds, which contain a carbonyl function on both sides of the azo bond, in contrast to aliphatic and aromatic azo compounds, possess a highly reactive N=N group. In recent years they have found wide... 

With concentrated mineral acids azobenzene gives red salts, as may be shown by pouring hydrochloric acid on it. Addition of hydrogen leads to the re-formation of the hydrazo-compound. Oxygen is added on and the azoxy-compound formed by the action of hydrogen peroxide or nitric acid. The synthesis of asymmetrical aromatic azo-compounds from nitroso-compounds and primary amines was discussed above. 

Ghandehari, H., Kopeckova, P., and Kopeck, J., In vitro biodegradation of pH-sensitive hydrogels containing aromatic azo bonds. Biomaterials, 18 861-872 (1997). 

Weber EJ, Wolfe NL. 1986. Kinetic studies of aromatic azo compounds in anaerobic sediment/water systems abstract. In 191st National meeting American Chemical Society Division of Environmental Chemistry 26 239-40. 

Aromatic Azo Compounds Reduction of aromatic azo compounds involves a four-electron process that proceeds through a short-lived intermediate, hydrazoben-zene, that ends with complete reductive cleavage of the azo hnkage and formation of aromatic amines. 

Other inhibitors include sulfur, carbon, aromatic azo compounds, and chlorophosphines [Nigenda et al., 1977 Uemura et al., 1977]. 

The ARS Technologies, Inc., Ferox process is an in situ remediation technology for the treatment of chlorinated hydrocarbons, leachable heavy metals, and other contaminants. The process involves the subsurface injection and dispersion of reactive zero-valence iron powder into the saturated or unsaturated zones of a contaminated area. ARS Technologies claims that Ferox is applicable for treating the following chemicals trichloroethene (TCE), 1,1,1-trichloroethane (TCA), carbon tetrachloride, 1,1,2,2-tetrachloroethane, lindane, aromatic azo compounds, 1,2,3-trichloropropane, tetrachloroethene (PCE), nitro aromatic compounds, 1,2-dichloroethene (DCE), vinyl chloride, 4-chlorophenol, hexachloroethane, tribromomethane, ethylene dibromide (EDB), polychlorinated biphenyls (PCBs), Freon-113, unexploded ordinances (UXO), and soluble metals (copper, nickel, lead, cadmium, arsenic, and chromium). 

Aromatic azo polymers have been prepared from aromatic diamines using cupric ions as catalyst for an oxygen-oxidation in pyridine or dimethylacet-amide-pyridine solution [82],... 

Since A,A -disubstituted hydrazines are readily available from a variety of sources (see Volume I, Chapter 14), their dehydrogenation constitutes a widely applicable route to both aliphatic and aromatic azo compounds. Such oxidative procedures are of particular value in the aliphatic series because so many of the procedures applicable to aromatic compounds, such as the coupling with diazonium salts, have no counterpart. The oxidation reactions permit the formation not only of azoalkanes, but also of a host of azo compounds containing other functional groups, e.g., a-carbonyl azo compounds [83], a-nitrile azo compounds [84], azo derivatives of phosphoric acid [85], phenyl-phosphoric acid derivatives [86],... 

The oxidation of both aliphatic and aromatic azo compounds to the corresponding azoxy derivative may be carried out with a variety of reagents. While older techniques favored chromic or nitric acid as the oxidizing agent, newer methods make use of various organic peracids or hydrogen peroxide. In the oxidation of aliphatic azo compounds, relatively weak peracids are favored to reduce the possibility of acid-catalyzed isomerization of azo compounds to hydrazones. Under controlled conditions cis azo compounds may be converted into cis azoxy compounds. 

The problem of the position which the entering oxygen will occupy on oxidation of an azo compound has not been fully resolved. There is evidence that, in the case of some aliphatic azo compounds in which one side of the azo bridge is a methyl radical and the other side is a more complex aliphatic radical, the final azoxy compound bears the oxygen on the nitrogen atom farthest from the methyl radical. The effect of substituents on the oxidation of aromatic azo compounds has not been studied extensively. 

The oxidation of a fluorinated aromatic azo compound has been carried out with a mixture of 89% hydrogen peroxide and trifluoroacetic anhydride it may be considered a trifluoroperacetic acid oxidation [29],... 

Perhaps the best-known method of preparing aromatic azo compounds involves the coupling of diazonium salts with sufficiently reactive aromatic compounds such as phenols, aromatic amines, phenyl ethers, the related naphthalene compounds, and even sufficiently reactive aromatic hydrocarbons. Generally, the coupling must be carried out in media which are neutral or slightly basic or which are buffered in the appropriate pH range. The reaction may also be carried out in nonaqueous media. While some primary and secondary aromatic amines initially form an A-azoamine, which may rearrange to the more usual amino-C-azo compound, tertiary amines couple in a normal manner. 

CAUTION Derivatives of p-dimethylaminoazo benzene (DMAB, butter yellow) are known carcinogens [lb, c]. Other azo compounds, especially aromatic azo compounds, should be handled with great caution. 

The melting points of some symmetrical aromatic azo compounds, prepared from the corresponding aromatic nitro compounds, are given in Table VII. 

Melting Points of Symmetrically Substituted aromatic Azo Compounds... 

It must be reiterated that, whereas aromatic azo compounds are relatively stable thermally and can be subjected to typical reactions of aromatic compounds [67, 68a, 88], the aliphatic azo compounds may be substantially less stable thermally. Aliphatic azo compounds, such as oc,a -azobis(isobutyro-nitrile), do decompose on heating and are used as free radical sources. Hence adequate safety precautions must be taken in handling them. This, by the way, does not mean that aliphatic azo compounds have not been subjected to distillation and to vapor phase chromatography. Many have been distilled and, as will be pointed out in a subsequent section, their preparation by isomerization of hydrazone depends on a distillation technique. 

The possibility exists that strong acidic reaction systems such as hydrogen peroxide in glacial acetic acid may cause isomerization to hydrazones, particularly in the oxidation of aliphatic or aliphatic-aromatic azo compounds. Therefore the much milder perbenzoic acid in an inert solvent has been suggested as an oxidizing agent [7]. Peracetic acid (40% solution) has also been used in conjunction with an indifferent solvent [3, 6]. 

Azo or diazene compounds possess the —N=N— grouping. Aliphatic azo compounds of the type R—N=N—H appear to be highly unstable and decompose to R—H and nitrogen. Derivatives of the type R—N=N—R are much more stable and can be prepared as mentioned above by oxidation of the corresponding hydrazines. Aromatic azo compounds are available in considerable profusion from diazo coupling reactions (Section 23-IOC) and are of commercial importance as dyes and coloring materials. 

It will be shown in later sections that side groups such as aromatic azo or biphenyl units generate liquid crystallinity metal phthalocyanines and tetracyanoquino-dimethane generate electroactive domains and polyaromatic units such as naphthyl or anthracene groups alter the physical properties markedly as they attempt to align then-molecular axes or form stacks of side groups. 

The cathodic dehalogenation can be carried out by an indirect route for example, by reducing aromatic azo compounds 436 439). 

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