The Addition of Hydrogen Bromide to Unsymmetrical Alkenes Introductory Experiments in NMR Spectroscopy and Mechanistic Chemistry Trevor M. Brown, Alan T. honsfield, and Robefl Ellis Derbyshire College of Hlgher Education, Derby, England DE3 1GB Once the preserve of the research student, nuclear magnetic resonance is increasinelv .. - beine used as a routine tool bv undergraduate chemistry students. As an introduction to the techniaue. rather than use "off the shelf' chemicals to illustrate chemical shift and coupling phenomena we center our work around the addition of hydrocen hromide t o unsymmetrical alkenes and use the couplkg patterns in the proton NMR spectra t o establish whether the addition product is consistent with the Markovnikov rule. Thus students are presented with an attractive blend of simple investigative/interpretative, first-order, NMR work and interesting mechanistic chemistry. The preparation of dry gaseous HBr would necessitate complex assemblies of laboratory glassware, add considerably to the time scale of the exercises, and would not he without some attendant risk. However, hydrogen hromide dissolved in ethanoic acid (as a 30% solution of HBr) is an attractive, commercially available, inexpensive reagent that appears to duplicate gaseous HBr chemistry and that deserves to be more widely known. The use of this mixture for performing hydrobrominations has a long history1 but appears to have been overlooked by most writers of undergraduate practical texts. The experiments are performed on asmall scale using0.25 mL of the alkene and about 1mL of the HBrIethanoic acid solution. The other reagents are inexpensive, and the whole procedure costs less than 1 0 a~ run. Using test tubes and Pasteur pipet as the main items of glassware, each sample of bromo-alkane can be produced sufficiently dry for NMR analysis in less than 10 min. The cost is reduced yet further by using tetrachloromethane as the NMR solvent rather than deuterochloroform. This is added midway through the preparation, thus giving students a reasonable volume of liquid to manipulate during the purification stages. A trace oftet~ameth~lsilane can be added to the NMR sample to serve as internal standard. Alternatively the spectrometer can he set up beforehand using TMS as an external standard. Result and Dlscusslon Both stvrene (CcHsCH=CH?) and vinvl acetate (CHnCO~CH=CH,) give products that suggest ;hat Markovnikov addition hasoccurred (aauartet and doublet associated wlth -CH-CH3 couplingj. fn the case of methyl vinyl ketone (CH3COCH=CH2) a n d m e t h y l a c r y l a t e (CH3OCOCH=CH2) the presence of two distorted triplets indicates the presence of -CHz-CHz-, which is suggestive of anti-Markovnikovaddition of HBr. Clearly the presence of a carbonyladjacent to the double bond has asignificant effect, and we encourage our students to speculate on the various
518
Journal of Chemical Education
possible reaction pathways2. The reagent seems to add to most olefinic compounds so i t is worthwhile attempting its reaction with any conveniently available alkenes in addition to those mentioned above. For example, we have used cyclohexene and l-methyl-l-cyclohexene. NMR spectroscopy readily shows that addition has taken place (absence of olefinic protons), but in the latter case the regiospecificity of the reaction is difficult to deduce from the pattern of peaks. Students who find themselves underextended by the above exercises can profitably explore the methyl metbacrylate [(CH3C02(CH3)C=CH2]/HBr reaction. The alkene does not react as rapidly as those mentioned above. Some hours are needed for a complete reaction, and comparison with a reference spectrum of the starting material may he necessary to establish which peaks are due to product and not methyl methacrylate. The mode of HBr addition is less clear-cut, and the interpretation of the spectrum usually results in some useful discussion. Experimental Procedures
Pipet the alkene (0.25 mL) into a dry 2.5- X 15-cm test tube, and add an excess of hydrogen hromide in ethanoic acid (1.0 mL) (care: corrosive). Swirl to mix for about 15 s. The reaction is mildly exothermic, but no external cooling is required. Dilute the mixture with tetrachloromethane (2-3 mL) add tap water (10 mL), and swirl to extract the ethanoic acid and unreacted hydrogen bromide. Remove the tetrachloromethane layer using a Pasteur pipet, and transfer it to a second test tube. Add saturated sodium hydrogen carbonate solution (10 mL), and swirl/shake to extract any residual acid. Remove the CCb layer as before, and transfer it to a specimen vial. Add about 1' 3 volume of anhydrous sodium sulfate t o achieve rapid drying, and in 2 min or so the solution is ready for NMR analysis (our students routinely run samples on a JEOL-PMX6O 60-MHz spectrometer). As the HBrIethanoic acid mixture fumes in air and as the extraction involves tetrachloromethane, it is advisable to carry out most of these preparations in a fume hood. Acknowledgment The authors are indebted t o the Business and Technician Education Council (HNC Chemistry) for drawing the attention of the authors to the versatility of the HBdethanoic and reagent.
' E.g.,Ashworth,F.; Burkhardt, G. N. J. Chem. Soc. 1928,(11),1798.
Morrison, R. T.; Boyd, R. N . Organic Chemistry,5th ed.; Allyn and Bacon: Boston, 1987; Chapters 8 and 31.
