George Giaros Russell Sage College Troy, New York 12180
The Oxidation of Primary Alcohols to Aldehydes with Pyridinium Chlorochromate An organic chemistry experiment
Aldehydes a r e a n important class of organic compounds a n d as such their preparation is important in t h e i n t r o d u c t o ~ organic c h e m i s t 6 course. s t u d e n t s , however, are often led believe t h a t primary alcohols m a y b e oxidized to aldehydes with acidic dichromate if only the conditions of t h e reaction a r e controlled (I).T h i s usually m e a n s t h a t t h e aldehyde is distilled off a s it is produced to avoid its further oxidation t o a carhoxylic acid. T h e r e is a published procedure for t h e preparation of propanal from i-propanol (2) although m a n y laboratory manuals avoid t h i s experiment. T h e procedure recommends a three-neck flask, fractionating column, d r o p ping funnel, two thermometers a n d several clamps, rings, a n d ring stands, h u t its major drawback is its lack of generality. Dichromate oxidation of primary alcohols to aldehydes is limited to t h e preparation of aldehydes w i t h boiling points under 100°C (3). Collins reagent, dipyridine chromium (VI) oxide in methylene chloride, has been for years the method of choice for the oxidation of primary alcohols to aldehydes ( 4 ) . This reaction is general far the oxidation of a large number of primary alcohols and gives good yields of aldehydes, hut it is impractical for use in undergraduate laboratories. Collins reagent must be prepared and used with considerable care; it is hygroscopic and requires that a large excess of reagent be used. Recently Corey and Suggs (5)have described the use ofpyridinium chlorochromate (CsH5NHCr03Cl) for the oxidation of primary alcohols to aldehydes. Pyridinium chlorochromate is easily and safely prepared (51,stable for extended periods of time at room temperature, is not hygroscopic, and is also commercially available. The experimental orocedure utilizes a simole reflux aooaratus. reouires onlv a We have found that pyridinium chlorochromate can he readily utilizd in the undergraduate laboratory to illustrate the preparation of aldehydes from alcohols. The equipment is easily constructed, the reaction and purification by distillation are straightforward, and the solvents may-he used without special drying. m he reaction is mildly exothermic so a reflux condenser should be used to prevent the escape of methylene chloride int,o the atmosphere, although the reaction mixture is not refluxed. Last year at Sage, students taking Organic Chemistry I1 obtained the following yields of aldehyde from the appropriate alcohol: pentanal, 15-20%; hexanal, 15-60%; and benzaldehyde, 30-80%. I t was noted that the preparation of low hoilingal-
410 1 Journal of Chemical Education
dehydes such as hutanal and pentanal resulted in very low yields of isolated product because of the difficulty in carrying out a fractional distillation on the mixture of methylene chloride, ethyl ether, and aldehyde. Infrared analysis of the isolated products, however, indicated that the aldehyde was not contaminated by either carhoxylie acid or unreacted alcohol. We have included the procedure for the preparation of benzaldehyde since it was obtained in the highest yields and has the least ohjectionahle odor. The procedure can, of course, be applied to the preparation of other aldehydes and is essentially that described by Corey and Suggs ( 5 ) with only minor modifications. Procedure; A 500-ml round-bottom flask is charged with 32.3 g (0.15 mole) pyridinium chlorochromate, 150 ml methylene chloride (Fisher Certified Reagent used directly from the bottle), and fitted with a reflux condenser. Benzylalcohol(10.8g, 0.1 mole) in 20ml methylene chloride is added slowly through the condenser with swirling' to the susoended ovridinium chlorochromate. The reaction mixture turns , hn.wn-l,lack and the mrthylrw chloride (1,p 10°('1 may begin ro hod gentlv After nllown~the rm, Iiun 1') pn,cecd with rrrasitm4 zwirlmg for 1.5 hr, 200 ml of anhydrous ethyl ether (Mallinekrodt AR used without further treatment) is added, and the supernatant liquid decanted from the gummy black residue. The residue is washed twice with 25-ml portions of ether and the combined ether-methylene chloride solution is filtered through a l-cm pad of alumina (or Florid) in a Buchner funnel. The solvent is removed bv distillation and the oil that remains (which may be colorless or hrown-green) is purified by simple distillati~n.~
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Literature Cited (1) Morrison, Robert T.. and Bgvd, Robert N., "Organic Chemistry,"3?d Ed.. AUyn and Bacon,Ine.,Baton. 1973, pp. 52&5B,Temsydr. AndrewL.,"ConkmporaryO~anic Chemistry: W. B. Seundera Company, Phiisdrl~hia, 1976. p. 308. (2) Swinehart. James S., "Organic Chemistry: An Experimental Approach,(( AppktonCentury-Crofts, New York, 1969, p. 312. (3) Streitwierer. Jr., Andrew, and Heathcock, Clayton H.,"Introduction UI Onanic Chemistry.l'MsrmillsnPubiishingCo.,Inc..New York, 1963,pp. 232and 357. (41 Collinr, J. C., Hem. W. W.,and Frank, F. J., Tetrahedron Latr., 3363 (1968):Collina. J. C..and Hess. W. W., Orp. S y n . 52.5l19721. ( 5 ) Corey, E. J.,andSugg~,J. W., Telroh~dronI,ett.,2647 (1975).
Presented at the 7th Northeast Regional Meeting of the American Chemical Society, held at the State University of New York-Albany, August 1976. 'While the use of a magnetic stirrer is desirable it is not essential. Zhrey and Suggs recommend the use of a short Vigreua column, but we found it unnecessary for the preparation of benzaldehyde.
