Azo compounds azobenzene

The simplest azo compound, azobenzene, exists as a mixture (Scheme 4-18) of a stable trans (4.19) and an unstable cis (4.20) form [38,39]. Formation of the cis isomer is induced by exposure to light, the quantum yield of the process depending upon the wavelength of the light employed [40]. The proportion of cis isomer can be appreciable in an equilibrium mixture. Thus a concentration of 24% of this unstable form builds up within a few hours when an acetic acid solution of azobenzene is exposed to sunlight in shallow white trays. Reversion to the trans form occurs readily on heating and is catalysed by a variety of substances that can function as electron donors or acceptors [41]-... 

These compounds, RN=NR are commonly called azo compounds azobenzene, PhN=NPh is one of the best known. As ligands they can act in 171-, rf-, and bridging modes 171 ligands are trans but the bridging modes can have cis or trans groups. 

Two phenyl rings separated by an azo (-N=N-) bond known as Azobenzene, functions as the base molecule for a wide class of aromatic azo compounds. Azobenzenes almost cover the full rainbow, and up to and almost 70 % of the world s commercial dyes are still azobenzene-based. These chromophores are adaptable molecules and have received much attention, in both fundamental and applied research. Azobenzenes can be categorized into three spectroscopic classes azobenzene-type molecules, aminoazobenzene-type molecules, and pseudo-stilbenes. 

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... 

Unsymmetrical azo compounds must be used to generate phenyl radicals because azobenzene is very stable thermally. Phenylazotiiphenylmethane decomposes readily because of the stabihty of the triphenylmethyl radical ... 

Nitrobenzenediazoate can be considered as an azo compound comparable to an azobenzene having one electron acceptor and one donor on each side of the azo group the acceptor-donor relationship is more dominant in the (Z) -> (E) diazoate pair than in the diazohydroxide pair. The N=N rotation mechanism of the diazoate pair is therefore the favored process (E = 84 kJ mol-1 Lewis and Hanson, 1967). On the other hand, 4-C1 is not a substituent with a —M effect therefore it does not reduce the double-bond character of the N = N bond and the mechanism involving inversion at the N((3)-atom becomes dominant. The activation energy of the latter process (E = 104 kJ mol-1 Schwarz and Zollinger, 1981) is higher than that of the N = N rotation mechanism for the 4-nitro derivative, but it is reasonable to assume that it is lower than that for N = N rotation in the 4-chloro derivative. Furthermore, one can conclude that N-inversion is more favorable in the diazohydroxide than in the diazoate. ... 

Mechanistically these isomerizations have been investigated very little, in contrast to those of azo compounds such as azobenzene and its derivatives. This subject is not... 

As already discussed in Section 7.4, hexamethyidisiiane 857 (which is produced on a technical scale), in the presence of catalytic amounts of tetrabutylammonium fluoride di- or trihydrate in THF, reduces aromatic heterocyclic N-oxides such as pyridine N-oxide 860, quinoline N-oxide 877, or isoquinoline N-oxide 879 to the heterocycles [95] and nitrones to Schiff-bases. Aromatic nitro compounds such as nitrobenzene are reduced analogously to azo compounds such as azobenzene [96]. As mentioned in Section 7.5, secondary aliphatic nitro groups are reduced to oximes. 

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. 

Catalytic hydrogenation of azo compounds over Pt or Ra-Ni often leads to hydrogenolysis. 551 Catalytic transfer hydrogenolysis using cyclohexene and Pd is also used for the conversion of azobenzenes to anilines.552... 

Azobenzene, which contains the chromophore group —N=N—, and is the parent substance of the azo-dyes, is a very stable compound, capable of being distilled without decomposition. The group N=N between the two aromatic nuclei is very firmly bound, although in most other azo-compounds this is not the case. The fastness of the azo-dyes is thus explained. 

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. 

The azo-compound corresponding to azobenzene, azomethane CH3.N=N.CH3, is a colourless explosive gas (at low temperatures a pale yellow liquid) obtained by dehydrogenation of the corresponding hydrazine (hydrazomethane) (Thiele, Ber., 1909, 42, 2575). 

In its simplest form this reaction, by means of which the extremely numerous technical azo-dyes are manufactured, consists in condensation of aromatic diazo-compounds with phenols or aromatic amines to form azo-compounds. From the labile diazo-system the very stable azo-complex is produced. The azo-dyes, therefore, are, without exception, derivatives of azobenzene or else of azonaphthalene, etc. 

Azobenzenes, (29), and analogous heteroaromatic azo compounds, (30), are in aprotic solvents reduced in two sequential one-electron steps to the radical anion and the dianion [61-66]. Disproportionation of the radical anion to the dianion is favored by the presence of Li+ [67]. The dianion is considerably more basic than the radical anion, and the dianion is only stable in very dry nonacidic solvents [64, 65, 67, 68]. Both the dianion and the radical anion derived from (29) have been used as EGBs. The anion resulting from protonation of the dianion is less basic (by several pK units) than the dianion but more basic than... 

Atrazine under Aromatic Amines and Diamines Azobenzene under Azo Compounds Azo Compounds... 

Aliphatic amines have been oxidized to aliphatic azo compounds with iodine pentafluoride [80-81], In the aromatic series, this reagent evidently gives rise to tarry products when used to oxidize amines, although it has been used to oxidize hydrazobenzene to azobenzene. 

The N=N stretching vibration of a symmetrical trans-azo compound is forbidden in the IR but absorbs in the 1576 cm-1 region of the Raman spectrum. Unsymmet-rical para-substituted azobenzenes in which the substituent is an electron-donating group absorb near 1429 cm1. The bands are weak because of the nonpolar nature of the bond. 

Metal complex dyestuffs derived from tetradentate azo compounds (147) have very limited commercial significance but merit brief discussion on chemical grounds. The first dyestuffs of this type to be used commercially contained the o-hydroxy-o -carboxymethyleneoxydiarylazo system, e.g. (148). These were dyed on cotton and aftertreated with a copper salt to produce the copper complex dyestuff which was reported71 to be a triannelated species (149). Formation of complexes of this type has been confirmed105 in the case of 2-hydroxy-5-methyl-2 -carboxymethyleneoxy-azobenzene, which yields the complex (150) on treatment with copper acetate under mild conditions. Under more severe conditions, however, the complex (151) is produced. Loss of the carboxymethyl group from dyes of this type has also been shown105 to occur under dyebath conditions. This may be compared with the demethylation of o-methoxy-o -hydroxydiarylazo compounds (Section 58,2.3. l(iii)(a)) under similar conditions. 

Heilbroner and co-workers found the UV spectra of A-protonated 1,2-diazocine 8 and M-protonated (Z)-azobenzene were very similar. This and other spectroscopic and chemical evidence indicated that upon protonation (Z)- and ( )-azobenzene retain their configuration and are classical rather than bridged or nonclassical cations (60HCA1890). Haselbach and co-workers studied protonated 8 (and other azo compounds) using electron spectroscopy for chemical analysis (ESCA) and concluded that the cation has classical structure 69 as opposed to a nonclassical structure (70) (72HCA705). 

Diazenium dications may also be considered gitonic superelectrophiles, but only a limited number of them have been prepared and studied.97 In principle, diazenium dications could be prepared by double alkylation or protonation of various azo-compounds. To date, only monalkylation has been achieved with strong alkylating agents (eq 57)97a However, Olah et al. showed that diazenium dications can be produced by double protonation of azobenzenes in FSQ3H-SbF5 at low temperature.97b It has also been demonstrated that the diazenium dications are prepared from electrochemical and chemical oxidations (eqs 58-59).97c,d... 

---