Azo compounds, preparation

Whilst azo compounds prepared from diazonium salts and phenolic or keto-enol coupling components are often depicted in the hydroxyazo form (4.11), an alternative tautomeric structure can be drawn for such compounds (Scheme 4.19). This ketohydrazone tautomer (4.21) can, in cases where the azo and hydroxy groups are located on adjacent carbon atoms, exhibit hydrogen bonding between the two groups as shown. Similar pairs of structures, but without hydrogen bonding, can be drawn for p-hydroxyazo compounds. 

Table II lists yields and properties of several other olefinic azo compounds prepared by this method.

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

Many common synthetic dyes, such as alizarine yellow R, para red, and Congo red, are azo compounds, prepared by the diazonium coupling reaction described in Section 25.15. 

Azo-compounds can be obtained by reduction of nitro-compounds, or by oxidation of hydrazo-compounds. They are usually prepared, however, by reacting a phenol or amine with a diazonium salt. The coupling usually takes place in the position para to the hydroxyl or amino group, but if this position is occupied it goes to the ortho position, e.g. 

Aminoazobenzene is a member of the large class of azo-compounds, all of which contain the characteristic grouping, C-N N-C. Azo-compounds are of considerable technical importance, as they are all coloured, and the majority possess considerable stability. They may be prepared by the following methods ... 

The most noteworthy reaction of azo-compounds is their behaviour on reduction. Prolonged reduction first saturates the azo group, giving the hydrazo derivative (C NH-NH C), and then breaks the NH NH linkage, with the formation of two primary amine molecules. If method (1) has been employed to prepare the azo-compound, these two primary amines will therefore be respectively (a) the original amine from which the diazonium salt was prepared, and (6) the amino derivative of the amine or phenol with which the diazonium salt was coupled. For example, amino-azobenzene on complete reduction gives one equivalent of aniline, and one of p-phenylene diamine, NHaCeH NH benzene-azo-2-naphthoI similarly gives one equivalent of aniline and one of... 

Azo compounds (ArN=NAr) are prepared bj the interaction of a diazonium salt with a phenol in the presence of sodium hydroxide, for example ... 

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 type of initiator utilized for a solution polymerization depends on several factors, including the solubiUty of the initiator, the rate of decomposition of the initiator, and the intended use of the polymeric product. The amount of initiator used may vary from a few hundredths to several percent of the monomer weight. As the amount of initiator is decreased, the molecular weight of the polymer is increased as a result of initiating fewer polymer chains per unit weight of monomer, and thus the initiator concentration is often used to control molecular weight. Organic peroxides, hydroperoxides, and azo compounds are the initiators of choice for the preparations of most acryUc solution polymers and copolymers. 

Although this reduction is more expensive than the Bnchamp reduction, it is used to manufacture aromatic amines which are too sensitive to be made by other methods. Such processes are used extensively where selectivity is required such as in the preparation of nitro amines from dinitro compounds, the reduction of nitrophenol and nitroanthraquinones, and the preparation of aminoazo compounds from the corresponding nitro derivatives. Amines are also formed under the conditions of the Zinin reduction from aromatic nitroso and azo compounds. 

Although distibenes, the antimony analogues of azo compounds, have never been isolated as free, monomeric molecules (130), a tungsten complex, tritungsten pentadecacarbonyl[p.2-Tj -diphenyldistibene] [82579-41-7] C2yH2Q025Sb2W2, has been prepared by the reductive dehalogenation of phenyldichlorostibine (131) ... 

Various substituted N-hiomo- and A/-chloroureas have also been prepared (97). These compounds are useful for synthesis of oxazoUdinones, and also hydrazine, hydrazo, and azo compounds. A/-Bromourea [51918-81 -1] is useful for selective oxidation of sugar derivatives (98). 

The reaction is very common in pyrazolone chemistry. Since alkoxypyrazoles and tautomerizable pyrazolones undergo this reaction and 3-pyrazolin-5-ones, like antipyrine, do not, it is assumed that the reaction takes place at C-4 of the OH tautomer. Pyrazolone diazo coupling is an important industrial reaction since the resulting azo derivatives are used as dyestuffs. For instance, tartrazine (Section 4.04.4.1.3) has been prepared this way. 3,5-Pyrazolidinediones react with aryldiazonium salts resulting in the introduction of a 4-arylazo group. As has been described in Section 4.04.2.1.4(v), diazonium salts couple in the 3-position with indazole to give azo compounds. 

There are many references in the patent literature to azo dyes prepared from 4- and 5-aminoisothiazoles, 3-, 5- and 7-amino-1,2-benzisothiazoles, and their quaternized derivatives. These are particularly useful in the dyeing of synthetic fibres. Isothiazole compounds have also been suggested for other industrial purposes, such as corrosion inhibitors, fireproofing agents, additives in rubber vulcanization, photographic chemicals and fluorescent whiteners in detergents. 

The only practical method of preparing 1,4-aminonaphthol is from a-naphthol through an azo dye, the nitroso compound not being readily available. The majority of investigators have reduced technical Orange I with stannous chloride Mi.is.is.ir.is by the procedures discussed above, and benzeneazo-a-naphthol has been reduced by the same reagent. In order to make possible the use of crude, technical a-naphthol a method has been developed for the preparation of the benzeneazo compound, its separation from the isomeric dye coming from the d-naphthol present as well as from any disazo compound by extraction with alkali, and the reduction of the azo compound in alkaline solution with sodium hydrosulfite. The process, however, is tedious and yields an impure product. 

The general method used by the submitters has been reported by others for the preparation of other azo compounds. 

The azo compounds A and B were prepared and the thermal and photochemical behavior of these materials was investigated. The results are summarized in the equations below. Discuss how these results m relate to the photochemical di-rc-methane rearrangement. (See Section 12.1.4 for some indications of the reactivity of... 

The reactivity of the 5-position of 2-aminothiophene in diazo coupling, which is present also in the acylated derivatives, complicates the formation of a diazonium salt from 2-aminothiophene. Thus Steinkopf and Miiller obtained only an azo dyestuff, although they proved, through the isolation of small amounts of 2-thienyl diazonium chloride, the diazotizability of 2-aminothiophene which had earlier been denied. However, recent Russian work claims the preparation of 2-thienyldiazonium chloride by treating the double salt in 10% hydrochloric acid with sodium nitrite. Amazingly high yields (over 90%) of azo compounds were then achieved by coupling the diazonium salt solution with y9-naphtol, w-toluidine or with the 2-aminothiophene double salt. These authors have also studied the... 

Almost all dyes are quinones or azaquinones of compound 2. The majority are used for thermal transfer processes a recent example is compound 306 (98USP5792587). The second major class are azo dyes, prepared either from aromatic diazonium salts as in compound 307 (95GEP4319296) or from a triazolopyridine 3-diazonium salt as in 308 (81BRP2054630). 

In contrast to the aliphatic diazo compounds, which are invariably colored, all the diazirines so far prepared are colorless. The UV absorption of diazirines corresponds approximately to that of the aliphatic azo compounds. Diazirine shows in methanol an absorption maximum at 321 mja. The IR spectrum of the diazirines shows a band at ca. 1580 cm". ... 

A general method for the preparation of azo compounds is reduction of azines to the hydrazine, followed by oxidation (12,33,128,132). Platinum... 

In this chapter techniques of block copolymer preparation involving thermally labile azo compounds are reviewed. Upon heating, aliphatic azo compounds evolve nitrogen thus forming two carbon centered free radicals. 

Fluorinated 1,2-diazepines (23) can be prepared by the thermolysis of 2,4,6-trimethylphenyl azo compounds with elimination of HF from the Me and F ortho to the azo linkage [84CC832 88JFC(41)439]. The oxidation of these unsymmetrical diareno-1,2-diazepines gave N-oxides and diazepi-nones, depending on the oxidant [89JCS(P1)1117]. 

An important ramification of the photolability of azo-compounds is that, when using dialkyldiazenes as thermal initiators, care must be taken to ensure that the polymerization mixture is not exposed to excessive light during its preparation. 

Bamberger s main achievements were the rediscovery of Blomstrand s diazonium formula and the development of a large number of methods for the preparation of diazoates and azo compounds. In the end, Bamberger abandoned his negative attitude towards the stereoisomerism of the diazoates by reason of his own experiments, which demonstrated the similarities in the oxidation behavior of isomeric series of oximes and diazo compounds (Bamberger and Baudisch, 1912). 

A bicyclo[3.3.0]octane ring system 164 can be conveniently prepared by refluxing an acetonitrile solution of the azo compound 163 in the presence of excess of phenyl vinyl... 

Iodosobenzene diacetate is used as a reagent for the preparation of glycol diacetates from olefins,9 for the oxidation of aromatic amines to corresponding azo compounds,10 for the ring acetylation of N-arylacetamides,11 for oxidation of some phenols to phenyl ethers,12 and as a coupling agent in the preparation of iodonium salts.13 Its hydrolysis to iodosobenzene constitutes the best synthesis of that compound.14... 

ET-IR spectroscopy was employed to investigate the structures of the 1 1 complexes between Li" and the guanidine-substituted azo compounds pyiidine-2-azo-p-phenyltetramethylguanidine and 4,4 -bis(tetramethylguanidine)azoben-zene. Both Li" complexes exist as dimers in acetonitrile solution.The structural chemistry of potassium N,N -di(tolyl)formamidinate complexes has been investigated in detail. These compounds were prepared by deprotonation of the parent Af,N -di(tolyl)formamidines with potassium hydride (Scheme 13). The resulting adducts with either THE or DME display one-dimensional polymeric solid-state structures that exhibit /r-fj fj -coordinated formamidinates. 

Fig. 107.—Tensile strengths of natural rubber plotted against the degree of cross-linking with bis-azo vulcanizing agent (O), expressed as equivalent percent (pXlOO). Upper curve ( ) sample prepared using one equivalent percent of bis-azo compound plus monoreactive ethyl azodi-carboxylate for the total degrees of modification of the units indicated on the ordinate scale. (Flory, Rabjohn, and Shaffer.

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

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