The Synthesis of E-p-Bromostyrene An Experiment Illustrating the Use of IR Bending Modes To Distinguish E and Z Isomers and the Concept of Kinetic and Thermodynamic Controlled Reactions Laura A. Strom. James R. Anderson. and J o s e ~ hR. ~ a n d l e r ' California State university, Fresno, ~i93740-00;0
Two separate reports have appeared recently in this journal regarding t h e synthesis of P-bromostyrenes, Corvari, McKee, and Zanger (I)reported the synthesis of a mixture of E and Z-P-bromostyrenes starting from trnnscinnamic acid. This was reported as an example of a synthesis that produced a pleasant-smelling (the odor of hvancinth) rather than the all-too-usual foul-smelline product. Mestdagh and Puechberty (21 reported the spy the& of both the E andZ isomers by decarhoxylativeelimacid in butanone ination ol2,3-dibromo-3-phenylpropanoic (to vield the2 isorner~and m water (to vidd the E isomer). Thk work provides a good example of hbw the reaction solvent can influence the stereochemical course (and mechanism) of chemical reactions. We report here an extension of these works in which the E isomer is prepared via a different route, and IR spectroscopy is used to distinguish between the E and Z isomers. We also report t h a t i n a mixture of E and Z,Pbromostyrenes it is the E isomer that is responsible for the hyancinth odor (I). The sequence of reactions is illustrated below-the Z isomer (1)is prepared following the procedure of Mestdagh and Puechberty (2). The E isomer is then prepared from the Z isomer by addition of Brz followed by iodide ion promoted debromination (3).
g (0.036 moll of NaI in 30 mL of ethanol. Heat the solution at reflux for two-hours (the solution will darken as it reacts). When the reflux period is complete, evaporate off the ethanol using a hot water-bath. The product that remains should then be washed with a 5%sodium bisulfite solution, and then extracted three-times with methylene chloride. Dry the combined extracts, and evaporate the solvent from a tared flask. The product is an oil whose IR and lH NMR spectra should be recorded (4). Discussion Mestdagh and Zanger report the synthesis of Z-Pbromostyrene by addition of Brz to trans-cimamic acid to give 2,3-dibromo-3-phenylpropanoicacid followed by decarboxylative elimination (2). The product that is isolated is characterized by its 'H NMR spectrum: coupling constants are used to distinguish the Z from the E isomer (1, 2). IR is also useful in characterizing the isomers and illustrates the diagnostic value of IR bending modes: the Z-P-bromostyrene shows a C-H bend at 770 cm-', whereas the E isomer absorbs at 731 and 941 cm-I (5). The molecule, l-phenyl-1,2,2-tribromoethane, 2, is made from 1by addition of Brz. The 'H NMR of 2 shows a pair of doublets a 6 = 5.35 and 6.0 (J = 7 Hz). In the last step of the sequence, E-P-bromostyrene,3, is made from 2 by debromination via transition state 4; the
Experimental 1.2,Z-Tribromo-l-phenylethane(2)
In a 50-mL, round-bottom flask dissolve 1.66 g (0.0090 moll of Z-P-bromostyrene,1,in 12 mL of glacial acetic acid. Add to this solution 2.50 mL of a solution of Brz in glacial acetic acid (1.0 mL of Brz per 3.0 mL of acetic acid), and heat the solution at reflux for two hours. Caution: Bromine is very toxic. It should be dispensed i n a hood from a buret. After the reflux period is over, the solution is extracted three-times with 10 mL portions of methylene chloride. The extracts are washed with a 5% sodium bisulfite solution, and then with a 10% sodium bicarbonate solution until evolution of COzceases.The methylene chloride solution is then dried over anhydrous magnesium sulfate, filtered, and evaporated using a water bath. A sample of the product, 2, is saved in order to record its 'H NMR spectrum. The rest of the sample is used in the next step to prepare E-P-bromostyrene,3.
reaction is promoted by iodide ion in acetone solution. The product is characterized by its IR and 'H NMR spectra as described above for Z-P-bromostyrene. The concept of kinetic and thermodynamically controlled reactions is introduced and discussed (i.e., thezisomer is under kinetic control because of the requirement for anti-periplanar elimination,whereas, the more stableE isomer is the product of thermodynamic control). Finally, the students are asked to carefully smell both the Z and E isomers. They are generally surprised to learn that only the E isomer smells like hyancinth, and that it is the E isomer that is responsible for the hyancinthlike odor of a mixture of P-bromostyrenes (1). Literature Cited 1. Corn", L.: MeKee, J. R.;Zanger, M.J. Chpm Edue. 1981,68,161. 2. Mestdagh,H.; Pushberty,AL Chpm.Edue. IsB1,68,515.
3. The syntheaia followathe pmcedure described@ Mareheae. Modens aodNaso: Marcheae, G.;Modens G.; and Naao, F.'lbtrahaimn 1868.24.663. 4. The synthesis follows the prmedure desnibed byvoiogei: vogels ' 'lbtboob0 f R o ~ t i n r l O?gmn& Chpmistry.4UI ed.: h g m a n : New Yorlr, 1978; p 349. 5. Gmvenatein, E.;Lee, D. J. Am. ChPm Sor 19S3,75,2639.
In a 50-mL, round-bottom flask dissolve 1.20g (0.0034 acid, 2, and 5.40 mol) of 1,1,2-tribromo-l-phenylpropionic 588
Journal of Chemical Education
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