5. H. Goh University of Molaya Kuala Lumpur, Malaysia
Dihalocarbene Addition Reaction An undergraduate laboratory experiment
T h e importance of carbenes in synthetic and mechanistic organic chemistry is often emphasized in undergraduate teaching programs ( 1 ) . However, in the laboratory i t is frequently difficult to devise simple experiments t o illustrate some reactions of typical carhenes. For instance, the preparation of dichlorocarhene-olefin adducts as shown by the following equations HCCI,,
+ t.BuOCC$Coo-
- CCI,
CI?
/"
requires either rather stringent conditions (absence of air strong bases high temperature, and water, rate equipment) or uses extremely toxic chemicals. A simpie experiment involving the capture of dichlorocarbene, from chloroform and aqueous sodium hydroxide, by phenolate anion (Reimer-Tieman) is often quoted in textbooks, but this reaction unfortunately is rather untypical of ~ i ~ h generated l ~ in ~ such ~aqueous ~ solutions cannot be efficiently trapped by olefins: cyclegives at most 5% formation under these conditions. Recently however, the use of phase-transfer catalvsts has been reoorted to dramaticallv . (2-5) . . imorove . the yield of dichlorocycl~~ro~anation of olefins in aqueous-alkali solutions. Most tetraalkyl ammonium salts apparently are capable of acting as catalysts. Their catalytic activity lies in their solubility in both water a n d some organic solvents (e.g., dichloromethane). Thus the anion CC4- is readily taken into the organic phase, and its subsequent decomposition to CClz allows the latter to he trapped by the olefin in the same phase; whereas in the absence of a phase-transfer agent, reaction of CClz with hydroxide ions gives formate ions and carbon monoxide almost exclusively. Yields often turn out t o be surprisingly good despite the simplicity of the method; a near quantitative addition to cyclohexene has been reported by use of a n excess of dichlorocarbene reagent (4). Even with lower yields the obvious simplicity and convenience will make this method a competitive alternative to other methods. The experiment described below could be done in most 5 h r laboratory sessions and 40-60% yields have been obtained by students without .too much difficulty. The experiment not only illustrates the synthetic utility of carbenes but also t h a t of phase transfer catalysts which is now assuming more importance (6).
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678
/ Journal of Chemical Education
Experimental Reagents Redistilled cyclohexene, chloroform, 50% sodium hydroxide solution and tetraalkylammonium salt (any of the following readily available chloride or bromide salts is suitable-henzyl trimethylammonium, benzyl triethylammonium, benzyl dimethyl hexadecylammonium, cetyl trimethylammonium, tetrahutylammonium, and tetraethylammanium). Procedure To a 250-ml round bottom flask place 8.2 g (10 mmale) of eyclohexene, 24.3 ml (36 g, 30 mmole) of chloroform and 1 g of benzyl trimethvl ammonium chloride. Fit the flask with a water condenser and warm on a water bath. With shaking add in small portions of a total of 32 ml 50% aqueous sodium hydroxide (80 mmnle) down the condenser over half an hour (use a magnetic stirrer if available). Continue to reflux (shake occasionally) the reactian mixture for a further 1 hr. Cool and add slowly with swirling 30 ml of water dawn the condenser and then 30 m~ of ether (b.p. 40-60'C). Transfer to a separating funnel and separate off the lower aqueous layer. Dry the crude organic layer with 5 g sodium hydroxide pellets tor 15 min and then pass the crude mix~ ture~ through b 30~g of activated ~ ~ alumina dry column and elute with a further 90 ml of petroleum ether. Distill off the solvents over a water bath and finally distill the product preferably under re. duced pressure: 6-9 g, b.p, 197-8"C, 94-VC/35 mm or 78-yC/15 , urnax.,, 2920, 1444,1023 and 792 Further Experiments (a) 1,5-Cycloactadiene can be reacted instead of cyclohexene to give a high yield of solid product (7). (b) Dibromocarbene can similarly be generated and trapped; an excess of cyclohexene is used. (c) If a gas chromatography instrument is available competitive reactions using mixtures of alefins can be performed as a group project; an excess of olefin mixture is used. The relative reactivity of alkylated olefins towards dichlorocarbene is approximately as follows-monosubstituted: 1,2-disubstituted: 1,l-disub1,2,3,4-tetrasubstituted ; ; stituted: 1.2.3-trisubstituted: 1:5:35:40:80. Acknowledgment T h e author thanks the Chemical Society (London) for a grant and the University of Malaya for support. Literature Cited (11 (a1 Buehler, C. A , J CHEM. EDUC.. 19. 239 (19721 ibl M a s . R. A., Chsm En#. News, June 30.50 (19691. (21 M a k a r s . M.,snd Wawzyniewicz,M., TelmhsdmnLatl., 4659i19691. (31 Wobo~,W.P.,mdGokel,G. W.. TefmhrdronLert.. 1637 (19721. (41 J a h i , G . C., Singh, N., and Pande. L.M.. TetrohedmnLett.. 1161 119721. (51 Starks, G. M..J Amer Chem Sor.. 91,195 11971). (61 Menger, F. M . . J C. S. Reubma. 1.229119721. 171 Fieler. L. F.. "Orpanie Experiments." Raytheon. Lprin~ton.Mass., 1968. p. 209.
