The Addition of Hydrogen Bromide to Simple Alkenes Hilton M. Weiss Bard College, Annandale-on-Hudson,NY 12504 The addition of hydrogen bromide to alkenes is a major topic in all organic textbooks, and it illustrates the mechanism and synthetic potential of Markovnikov and antiMarkovnikov repioselectivitv. These reactions. however. are rarely prrfofmrd in underk~aduatelaboratories be: cause of the difficulties inherent in the use of rmsei~ushvdrogen bromide. An article in this Journal 7 1 ) has dkscribed the use of HBr in acetic acid as a n overlooked reagent for addition to alkenes. The alkenes used in that experiment were styrene, methyl acrylate, vinyl acetate, 1-methylcyclohexene, cyclohexene, and methyl vinyl ketone. Unfortunately, most of these alkenes are conjugated to functional groups which modify their reactivity, and they are not familiar to students in the first semester of the course when electrophilic addition to alkenes is introduced. The only simple alkenes used in that experiment aave - repiochemicallv unclear results. To investigate this reaction and to work within the range of alkenes commonlv encountered during the first semester of organic chemistry, we chose to investigate the behavior of simple alkenes having different alkyl substitution patterns. In addition, the experimental method was expanded also to include the preemptive destruction of peroxides in order to prevent the radical addition pathway
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I Other alkenes that were used include I-octene, Bmethyl-I-pentene, allyl benzene, 1-methylcyclohexene,and p-pinene. Clear, consistent results were obtained in every case.
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The utility of this reagent also is demonstrated here by the preparative scale synthesis of 1-bromohexane. In the interest of cost and simplicity, the alkenes that we have selected1 for the student experiment include cycloand 3,3-dihexene, 1-hexene, 2,4,4-trimethyl-1-pentene, methyl-1-butene. Students are asked to list the products expected from ionic and free radical addition of HBr to each alkene. With a very simple understanding of NMR, students also are expected to predict the multiplicity of the low-field resonance coming from the proton(s) on the brominated carbon atom in each case. Predictions are shown in Figure 1. Each of these reactions produces a predominant product that can be identified by NMR. The absence or presence and multiplicity of the peak at 3-4 ppm coming from protons on the brominated carbon atom clearly identifies each product. In most cases, the full spectra also may be compared to published spectra of the corresponding alcohols (2).The results of these reactions are quite unexpected and can stimulate some interesting discussion. Alkenes that can form tertiary carbocations react rapidly by an ionic mechanism to form Markovnikov addition products. No rearrangements are seen because only tertiary carbocations are formed initially Although these cations probably also react with the solvent, no acetates are found, presumably because these tertiary acetates are converted to the more stable bromides under the conditions of the reaction.
alkene
radical HBr
ionic
rearranged
same
same
Br
Br
k or
+-,,/,
3
Br
-
2
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-
-4
or
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aw 4 br
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Figure 1. Potential HBr addition products. Br or OAc
HBr + Zn +-,,/, HOAc
&
LA-. ' 7 7 B r -
XOI
,T,O
OAC 23% -~
3%
SmaN Scale Addition of HBr to Alkenes for NMR Analysis
Carefully put 3 mL of 30% (wt:wt) HBr in acetic acid solution2into a 6-in. test tube and add aooroximatelv 1mL (30 droos) of one of th' Hforementibned alkenes. swirl to mix these reagents and allow them to react for a few minutes. Dilute this mixture with 2-3 mL of carbon tetrachloride followed by 10 mL of water to extract the water-soluble carbon tetrachloride acids. layer Remove withthe a disposlower able pipet and transfer it to a second test tube. To this solution, carefully add 10 mL of saturated sodium bicarbonate solution to neutralize and remove anv remainine acids (beware of foaming). M ~ Xthese la; ers well with the pipet and transfer the CC14 solution to a clean dry test tube. Add a small amount of anhydrous sodium sulfate to absorb any remaining water and ~ usome t of this dried organic solution into Hn NMR tube for spec&al analysis. (We used a Hitachi-Perkin Elmer R-24 60MHz spectrometer.) Record the NMR spect r u m and analyze i t to determine t h e structure of the product. Synthesis of 18romohexane
HOAc
It was found that the scale-up of these reactions caused a temperature rise that resulted in a large proportion of ionic addition products. To prevent this, hexanes were added to slow the reaction that also was kept in an ice-salt bath. Twenty milliliters of the 30% HBr in acetic acid2 (0.1 mole) solution was put into a 50-mL Erlenmeyer flask that was cooled in an ice-salt bath at -10 "C. To this was added 25 mL of hexanes followed by the dropwise addition of 8.4 m~ (0,067 mole) of l-hexene, ~f~~~20 min of stirring, 30 mL of cold water was added and the entire contents of the flask was transferred to a separatory funnel. The aqueous laver was removed, and the organic layer was washed with 20 mL of water, dried over p&assiui carbonate, and filtered into a distilling flask. Simple distillation afforded 7.9 g of 1-bromohexane (72%)with a boiling range of 152-154 "C. Gas chromatographic analysis showed the product to be 90% of the 1-bromoisomer with 7% and 3% of the 2- and 3-bromo isomer, respectively.3
59% Br 23% OAc 18% Figure 2. Product distributionsfrom the polar additions to monosubstituted alkenes. Those alkenes that cannot form tertiary cations react too slowly by the ionic mechanism. In these cases, the addition occurs by a radical chain reaction initiated by peroxide impurities present in the alkenes. The free radical mechanism produces anti-Markovnikov products. In an interesting extension of the ex~e-ent, a pinch of zinc dust may be added to the HBr in acetic acid about 60 s before adding the alkene. The zinc-acid combination is quite effective at reducing -.oeroxide initiators thus orevent'i- the radical addition. The alkenes that are incapable of forming stable tertiary carbocations remain slow to react and NMR spectra of these products show wnsiderable amounts ofunreacted alkenes. Capillary gas chromatography (preferably GC/MS) can be used to identify the addition products that are formed. Only products arising from an ionic addition are seen (Fig. 2). The major reanangement product also may be synthesized easily by the addition of HBr to 2,3-dimethyl-2butene. Experimental Caution: Perform work under the hood. Carbon tetrachloride is carcinogenic. Other alkyl halides also are suspect.
HBr in acetic acid is corrosive.
Literature Cited 1. Brown, T. M.: Dronsfeld, A.T.;E116, R. J. C k m . Edue. 1990.67.518. 2. Pouehert,C. J.AldriehLibroryo/NMRSpeefm;Ald~chChemicalCompany:Milwaukee, WI 53233.
'Available from Aldrich Chemical Company, 1001 West Saint Paul Avenue, Milwaukee, WI 53233. "The isomeric products presumably arose from an initial radical induced allyllic rearrangement of the 1-hexene.See Gale, L. H. J. Am. Chem. Soc. 1966.88,4661.
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