A Mild and Convenient Oxidation of Alcohols Benzoin to Benzil and Borneol to Camphor Thomas S. Straub La Salle University, Philadelphia, PA 19141
The oxidation ofbenzoin to benzil is a popular experiment in elementary organic laboratory texts whether with the semimicro (1-5) or microscale (6-8) approach. The oxidizing procedure usually recommended (1,2, 5, 7, 8) requires heating benzoin with nitric acid for one hour until nitrogen oxide fumes are no longer produced. This procedure is potentially hazardous and environmentally unfriendly. It produces a product that is often contaminated by unreaded benzoin. Also, it is not a general procedure for the oxidation of secondary alcohols. An alternate procedure involving refluxing aqueous copper(I1) acetate as the oxidizing agent has been suggested (3,4,6), but unreacted benzoinis a commonimpurity in the product (5). Although oxidation with copper(I1) acetate auuears to have little advantage over the nitric acid oxidatibn, a recent pnwdural modlijcation may change this 9,. The beet solutlon is to find a general vroccdurc to substitute for the textbook methods. ~ e t h o d using s chromium reagents were automatically eliminated (10).The oxidation of secondary alcohols by bleach in acetic acid (10) seems ideal and readily adaptable to this system, hut repeated attempts under a wide variety of conditions gave little or no product. However, benzil can he formed from benzoin in10 min in almost quantitative yield when oxammonium ion is used as the catalytic oxidizing agent. Bleach with a catalytic amount ofbromide ionis used to continually regenerate the oxammonium ion (11).No molecular chlorine is generated, so traps and heating are not required. The procedure is reported to he quite general (11).There was no difficulty adapting the procedure to the oxidation of borne01 to camphor or working a t semimicro or microscale. The use of oxammonium salts to oxidize primary alcohols to aldehydes and secondary alcohols to ketones is well-estahlished (12). The reagent of choice appears to he the nitrosonium ion derived from 4-methoxy-2,2,6,6tetramethyl-piperidine-1-oxyl(11,131, although 2,2,6,6tetramethylpipendine-1-oxyl(TEMPO) was the first reagent used (12). It is commercially available and proved more than adequate as a reagent for these oxidations. The mechanism of oxidation with TEMPO is under study (14, 151, and a scheme has been proposed for the use of bleach to regenerate the catalytic oxammonium ion (11).Current evidence suggests that the alcohol and nitrosonium ion form an adduct with the loss of a proton, followed by an internal uroton transfer. Even with these restrictions two reaction pathways are possible dependingon the nature of the intermediate adduct (15,.TEMPO gives a much better yield of henzoin from henzil when usea as a catalytic oxidizing agent than a simple oxidizing agent (13). Adaptation to Our Labs The published procedure (11)is quite general and works well, hut changes were made for safety and convenience in the undergraduate laboratory Caution: The oxidizing agent TEMPO is known to be a toxic substance and a severe irritant that is readily absorbed throughthe skin(16).Its properties have not been
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Journal of Chemical Education
HOCl
+
KBr
=
HOBr
+
KC1
thoroughly investigated. It should be dispensed in solution in a fume hood and rubber gloves should be worn. Bleach is a n inhalation and contact irritant. It should be handled with gloves and dispensed in a hood. Benzil, henzoin, borneol, and camphor are irritants. Skin contact, inhalation, or ingestion should he avoided. Dichloromethane is a contact and respiratory tract irritant and a possible carcinogen. One of the wash solutions contains 10%hydrochloric acid. The catalysts were prepared as stock solutions, TEMPO in dichloromethane and KBr in water, to avoid dispensing small quantities of solids. The recornmended volume of reaction solvent, dichloromethane, was cut in half. This saved solvent and allowed the reaction to be monitored. Benzoin is poorly soluble in cold dichloromethane while benzil is quite soluble.
To minimize exposure and to facilitatehandling the small amounts needed, a stock solution of TEMPO in dichloromethane was prepared and stored in the refrigerator to be dispensed as needed. Stock solutions were still active a year after preparation, indicting that the stock solution is quite stable. TEMPO is separated from the reaction product by the heterogeneous nature of the reaction and by aqueous washes with reducing and oxidizing agents. The use of KBr as a second catalyst is strongly recommended. Bleach buffered with sodium bicarbonate is required for good yields. KBr forms HOBr, the agent that oxidizes TEMPO to the nitrosouium ion (11). This study confirmed that KBr had a very favorable effect upon the reaction. The reaction is effective with bleach buffered to a pH of 8.k9.5. Rather than adjust with a pH meter, we simply dissolved 1.0 g sodium bicarbonate in every 50 mL of bleach used. The generality of this method was confirmed by variation in scale and by the alcohol oxidized. The procedure is readily adaptable to both semimicro and microscale quantities, and the oxidation of borneol to camphor works as well as the benzoin oxidation to benzil. However, commercial bleach loses strength over time. It should be titrated, or fresh bleach should be purchased annually. Three sessions of organic chemistry laboratory students proved this exercise to be safe, inexpensive, and easy-to-complete. The students particularly enjoyed the color changes seen during the reaction and the workup: the orange of the TEMPO and oxammonium ion to the yellow of the benzil. The only difficulty arose when student-prepared benzoin was wet with ethanol recrystallization solvent because TEMPO is known to oxidize primary alwhols more rapidly than secondary alcohols (17). Experimental
Regular laundry hlcach was used. Other reagents were purchased from the Aldrich Chemical Co. or were studcnt prep:lrarions used without further purification. Infrared spectra were recorded on either a I'crkin-Elmer Model 727 spertn~meteror a hlattson Polaris FTIH spertromcter. The Semimicro Preparation of Benzil from Benzoin
A magnetically stirred solution of 6.37 g (30 rnmol) of benzoin in 50 mL dichloromethane is treated with 4 mL of 0.75 M aqueous KBr and 3 mL of 0.1 M TEMPO in dichloromethane. The solution was cooled below 15 C in a n ice bath, and the mixture temperature was monitored. A solution of 1.0 g of NaHC03in 50 mL of 5.25% (0.74 MI household bleach is added dropwise to the vigorously stirred mixture, while keeping the reaction temperature below 15 "C. Stirrine is continued for 10 min. The aaueous and organic layers &e separated. Then the organic layer is washed consecutively with 20 mL of 10% HC1 containing 0.250 g of KI,20 mL of 10%sodium thiosulfate, and 20 mL of water. The oreanic laver is dried over 1.0 e anhydrous MgSO,, filtered,and evaporated to dryness. ~ p r o d u & with 82 "C and helow 96'C is sufficiently a meltine ~ o i n above t subsequent reactions ( I ) , or the benzil can bk pure for readily recrystalized from aqueous ethanol. Yields from commercial benzoin are greater than 90%.
any
A Microscale Preparation of Benzil from Benzoin
A magnetically stirred solution of 106 mg (0.5 mmol) of benzoin in 2 mL of dichloromethane was treated with 50 mL of 1.0 M aqueous KBr and 50 mL of 0.1 M TEMPO in dichloromethane. The reaction mixture was cooled below 15 OC with stirring, and 0.9 mL of 5.25% bleach wntaining 0.1 g of NaHCO, for every 5 mL of bleach was added dropwise. The heterogeneous mixture was vigorously stirred for 10 min. The layers were separated with a Pasteur pipet. The organic layer was washed with 0.5 mL of 10%HC1 containing 125 mg KI for every 10 mL of acid, 0.5 mL of 10%sodium thiosulfate ,and 0.5 mLof water. Mixing was done by air bubbles from a squeezed pipet bulb. The final organic layer was filtered through a cotton-plugged Pasteur pipet containing 500 mg of anhydrous MgSO&.The eluant was collected on a watch glass and evaporated to dryness in a hood. Yield: 85 mg (81 %); mp: 86-88 "C. The Preparation of Camphor from Borneo1
The above procedure was followed using 1.0 g (6.5 mmol) of borneol, 15 mL of dichloromethane, 1 mL of 0.75 M aqueous KBr, 0.65 mL of 0.1 M TEMPO in dichloromethane, 11 mL of bufTered bleach (1.0 g NaHC03 for every 50 rnL of bleach). Yield: 0.85 g (86%);mp: 168-170°C in a sealed c a ~ i l l a w(lit. value: 174°C): IR: 2960(s). 1742 (s). 1043 (m).'~ubli"mationfailed to improve either the melting ~ o i nor t the IR s~ectroeram.which mve no evidence of the characteristic b&eol absorbances at 3300 (s), 1107 (m),or 1054 (m). Acknowledgment We thank the students and faculty, Robert F. Polek and William A. Price, of Chemistry 202 Laboratory, Summer, 1989, Spring and Summer, 1990, for their assistance in testine these Drocedures. We also thank the National Science Foundacon for a grant to La Salle University that allowed the purchase of the FTIR spectrometer. Literature Cited 1. Pavia, D. L.: Lampman. 0. M.; Kriz. G. S. I"tlod~di0" 10 0 r g ~ l " Y Laborat0ry Techniques, 3rd ed.; Saundem: Philadelphia, 1988: pp 806307. 2. Ault. A. Tehnlqu48 and Erp~timmtsfor Olgonic Chemistry. 5th ed.; Allyn and Baean: Boston, 1987;p 480. 3. Lehman, . I . W. OperotYnoi Orgo"& chamis*, 2nd ed.; Alb" and Bacon: Boston. 1988;50LL501. 4. Wileol. Jr., C. F. Erprimntol O g m Y Chomutry:A S m d l - 5 m h Appmoch: Maemills": NewYork, 1988: pp431-432. 5. W i i l i m m , K. L. Mocmscoie ondMirroacolP 0r.pnicErpetirnnts; Heath: Lexing. ton. MA, 1089;pP536539. 6. Mayo, D. M.; Pike,R. M.; Butcher, S. S. MYms&OlgonicLabomfory: Wiley: New York, 1986:pp 256258. 7. Pavia, D. L.; Lampman, C M.; &z, G. S.; Engel, R. 0. lnlrodveiran to Organic Lobomtory T e h n i q u m A MicmscaleApprmeh: Saunders: Philadelphia, 1990: pp
-%7_lfQ -. -
8. Williamson,KL.MicmsmieOrgonrcEzp~timn~~:Heath:Bos~n,1987;pp41~20. 9. D e p ~ mP.; , Bethegniea, 0.;M a x i m a l - k f e h . A. J. Chm. Educ 1988.65.558. 10. Mohrig,J.R.;Nienhlus,D.M.;Liek,C.F.;VsnZoeren.Csml:For,B.G.:Maha@, P. G. J Cham Educ 1885,62,519-521. 11. Anelli, P. L.; Banfi, S.; Montanan,F.:Wci, S. J. 0%.Chem. 1988,54,1970-297% Anelli,P L.; B f i , C.; Monfanati, F.: Quiri, S. J Org Chern. lfS7.52.2559-2562; Anell:, P. L.: Montane, F.; Quici, 8.J. OlgonYSynfhesi~1990,60,212-219. 12. Goluhw, V. A ; Rozsntsev, E. G.: N e i m , M. B. Isu.Akod. Nouk SSSR, Ser KAim. 1965,1927-1938:RuIl. h o d . h i . USSR lWB, 1898-1904. 13. Miyacawa, T:Endo, T.:Sh&aahi, S.; Okaaara, M.J O m Chem. 1885.50, 13321334. 14. Golubev, V A,; Borislsvskii. V. N.; Alekendmv, A. L. Ian. &ad N=uh SSSR, Ser KXm. 19'77,2025-2034;Bull. Aeod Sei. USSR 1977,1874-1881. 15. Semmelhack, M . L.; Schmid, C. R.; Cmtes, D. A. Tefrokdmn Left lW. 27,
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16. Material Safety Data Sheet, Aldrieh Chemical Co., June 30, 1989. 17. Semmehck, M. I'.; Chou, C. S.; Codes, D. A. J Am. Chem. Soc 1883, 105,
44924494.
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