Azobenzenes

Further nitration gives w-dinilrobenzene sulphonation gives w-nitrobenzene sulphonic acid. Reduction gives first azoxybenzene, then azobenzene and aniline depending upon the conditions. Used in the dyestufTs industry as such or as aniline. 

N-phenylhydroxylamine, PhNHOH and further reduction can give azoxybenzene, azobenzene, hydrazobenzene and aniline. The most important outlet commercially for the nitro-compounds is the complete reduction to the amines for conversion to dyestufTs. This is usually done in one stage with iron and a small amount of hydrochloric acid. 

Figure C2.4.6. Azobenzene stmcture modified with hydrocarbon chains [53, 54],...

Success of depositing compounds where an 18-carbon chain was attached to one end of an azobenzene group and various different hydrophilic groups attached to the other end has been reported in X and Z mode [52] and piezo-and pyroelectric effects were demonstrated. 

Many workers have used the cis-to-trans stmctural change referred to above and brought about by UV irradiation to change some physical parameter of the LB films fonned from azobenzene derivatives [55, 56, 57, 58, 59 and 60],... 

Nakahara H and Fukuda K 1983 Orientation of chromophores in monolayers and multilayers of azobenzene derivatives with long alkyl chains J. Colloid Interface Sol. 93 530-9... 

Jones R, Tredgold R H, Hoorfar A, Allen R A and Hodge P 1985 Crystal-formation and growth in Langmuir-Blodgett multilayers of azobenzene derivatives—optical and structural studies Thin Solid Films 134 57-66... 

Tredgold R H, Allen R A and Hodge P 1987 X-ray-diffraction and optical studies of Langmuir-Blodgett films formed from azobenzene derivatives Thin Solid Films 155 343-52... 

Nishiyama Kand Fujihira M 1988 Cis-trans reversible photoisomerization of an amphiphilio azobenzene derivative in its pure LB film prepared as polyion oomplexes with polyallylamine Chem. Lett. 1257-60... 

Nakahara H, Fukuda K, Shimomura M and Kunitake T 1988 Moleoular arrangements and photoisomerization of amphiphilio azobenzene derivatives in monolayers and multilayers Mppo/ Kagaku Ka/sh/1001-10... 

Liu Z, Loo B FI, Baba R and Fu]ishima A 1990 Exoellent reversible photoohromio behaviour of 4-ootyl-4 -(5-oarboxyl-pentamethyleneoxy)-azobenzene in organized monolayer assemblies Chem. Lett. 1023-6... 

Liu Z F, Loo B FI, Flashimoto K and Fu]ishima A A 1991 Novel photoeleotroehemioal hybrid one-way prooess observed in the azobenzene system J. Electroanal. Chem. Interfaclal Electrochem. 297 133-44... 

By condensing nitroso derivatives with primary amines. Nitrosobenzene and aniline, for example, readily give azobenzene. This method is seldom used... 

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... 

Azoxybenzene, CeH NOiNCeHs, and Azobenzene, CeH5N NCeH5. (Method 2, p. 208.)... 

The azoxybenzene in turn, when heated with iron filings, readily undergoes C4HjNO NC,Hs + Fe = C.HjNtNC.Hs + FeO further reduction to azobenzene. 

If diazoaminobenzene is dissolved in aniline with a small quantity of aniline hydrochloride and the mixture kept at about 40° for a short time, it is converted in good yield into />-amlno-azobenzene ... 

This view is supported by the fact that if diazominobenzene is dissolved in dimethylaniline in the presence of the hydrochloride of the latter, the main product is p-dimethylamino-azobenzene, CgHjN=NCgHgN(CH3)2 this is because dimethylaniline couples in the nucleus more readily than does aniline. The reaction is an electrophilic displacement of hydrogen by the diazonium ioii ... 

Dissolve 5 g. of finely-powdered diazoaminobenzene (Section IV,81) in 12-15 g. of aniline in a small flask and add 2-5 g. of finely-powdered aniline hydrochloride (1). Warm the mixture, with frequent shaking, on a water bath at 40-45° for 1 hour. Allow the reaction mixture to stand for 30 minutes. Then add 15 ml. of glacial acetic acid diluted with an equal volume of water stir or shake the mixture in order to remove the excess of anihne in the form of its soluble acetate. Allow the mixture to stand, with frequent shaking, for 15 minutes filter the amino-azobenzene at the pump, wash with a little water, and dry upon filter paper Recrystallise the crude p-amino-azobenzene (3-5 g. m.p. 120°) from 15-20 ml. of carbon tetrachloride to obtain the pure compound, m.p. 125°. Alternatively, the compound may be recrystaUised from dilute alcohol, to which a few drops of concentrated ammonia solution have been added. 

To prepare the hydrochloride, dissolve about 1 g. of the compound (which need not be perfectly dry) in about 8 ml. of alcohol. Add this solution to boiling dilute hydrochloric acid (10 ml. of the concentrated acid and 80 ml. of water). Boil for 5 minutes, filter the hot solution if necessary, and allow to cool. p-Amino-azobenzene hydrochloride separates in steel-blue crystals. Filter, wash with a little dilute hydrochloric acid, and dry. 

To recover the free base, dissolve the hydrochloride in the minimum volume of boiling alcohol, add concentrated ammonia solution dropwise until a clear solution results and the blue colour has become fight brown. Add water carefully untU a cloudiness appears, warm on a water bath untU the cloudiness just disappears, and allow to cool. Yellow crystals of p-amino-azobenzene separate on coofing. 

Further reduction in alkaline solution (say, with zinc powder) leads to azobenzene and hydrazobenzene ... 

Reduction of nitrobenzene in methyl or ethyl alcoholic sodium hydroxide solution with zinc powder leads to azobenzene or hydrazobenzene according to the proportion of zinc powder employed ... 

Hydrazobenzene may be oxidised to azobenzene by sodium hypobromite solution at 0°. 

Frequently the recrystallized azobenzeno has m.p. 61°, which is unaffected by recrystallisation from alcohol. Upon distillation from a. 50 ml. distilling flask fitted with a short air condenser, the m.p. is raised to 67-5° and the recovery is about 90 per cent. one recrystallisation from diluted alcohol (as above) then gives perfectly pure azobenzene of m.p. 68-5°. 

Method 2 (from hydrazobenzene). Prepare a solution of sodium hypobromite by adding 10 g. (3-2 ml.) of bromine dropwise to a cold solution of 6-0 g. of sodium hydroxide in 75 ml. of water immersed in an ice bath. Dissolve 9-5 g. of hydrazobenzene (Section IV,87) in 60 ml. of ether contained in a separatory funnel, and add the cold sodimn hypobromite solution in small portions. Shake for 10 minutes, preferably mechanically. Separate the ether layer, pour it into a 100 ml. distilling flask, and distil off the ether by warming gently on a water bath. Dissolve the warm liquid residue in about 30 ml. of alcohol, transfer to a small beaker, heat to boiling on a water bath, add water dropwise to the hot solution until the azobenzene just commences to separate, render the solution clear again with a few drops of alcohol, and cool in ice water. Filter the orange crystals at the pump, and wash with a little 50 per cent, alcohol. Dry in the air. The yield is 8 g. 

Support a 1500 ml. three-necked flask, equipped with a mercury-sealed stirrer and a double surface reflux condenser, on a water bath, and place a solution of 84 g. of sodium hydroxide in 185 ml. of water, 50 g. (41-5 ml.) of nitrobenzene and 500 ml. of methyl alcohol in the flask. Add 70 g. of zinc powder (1), start the stirrer, and reflux for 10 hours. The solution gradually assumes the reddish colour of azobenzene and then on further... 

If the methyl alcohol is distilled off before thorough cooling in a freezing mixture, the yield of hydrazobenzene is appreciably increased, but the product is considerably more coloured due to admixture with a trace of azobenzene. About 12 g. of impure hydrazobenzene may be recovered by distilling off the methyl alcohol from the hltrato after the colourless hydrazobenzene has been collected. 

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