Palladium Catalyzed Reduction of Nitrobenzene John A. Mangravite West Chester State College, West Chester, PA 19380 One of the more popular organic transformations that is carried out in the undergraduate organic laboratory is the reduction of nitrohenzene to aniline. This reaction, is usually nerformed as Dart of a multi-sten seauence used to oreoare sulfanilamide'or some other suifa diug. Tradition&; the reduction has been accom~lishedusine either iron or tin and hydrtxhloric acid. Buth ,,I ~lieaeprwed;res arc rnl htr Ieng hy. and uauallv inwlvr 3 ret'lux ~wricod.31 least one ;ream di>lillation, an extraction with either C H & I ~or ether, and a final distillation. in order to obtain Dure aniline. At least two laboratory periods are needed a n d t h e reduction step, especially in the case of iron, is somewhat messv. Reeve and Lowe ( 1 ) have developed an alternate proceduie in which the aniline is not isolated hut converted directly, after the reduction step, into acetanilide. While such a procedure is attractive, i t does not allow for IR and 'H-NMR analysis of aniline. T h e development, over the past ten years, of anumber of transition metal catalyzed reactions (2).which have increased the efficiency and yield of functional group conversions, has prompted us to investigate the applicability of palladium catalyzed reductions to the undergraduate laboratory. In this paper, we compare two PdIC reducing systems, to the FeiHCl and SnIHC1 reductions and suggest an efficient, clean, and inexpensive procedure for the conversion of nitrobenzene to aniline. Experimental
Comparison of Varlous Reducing Systems in the Conversion of Nitrobenzene to Anillne Product Yield (%) Crude Purified
Method (1) (2) (3)
Fe and HCI Sn and HCI Pd/C:
78.5 80.0
Et3NH+Hc0*(4) PdIC;
Costa Per Student
Time (hours)
-
75.0 77.0 83.5
$0.75 $5.60 $1.50h
7 4
-
80.0
$4.20b
3
6
rvclnhexene
'Total cost of all reagents used for conversion of 15.4 g of ninobenzene. This cost decreases each time catalyst is reused.
Results and Discussion The table presents a comparison of the two palladium catalyzed reductions with the FeIHC1 and SnIHC1 procedures. These comparisons are the result of three preparations each for all of the methods. The reduction with SnIHC1 is more attractive than the F e B C 1 reduction in that the overall yield is slightly higher and the product slightly purer, as evidenced by IR analysis. The crude SnIHC1 reduction product is only slightly yellow; whereas, the FeIHCI product was brown. Both ~roceduresreauire two lahoratorv s the SnIHC1 . ~ e r i o d and reduction is 5 to 10 times more expensive. As o u ~ o s e dto the traditional reductions. the Pd/C nrocedures &e an overall higher yield of pure aniline and rkquire onlv one laboratorv oeriod. Since the catalvst can be easilv recovered and reuskd with equivalent results, the procedures are cost competitive with the FeIHCl system. While we have successfully reused recovered catalyst two times, Entwistle e t al. ( 4 ) reDort that recovered catalvst could he reused UD to six times before any decrease in eificiency is noticed. The PdIC-triethylammonium formate reduction gives a slightly higher yield than the PdIC-cyclohexene system; however, we would recommend the latter procedure for the undergraduate laboratory. It is somewhat shorter requiring only a reflux period, a vacuum filtration, and a distillation, and it eliminates the need for the fairly toxic triethylamine. The initial addition procedure in the PdIC-triethylammonium formate system is also quite exothermic and might get out of control when carried out by an inexperienced investigator. &
Reduction with 5% PdlC and Triethylammonium Formate The following is a modification of the procedure of Cartese and Heck (31. Into a 250-mL, three-necked flask, fitted with condenser, addition funnel, and thermometer, was placed 15.4 g (0.125mole) of nitrobenzene, 0.30 g (0.1 mole%)of 5%PdIC, and 53 g (0.525 mole) of triethylamine.The mixture was stirred magnetically while 25 mL of 97%formic acid was slowly added over a l-hr period. On addition of the formic acid. the mixture fumed and started to heat uo. Remixture was stirred for an additional 1hr, after which time the mixture was coaled to roam temperature. Two layers formed. The bottom lay& was excess amine formate. Methylene chloride, 100 mL, was added to dissolve all components and the catalyst was removed by vacuum filtration. The methylene chloride solution was transferred to a separatoryfunnel and washed twice with 75-mL portions of water, and once with 75mL of asaturated NaClsolution. The organic layer was dried over anhydrous sodium sulfate, and filtered into a 125-mL RB flask. Distillation afforded a law boiling fraction containing methylene chloride and a trace of triethylamine,and a fraction, 9.7 g (83.5%) of pure aniline; b.p. 183-185', as evidenced by its IR spectrum. The recovered PdIC catalyst was washed with ethanol and air dried. This recovered catalyst was reused in a second and third preparation with equivalent results.
Acknowledgment The author wishes to thankProfessor Richard F. Heck for helpful discussion of palladium catalyzed reactions.
Reduction with 10% PdIC and Cyclohexene in Ethanol The followine is an adaotation of the oracedure of Entwistle et sl.
HAZARD WARNING Both nitrobenzene a n d aniline a r e acutely toxic. This euperimvnt i l i ~ u l d1,t. c : { r r i d
