Azobenzene sulfuric acid

Pseudo-stilbenes may emit fluorescence that is, contrary to true stilbenes, generally weak at room temperature and often weak even at low temperatures. Protonated azobenzene-type molecules and many protonated azo dye molecules emit strong fluorescence in sulfuric acid at 77 K with quantum yields of about 0.1. Inclusion of azobenzene in the channels of AIPO4-5 crystals provides complexation of the n-electrons and space confinement. This leads to emission by protonated azobenzene at room temperature. For their cyclopalladated azobenzenes, Ghedini et al. " report quantum yields of ca. 1T0 and lifetimes of ca. 1 ns. In contrast, donor/acceptor pseudo-stilbenes, if emitting at low temperatures or when adsorbed to surfaces, are weak emitters. In textile chemistry, it has long been known that azo dyes adsorbed to fibers may show fluorescence. ... 

The results reported in Reference 18 need some comments. First, Sharma and coworkers25, who studied oxidative voltammetry of aniline on graphite electrodes in acidic conditions, observed that in 0.1 M sulfuric acid the radical cation (4) undergoes not only head-to-tail and tail-to-tail but also head-to-head coupling to form p-aminodiphenylamine, benzidine and hydrazobenzene (23), respectively 23 is oxidizable to azobenzene (24). 

In a series of papers Lewis and co-workers have reported a comprehensive study of the photochemical reactions of azobenzenes in sulfuric acid. In the case of azobenzene itself, the products obtained were benzo[c]cinnoline (48%) together with benzidine (35%), and other azobenzenes gave products derived from the corresponding hydrazo compounds. The mechanism proposed is outlined in Scheme 2. There is an initial rapid establishment of a cis-trans equilibrium of monoprotonated azobenzene, with cyclization involving an excited form (14) of the cis-isomer, the lowest transition now being n-n. In the final step, dehydrogenation of the photocyclization product, 5,6-dihydrobenzo[c]cinnoline, is brought about by a second molecule of azobenzene, which is itself reduced to hydrazobenzene and... 

White io yellowish, odorless and bitter crystalline powder, mp 98-99.4 (Pfeifer) 102-104 from heptane (Tenor)- Eutectic temp of mixture with azobenzene 48. Very sol in water, liV sulfuric acid, 10% ammonium hydroxide easily sol in methanol, isopropano], n-butanol. chloroform and benzene sol in ether. Practically insol in hexane, hepiane. PKl . 79. uv max (methanol) 222, 270, 307 nm (log e 4.28, 3.83, 4.28). Very stable except under extremely alk conditions. LDW in mice, rats (mg/kg) 115, 76 i.v. 380, 235 orally 155, 100 i.p. 132, 100 s.c. (Fuller). Hydrochloride, C)( HieClN5, mp 212". 

The irradiation of azobenzene in 66% sulfuric acid follows this reaction mechanism. The reaction scheme of the partial reactions is written according to Section 3.1.4... 

Fig. 5.22. Absorbances plotted versus time of irradiation for the reaction of rranj-azobenzene in 66% sulfuric acid. ao = 5.5l x 10" mol I", /q= I0.36x I0" Einstein cm" s". The experimental absorbances are shown as dots. The line gives the calculated absorbances using the...

Fig. S.23. Absorbances versus irradiation time for cis-azobenzene in 66% sulfuric acid. bo = 5.51 X 10 mol I", Iq = 10.36 x 10 Einstein cm s . The experimental absorbances are shown as dots. The line represents absorbances calculated using constants zxj determined in the trcms-azobcnzene irradiation.

Fig. 5.24. EDQ-diagram of the photoreaction of azobenzene in 66% sulfuric acid, azobenzene O, irradiation of cts-azobenzene.

Pseudo-stilbenes may emit fluorescence that is, contrary to true stilbenes, generally weak at room temperature and often weak even at low temperatures. Protonated azobenzene-type molecules and many protonated azo dye molecules emit strong fluorescence in sulfuric acid at 77 with quantum... 

In a reaction analogous to that of stilbene, protonated azobenzene iso-merizes at room temperature under irradiation with ( )e, z = 0.27 and = 0.25. Subsequently, the dehydro-photocyclization starts from the photo-stationary E/Z mixture to give protonated benzo[c]cinnoline with a yield of (j) = 0.02. This is parallel to the stilbene-to-phenanthrene reaction, and it proceeds not in concentrated sulfuric acid where the thermal Z -> E reisomerization is fast, but best in ca. 66% acid. Protonated benzo[c]cinnoline and hydrazobenzene are formed, and the latter undergoes the benzidine rearrangement. The complexation with metal ions in azobenzene crown ethers under participation of the n-electrons of the azo group leads to an increase of the E Z quantum yield from 0.25 of the metal free compound to 0.4 to 0.6 in the Ba and Ca "" complexes. The Z E yield decreases from 0.18 to 0.13 and 0.05. If the crown is larger, the values increase to 0.35 and (pz-iE = 0.45. " ... 

Rau H, Crosby AD, Schaufler A, Frank R. 1981. Triplet sensitized photoreaction of azobenzene in sulfuric acid. Z Naturforsch, A Phys Sci 36(11) 1180 1186. 

Classification Azobenzene Empirical C23Hi8CIN4Na07S2 Properties YIsh.-brn. powd. sol. in water, alcohol, alkalis, cone, sulfuric acid si. sol. in ethanol, acetone insol. in other org. soivs. precipitated by dil. acids m.w. 584.98 Toxicology TSCA listed Uses Dye for wool, silk in the isolation of polymyxin... 

At the end of the first forward scan, three oxidation products II, IV and azobenzene are formed. This results in the appearance of three reduction peaks during the first reverse (reduction) scan. Cyclic voltammograms obtained for the oxidation of ortho- and /neto-toluidines in 0.1 M and 2.0 M sulphuric acid are shown in Figure 13.1(a)-(d) and the voltammogram obtained for the oxidation of/ -toluidine in 0.1 M sulfuric acid is shown in Figure 13.1(e). The concentration of toluidines was 5 X lO M [80]. All the three toluidines oxidize in a single step and it involves a two-electron change in the aqueous medium. The oxidative voltammetric behaviour of o- and w-toluidines is very similar to that of... 

The scattered measurements of basicities of azobenzenes have been reviewed by Jaff6 et al. (194,368,370). Some 32 mono- and di-substituted azobenzenes were studied by him as indicators in 20% ethanolio aqueous sulfuric acid generating an acidity function which was tied to the aqueous Ho scale through measurements on p- and 0- nitroaniline under the same conditions. An excellent matching of Ho values for different members of the series shows that a good acidity function is possible in this medium for molecules of similar structure. 

The azoxybenzenes may be considered as oxides of the azobenzenes and it may be seen through comparison of Table I with Table IV that there is nearly the same difference between the basicities of comparable azo- and azoxybenzenes as between pyridines and their oxides. Accordingly, the azoxybenzenes are quite weak bases. To our knowledge the recent study in 20% etbanolic aqueous sulfuric acid by Hahn and Jaff6 (160) is the only quantitative investigation of these compounds. They conclude that protonation occurs on oxygen and discuss the application of various substituent parameters to the azobenzene and azoxybenzene systems. 

It has been known for some time that irradiation of azobenzene (324) in either 22 N sulfuric acid350 351 or acetic acid with added ferric chloride 352 yields benzo[c]cinnoline (325). This is accompanied by the formation of an almost equal quantity of benzidine (326), undoubtedly arising by rearrangement of hydrazobenzene (327). The mechanism of this reaction differs, therefore, from that of the stilbene cyclodehydrogenation, and azobenzene itself functions as the hydrogen acceptor. Yields of not more than 50% of benzo[c]cinnoline are generally observed. 

Cu2(L )(NCS)2] (66), which has been shown (c.f. Fig. 8) [43] to contain two tetrahedrally coordinated Cu(I) ions held 2.796(8) A apart and linked inter-molecularly via the sulfur atoms of the thiocyanate ions. For the complexes 63 and 64, the reduction product is the diamagnetic complex [Cu2(L )(MeCN)2](Y)2 (67) (Y = CIO4 or BPh4) in which each three coordinate Cu(I) ion is bonded to two of the four macrocyclic nitrogen atoms and to the nitrogen of one of the two MeCN molecules (Fig, 9) [43]. In the presence of certain substrates the reduction of 63 or 64 is accompanied by substrate oxidation. For example, PhSH, PhC=CH, hydrazobenzene, catechols, hydroquinone, and ascorbic acid afford PhSSPh, PhC=CCsPh, azobenzene, o-quinones, p-quinone, and dehydroascorbic acid, respectively, together with the reduced species 67 and/or other copper complexes... 

Likewise, diphenyl selenoxide has been shown to convert aromatic amines to the corresponding azobenzenes, while, in contrast to their sulfur analogs, benzeneseleninic acid and its anhydride are well established as versatile oxidants in organic chemistry. ... 

Tin or tin(n) chloride,171 zinc in an acid medium,172 sodium dithionite,173 and sulfur dioxide174 are also effective for reducing A-oxides. Azoxy compounds are converted into azo or hydrazo compounds by relatively mild reducing agents azoxybenzene is reduced to hydrazobenzene by zinc in alkaline solution10 and to azobenzene (99% yield) by lithium tetrahydridoaluminate.35... 

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