Laurence Huestls Pacific Lutheran University Tacoma, Washington 98447
Sodium Perborate Oxidation of an Aromatic Amine
A synthesis which we have used for several years i n our organic chemistry laboratory involves t h e sodium perborate oxidation of aromatic primary amines t o t h e corresponding azo compound. 2NaB03. 4H20
+ 2ArNH2
-
2NaB02
+ ArN = NAr + 10Hz0
This reaction was first described i n 1952b y Mehta and Vakilwala' a n d its major virtues for laboratory use a r e its: (1) simplicity, (2) colorful product, (3) use with a wide variety of relativel; inexnensive ~ a r a - s u b s t i t u t e danilines. a n d (4) use of the easily h k d l e d sddium perborate as a n oxidizing agent. Furthermore. students often obtain better vields of t h e azobenzenes than were reported by Mehta a n d Vakilwala. T h i s usually leads t o questions about the nature of the reaction and upon learning t h a t the mechanism and even the nature of t h e oxidizine is unknown. t h e reaction assumes new sie. soecies . nificance and provides a n &ellent opportunity for speculation on the nature of the reaction and a review of factors which affect t h e yield. A number of these factors were investigated durine oxidation of P-chloroaniline a n d two of them, oxidant-;mine ratio and solvent, were found t o he important. Sodium Perborate-Aromatic Amine Ratio Rather than restrict the oxidant and amine to a 1:l ratio as was previously described,' a wide range of oxidant-mine ratioa were used with the results shown in the graph. These results show that the yield can be considerably improved over the reported 48%and that the yield is linear with respect to sodium perboratesolong as thesodium perborate is the limiting reagent. The least squares straight line does not begin at zero and it appears that this deviation is due to the solubility of 4.4'-dichloroazobenzene in acetic aeid at the temperature at which the azo-compound is collected (20°C). The least squares line intermots - of ~~.~at -3.6% and calculation of the statistical error at a dezree confidence of 0.95' gave the intercept as -3.6% + 1.9. In terms of sduhil~tyof 4.4'-dichll~roarohenzene.this represents a value ofO.13 + 0 0: gIli10 ml of acetic acid at 20°C. When the solubility of this compound wasdetermined experimentally by saturating acetic aeid with 4.l-dichloroazobenlene ar 20°C followed hy evaporation IL the acetlr arid frum an alrquut oi this aaturawd solution. the snluhility uazfound t~~beO.IReI100mlofareticacid. A~~h~wn.thevieIdof the azo-comnound is noioneer linear with resnect to sodium verborate " when the oxidant is stoichiometrieslly in excess. One probable cause of this decreasing rate of yield is further oxidation of the azo-compound to the correspondingazoxy-compwndand such a product can be isolated from the acetic acid filtrate (see general procedure). ~~
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Solvents The number of comwunds which can function as "solvent" is limited since the oreani'c medium must dissolve both sodium oerho~. rate and the amine. Tnak with 1,2-propanediol,acetonitrile, hexamrt hylphosphoramide, dimethyl sulfoxide. et hylene rarhonale, and tetramethylene sulfone either would not dissolve sodium perborate or the sodium perborate reacted with the solvent often with oxygen evolution. In the case of acetic acid, it too is more than a solvent as indicated by the precipitation of only sodium borate when a hot solution of sodium perborate in acetic acid was cooled. Thus it appears ~~~
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OXIDANT-AMINE
RATIO
Yield of 4,4'dich!oroazobenzena versus ratio of sodium perbarate to 4410rmnillne. that sodium perborate is oxidizing acetic acid to peracetie acid which is the probable oxidizing agent in the formation of the am-compounds. An analogous formation of peracetic acid occurs when acetic acid is warmed with 30% hydrogen p e r o ~ i d eFurthermore, .~ the oxidation of primary amines to m-compounds by peracetic acid has been noted in the oxidation of 1,l-diethylhydrazine to tetraethyltetrazine;'
General Procedure for Oxidation of Aromatic Amlnes Oxidation of 4-Chloroaniline to 4.4'-Dichloroazobenzene. A mixture of 2.31 g (15.0 mmole) of sodium perborate in 20 ml of glacial acetic acid was heated to 80'C for 10 min. The resulting solution was added rapidly to a solution of 1.28 g (10.0 rnmole) of recrystallized 4-chloroaniline (m.p. 70-1lDC) in 10 ml of glacial acetic acid which had been heated to 60%. The sodium perborate solution flask was washed with 5 ml of glacial acetic acid (20°C) and this acid wash was added to the reaction mixture. The 35 ml of reaction solution was kept s t approximately 60% far 1.5 hr. It rapidly became dark red and crystals began to form inahout 5min. After the 1.5hr heatingperiod, the mixture was cooled to20°C and vacuum filtered togive a brownish orange mass of crystals. These crystals were washed with water, 5 ml of glacial acetic acid, and again washed with water. After air drying, the 0.877 g (69.6%)of bright orange needles melted at 164186T (m.~. 187'C1).
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'Mehta, 5.M., and Vakilwala, M. V., J.Arner. Chem. Sac., 74,563 i1957.1 %Freund,J. E., "Modern Elementary Statistics," 44th ed. PrenticeHall, Englewwd Cliffs, 1973 3Fieser, L. F., and Fieser, M., "Reagents for Organic Synthesis," John Wiley, New York, 1967 Vol. I, pp 459-460 'Horner, L., and Fernekess, H., Ber., 94,112 (1961)
Volume 54. Number 5, May 1977 / 327
