Vol. 74 No. 1 January 1997 • Journal of Chemical Education 109

In the Laboratory

Reactions of Bromine with DiphenylethylenesAn Introduction to Electrophilic Substitution

Ronald M. Jarret, Jamie New, and Kalliopi KaralioliosDepartment of Chemistry, College of the Holy Cross, Worcester, MA 01610

The chemistry department of Holy Cross College hasinstituted a discovery-based curriculum for generalchemistry (1) and organic chemistry (2) courses, in whichthe laboratory (not the lecture) drives the courses. Quiteoften, traditional experiments (designed for verificationof a concept) can be conducted to promote student dis-covery (introduction of a concept). In subsequent lectures,class results are pooled and expanded upon, to cover theusual material associated with a particular topic. Inmany of our organic chemistry experiments, studentsdiscover some mechanistic aspect of a reaction that hasbeen previously introduced in lecture (3). These includecarbanion involvement in nucleophilic aromatic substi-tution, carbocation involvement in alcohol dehydration,and bromonium ion involvement in bromine addition toalkenes.

The discovery approach works best when centeredaround a specific question (such as whether bromine re-acts with alkenes by syn, anti, or random addition). Stu-dents are encouraged to predict the answer—based ontheir understanding of related topics, such as the stereo-chemistry of other addition reactions to alkenes: H2/Pd,HBr, Hg(OAc)2/H2O—and to propose methods that testand substantiate their prediction. This situation is notideally suited for a new reaction to be discovered by stu-dents in a meaningful way. We present the use of a tra-ditional experiment (conducted in under 3 hours) thatnot only allows students to discover the stereochemis-try of bromine addition to alkenes but, more importantly,has been expanded to allow for the student discovery ofelectrophilic substitution. This serves as an excellentspringboard for follow-up experiments on, and discus-sion of, electrophilic aromatic substitution.

Discussion

Students are told that when bromine reacts with analkene (e.g., ethylene), an addition takes place to form avicinal dibromide (e.g., 1,2-dibromoethane). They areasked to consider whether the stereochemistry of theaddition is syn (as with H2/Pd), anti (as with Hg(OAc)2/H2O), or random (as with HBr). They are further chal-lenged to design an experiment that will distinguishamong the three possibilities. Students readily suggestthat the starting alkene should be one that will gener-ate two stereocenters upon addition of bromine. Withfurther prodding, students suggest that the diastereomerof the starting alkene should also be studied; this willestablish if the most stable product is being formed or ifa particular transition state is required. We also need ameans of identifying which stereoisomer or stereoiso-mers are formed.

These general ideas are translated into the reactionsof bromine with stilbenes (Scheme I). Students are askedto determine whether the meso stereoisomer or (±) ste-reoisomer of 1,2-dibromo-1,2-diphenylethane is the morestable product. The potential products are solids withmelting points that differ by nearly 100 °C and are re-

solvable with gas chromatography. Experimental proce-dures for the reactions appear in a number of organicchemistry laboratory texts (4).

H H

H

H

BrH

H Br

BrH

H Br

Br2

Br2

meso stereoisomer if syn addition(±) stereoisomer if anti addition

(±) stereoisomer if syn additionmeso stereoisomer if anti addition

Scheme I

To round out the experiment, the remaining isomer1,1-diphenylethylene is also considered and reacted withbromine under the same conditions. It is used to empha-size the importance of experimental design in testing amechanism and, of course, for its unexpected results.Thus, one third of the class works with 1,1-diphenyl-ethylene, one third works with cis-1,2-diphenylethylene,and one third works with trans-1,2-diphenylethylene.The products are analyzed with GC-MS, rather than bymelting point.

Results

Students starting with trans-1,2-diphenylethyleneobserve meso-1,2-diphenyl-1,2-dibromoethane as themajor product (with about 5% of the (±) stereoisomer byGC). This observation is consistent either with forma-tion of the more stable product or with anti addition. Stu-dents starting with cis-1,2-diphenylethylene observe (±)-1,2-diphenyl-1,2-dibromoethane as the major product(about 5% meso by GC, after filtration). This is also con-sistent with anti addition and rules out the possibilitythat the more stable product is always being formed. Thereaction is highly stereoselective. Students propose a bro-monium ion (like the mecurinium ion in oxymercurationof alkenes) to explain the observations. They are told thatthis is the normal course of action for bromine additionto alkenes but they are also warned of cases where thestereoselectivity of the reaction is not maintained (e.g.,dihydropyran and cis-di-tert-butylethylene are excep-tions listed in common textbooks) (5). This leads us intothe discussion of surprise results when 1,1-diphenylethylene is reacted with bromine.

meso stereoisomer if syn addition(±) stereoisomer if anti addition

(±) stereoisomer if syn additionmeso stereoisomer if anti addition

110 Journal of Chemical Education • Vol. 74 No. 1 January 1997

In the Laboratory

The explanation for the formation of 1,1-diphenyl-2-bromoethylene from 1,1-diphenylethylene, under thesame conditions, is less obvious. A common student re-sponse is to propose that the substitution proceeds viafree radicals (as they have seen with alkanes and Br2).Arguments against this idea are readily made and it isabandoned. Students are instructed to see if they canmodify the established mechanism for bromine additionrather than come up with an entirely new one. The re-vised mechanism describes the substitution as an addi-tion, followed by an elimination (Scheme II). It is specu-lated that the phenyl–phenyl interaction on going fromsp2 to sp3 hybridization, makes bromonium ion forma-tion and subsequent addition of Br{ a less likely processthan elimination. The students are told that they arelikely to encounter this reaction again whenever stericinteractions make addition less likely or, more impor-tantly, when there is a strong driving force to re-formthe π-bond (e.g., electrophilic aromatic substitution).

H

H

Br

H

Br

H

H

Br+

HH

+ HBr

Br–Br

Br2

? ?

Br–

Br–×

Scheme II

Conclusion

Pooling the results obtained from the reaction be-tween bromine and the 1,2-diphenylethylenes allows stu-dents to discover the mechanism of anti addition, whichis common to most situations. Expansion of this experi-ment to include 1,1-diphenylethylenes allows studentsthe opportunity to discover the electrophilic substitutionreaction. This sets the stage for electrophilic aromaticsubstitution.

Experimental Procedure

1,2-Diphenyl-1,2-dibromoethanes are prepared from1,2-diphenylethylenes (4). They are analyzed with GC-MS, using the following separation method: 1 min atstart temperature of 110 °C; ramp 35 °C/min to a finaltemperature of 250 °C, maintain for 1.5 min (with a 12m × 0.2 mm cross-linked methyl silicone gum column0.33 mm film thickness). Under these conditions, themeso stereoisomer has a retention time that is 0.1 minshorter than the (±) stereoisomer. While the mass spec-tra of both isomers are quite similar, spiking experimentswith readily available authentic materials easily distin-guish between the signals.

1,1-Diphenyl-2-bromoethylene is prepared from 1,1-diphenylethylene under the same conditions as thoseused to prepare 1,2-diphenyl-1,2-dibromoethane from1,2-diphenylethylene. It is analyzed with GC-MS, usingthe same method for separation as 1,2-diphenyl-1,2-dibromoethane. It has a molecular ion of 258 (and anm+2 ion of 260). We have added the compound to ourdata base so that students readily identify their prod-uct through a library search. It has a melting point of42–44 °C (lit: 50 °C) (6) and the 1H-NMR spectrum showstwo signals (δ: 6.8, 1H (s); 7.4, 10H (m)).

Safety and Disposal

Avoid contact with the pyridinium bromide per-bromide. Ether should be disposed as a toxic waste. Ace-tic acid and methanol should be burned in an incinerator.

Acknowledgments

We would like to acknowledge support from the Na-tional Science Foundation (USE-8852774 and USE-9052318) and from the College of the Holy Cross. Wewould like to thank Paul D. McMaster for incorporatingand testing this experiment in his course.

Literature Cited

1. Ricci, R. W.; Ditzler, M. A. J. Chem. Educ. 1991, 68, 228–231.2. Jarret, R. M.; McMaster, P. D. J. Chem. Educ. 1994, 71, 1029–1031.3. Jarret, R. M.; New, J.; Patraitis, C. J. Chem. Educ. 1995, 72, 457–

459.4. (a) Lehman, J. W. Operational Organic Chemistry; Allyn & Bacon:

Boston, 1988; (b) Nimitz, J. S. Experiments in Organic Chemistry;Prentice Hall: Englewood Cliffs, NJ, 1991.

5. Kemp, D.; Vellaccio, F. Organic Chemistry; Worth: New York, 1980.6. Buckingham, A., Ed. Dictionary of Organic Compounds, 5th ed.;

Chapman and Hall: New York, 1982; Vol. 1, p 795.