A Quick and Effective Demonstration of Anti-Markovnikov Addition to Alkenes 'rho n-o nf hvrlroom ...u-....-. ..,-.-- hrnmide rlis~nlvedin ethnnoic acid has reeentlv been recommended' as a useful reaeent for dcmonrtratmgthe Marknvn~kovadditionnfIIBrtoalkenes.Wr have recently found thnt,dcsplte the high polarity afthesolvent, ifsufficltnt free-radical >nrtlaturconcrntrnrinn 1s used then dcmnnstrableadditaon prorrcds wa the anti-Mnrkomlkuv roule As hefwr, tetrachloromethane 1% the ro-solvent that mnplilics the isomer detection by proton magnetic resonance spectroscopy. The reaction is quick and can be performed in less than 10 min. The somewhat complicated second-order 60-MHz NMR pattern for the - C H r C H e protons in the anti-Markavnikov product 1s mstintly r e ~ n ~ m u a brithrr lr by nmpuriwn with the spectrum of an outhentic sample ur by rrfrrrnce too published collerlion o f s p e c t r s Z hiintcgrsrinn tracecan be used toderivethe ratloofthe two a d d m m products as the duublcl irom the -CH?urntons M:wkornik~vaddition, ix uaefully distant from the other signals in the soectrum. Usins ..the auantitiesieiven below. students rermlarlv find that more than 80% of the isomer mixture is 2.hn,moethylhrncme. the anti-\lnrkovn!ktn pruduct Whm used as o d a s s prnrtlral it i r instrurtwc to compare the product ratlo wing amnller amounts of imtmtor and sulvrnts of dlNerent pulanlyhnilmg pomt. Funher cxtpnslon ran bc ochiwed by mvcsfignrlng the e l l k t of Hllr rthsmnr a c d solution in the ndd with subacquent heating. Experimental. Clamp a dry standard test tube (1.8- x 15-cm) in a fume hood, and add tetrachloromethane (3.0 mL), dibenzoyl peroxide (0.25 g) (freed from adherent water by pressing it briefly with filter paper) and styrene (phenylethene) (0.07 mL). Warm the mixture (waterbath)so thatthe solvent gentlyboils without thevapors leaving the top of the tube, and add hydrogen bromide in ethanoic acid solution (1.0 mL) dropwise while maintaining the gentle bailing. (Caution: t h e HBrIethanoic acid m i x t u r e is corrosive and f u m e s strongly in air.) Remove from the heat and add water (5.0 mL). Stopper and shake briefly to extract the ethanoic acid and unreacted HBr. Transfer the lower CCh solution (Pasteur pipet) to a second tube and shake with saturated sodium hydrogen carbonate solution (3.0 mL). Finally, transfer the organic layer to a dry test tube and add about a third of its bulk of anhydrous sodium sulfate to achieve rapid drying. In about 2 min the solution becomes optically clear and sufficiently dry for NMR analysis. Tetramethylsilane can be added as internal standard or a n external standard can be used. As t h e CClr solution still c o n t a i n s significant a m o u n t s of u n r e a c t e d peroxide, n o a t t e m p t s h o u l d be m a d e t o purify t h e p r o d u c t s b y distillation. ~~
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'~rawn,T.M.: Dronsfield,A.T.; Ellis, R. J. Chem. Educ1990, 67(6),518. 2~oucherl C. J.; Campbell, J. R. The Aldrich LibmyofNMR Spectra;Aldrich Chemical, 1974, Volums 4; p 39.
Trevor M Brown Alan T. Dronsfield Ian Hitchcock Derbyshire College of Higher Education Derby, DE3 IGB, U.K.
Volume 68 Number 9 September 1991
785
