Preparation of t-Nitrosobutane Dimer Palle E. lversen University of Aorhus DK-8000Aorhus C, Denmark
A combined experiment in organic
I
synthesis a n d electrochemistry
The growing interest in organic electrochemistry has recently been reflected by the publication of three books (1-3) on this subject and certainly more will appear. However, there is still a large gap between the present state of research in the field and that covered in common textbooks of organic chemistry, even the more advanced ones. We have tried a t the University of Aarhus to incorporate some electrochemical experiments in our third year laboratory course in organic chemistry while exposing the student to a variety of synthetic, analytical, and spectroscopic techniques in one integrated course. The present experiment has been designed to demonstrate different oxidation stages of aliphatic nitrogen compounds and includes two different chemical oxidations, one electroreduction (electrosynthesis), and one electrooxidation (anodic polarography). The final product can be used directly in a later esr-experiment on trapping of primarilv formed radicals as nitroxides (4). Experiment The experiment consists of four steps, each repre-. senting a general synthetic or analytical method. (7) W(V1)-Catalyzed HzOz-Oxidation of t-Butylamine to t-Nitrobutane ( 5 )
t-BUN&
- H/&
Ns2W0,
t-BuNO
t-BuNO,
(2) Cathodic Reduction of t-Nitrobutane to t-Butyihydroxyiamine in Acid Medium (6)
- ZP-
+ m+
EXOH, HCI
[t-BuNO]
2-
+ an+
t-BuNHOH
As C-nitroso compounds are generally more easily reduced than the corresponding C-nitro derivatives, i t is not possible to stop the reduction a t the nitroso stage. However, the catholyte is already blue before the current is turned on, because the product from step (1) is used without purification and contains a small amount of the blue t-BuNO monomer.
For example, Eastman Kodak, :I1014
+
(4) Br2-Oxidation of t-Butylhydroxylamine to t-Nitrosobutane in Alkaline Medium and Dimerization (9)
A 20% excess of bromine (based on crude, moist hydrochloride from step (2)) is used for the oxidation and gives very satisfactory yields. A skilled worker will get 70-8070 in step ( I ) , about 90% in step (2) (correspondingly 60-70% in step (3)), and 7585% in step (4). The scale has been chosen so that even unskilled students will get some product from the last sten of the series. If desired. some laboratory time might be saved by omitting step (1) and preparing larger bGch of t-nitrobutane (10) or using a commercial product.'
a
Exoerimental Procedure
The product is a mixture of t-nitroso- and t-nitrobutane which can be used directly in the following electrolytic step (2).
t-BuNO,
(3) Quantitative Determination by Anodic Polarography (7) The overall yield of steps (1) (2) is conveniently determined by means of the anodic wave of the hydroxylamine in alkaline medium. The polarographic analysis is most practically done during the evaporation of the catholyte from step (2) to isolate the t-butylhydroxylamine hydrochloride which is used directly for the last step (4). The mechanism for the electrode reaction of step (3) has recently been discussed (8).
Apparatus The electrolytic equipment has been described earlier (11). For the present experiment an H-type cell made from 250-300-ml conical flasks (175-200-ml cathalyte volume) is adequate. The agar plug is omitted, and the Ag/AgCl reference electrode is filled with 4 N hydrochloric acid instead of saturated aqueous patassium chloride to avoid contamination with inorganic material if the pure hydrochloride should be wanted (e.g., analytical sample for step (3)). A Radiometer PO4 instrument was used for the polarographic analysis. (7) Oxidation of t-Butylamine
To a 100-ml 3-necked flask equipped with stirrer, reflux condenser, and dropping funnel, 1 g of sadium tungstate (Na2W01 2H10) and 20 ml of water are added. The salt is partially dissolved with stirring, and 10 ml of methanol and 10.5 ml (1.3 g = 0.1 mole) of analytical grade t-butylamine are added. During 3&45 min 40 ml of fresh 35% aqueous hydrogen peroxide (-0.48 mole) is added with vigorous stirring, cautiously in the beginning because of the exothermal reaction. The reaction is run at 40-
Volume51, Number 7. July 1974 / 489
4 5 T , but the temperature is not critical, and the reaction mixture can he cooled in a water hath if the reaction becomes taa violent, and the gaseous monomer t-nitrosobutane starts to escape through the condenser. Ether can be added through the condenser, if solid dimer t-nitrosobutane is formed in the condenser. After addition of the hydrogen pemxide the mixture is stirred for 1 hr. Five milliliters of ether is added and then 10-15 g of sodium chloride is added to salt out the product. The mixture is poured into a 100-ml separating funnel, and the flask and condenser are washed with 10 ml of ether which is added to the separatory funnel. The hlue organic layer is separated, and the aqueous phase extracted with 10 ml of ether. The combined ethereal solutions are washed with 15 ml of 4 N hydrochloric acid to remove unreacted amine and finally with 10 ml ofwater.
(2) Electrolytic Reduction The three compartments of the cell are filled with a 1:1 mixture of 4 N hydrochloric acid and ethanol, and the hlue ethereal solution from step (1)is added to the cathode compartment in such a way that all liquid levels have the same height. The medium has been chosen to give a reasonable solubility of the organic substrate, a good conductivity, a harmless anode reaction, and to facilitate the work-up (12). For the present electrolysis the cell is not purged with nitrogen before and during the electrolysis as this would cause some of the volatile monomer to escape reduction, The electrolysis is performed overnight (18-20 hr) in a water bath at -0.9 V versus Ag/AgCI. The next day the clear colorless catholyte is poured into a 250ml volumetric flask after separation of the mercury which (together with the cell) is washed with a little ethanol which is added to the catholyte and the volume adjusted to 250.0 ml with water. (Remember to shake the flask!). A 0.5-ml aliquot is withdrawn, diluted to 10.0 ml, and analyzed by anodic polamgraphy. The rest of the catholyte is evaporated to dryness in a weighed flask on a rotatory evaporator. This takes about 1 hr during which the anodic polarographieal determination can he made.
(3) Polarographic Analysis Firsr a double derermination of the contents ol t.burylhydroxy. lnmmr in rhr diluted 10 U-ml inlution from step 121 IS made by o h l i n g a 1.11-rnl alquor to 25 0 ml uirh aoueous alkaline sodrum sulfite ~(7)and recoiding the anodic pala&ams from -0.1 to -0.9 V. Then a standard curve is constructed from polamgrams of aqueous solutions with a known concentration of pure tbutylhydroxylamine hydrochloride. Usually 0.5, 1.0, 1.5, and 2.0 ml aliquots of a solution of 10-15 mg/lO.O ml are diluted to 25.0 ml with the alkaline sulfite solution and the polarograms recorded. However, it is advisable to start with the 1.0-ml standard and compare the wave height with the height of the unknown to
490 / Journal of Chemical Edocafion
he sure that the unknown will fall within the range of known concentrations. The wave height of the standards are plotted against concentration (it is convenient to use mg/25 ml) to give a straight line (7) from which the unknown concentration is determined; the analytical yield of steps (1) and (2) is calculated and compared with the electricity consumption and the yield of crude t-butylhydroxylamine hydrochloride fmm the evaporation.
(4) Oxidation of t-Butylhydroxyiarnine The weight of the crude moist hydrochloride from step (2) is determined, and 4 equiv of sodium hydroxide (based an t-BuNHOH HCI, mol wt 125.5), 30 ml of water, and 1.2 equiv of hromine are placed in a 100-ml 3-necked flask equipped with stirrer and dropping funnel. The mixture is cooled in an ice hath with stirring, and the hydrochloride dissolved in 20-25 ml of water is added over a 5-min period. A blue color develops immediately and gradually disappears, while the practically colorless dimer tnitrosobutane separates during further 1-2 hr of stirring on the ice hath. The product is filtered with a Biiehner funnel (greater lumps broken into smaller pieces), washed 2-3 times in the funnel with cold water, and dried in a vacuum desiccator over calcium chloride. The solid compound has a high vapor pressure and will disappear completely if air drying at room temperature is attempted. The desiccator is evacuated a t an aspirator at room temperature, hut not for an extended period. Yield about 5 g, mp 78-9°C (uncor, closed tube); lit. 83-4°C (9). The substance should he kept tightly stoppered in the refrigerator.
.
Acknowledgment T h e a u t h o r expresses his sincere gratitude for t h e skillful a n d interested cooperation of a n u m b e r of s t u d e n t s a n d t h e following technicians: Mr. J. Andersen, Mrs. K. Skov, Mrs. I. Ottzen, Miss E. Philip, a n d Mrs. B. Villadsen, during t h e working o u t a n d checking of t h e procedure. Lileralure Ciled (1) Bsizex, M. M . (Editor), "Oqanic Electcochemistry, An Introdunion and A Guide." Marcel Dekkerlne., NewYark. 1973. (2) Fry. A. A.. "Synthetic Organic Eleefroehemirtry." Harper and Row. New York. Evanston. San Raneiro, London. 1972. (31 Ebemn. L.. and SchBter, H.. "Organic Electroehemistry," Fortach,. Cham. For8eh.. Vol21,Sp~nger-Verlag.Berlin-Heid~lborg-Nw York. 1971. K..AcLo Chem Srond.. 23.522 (19691. I41 Forshult, S., Lagercranfr. C., and TOTTJOII. I51 Sfowell, J. C.. J. Org. Chem.. 36. 3055 l1971l. 16) Iucrren.P.E..sndLund. H.. T~rrohedmn1,df..4027.(1967). (7) Iwrsen,P.E.,andLund,H.,Anni. Cham.. d l , 1322(19691. (81 Iversen, P.E., C k m . Rsr., IO4.2195(19711. (91 Emmons, W.D.. J.Amer Cham. S o r . 79.6522(1957). (101 Calder. A,, Torrestor. A R . . end Hephum, S . P.. O r 8 Synth.. 52.78(19721. (111 1verson.P. E.. J. CHEM.EDUC., dR,136(197L1. (121 Iversen. P. E..ArtaChem. S
