Voltammetric Monitoring of Br- and Br3- Concentrations during the Bromination of Styrene by Br3A Laboratory Experiment Annie Desbene-Monvemay ITODYS, L.A. 36, Universite Paris VII, 1 rue Guy d e La Brosse, 75005 Paris, France J a c q u e s Betthelot' and Paul-Louis Desbene Laboratoire d e Chimie Organique Structurale. U.A. 455, Universite Pierre et Marie Curie, 4 Place Jussieu, 75005 Paris. France
A previous paper2 has reported thesimple and easy bromination of styrene using tetrabutylammonium trihromide (TBABr3) (1) which does not display the drawbacks involved in the use of toxic and corrosive bromine.
This mild and safe bromination is easily carried out a t room temperature in various solvents and may be used as a aualitative and ouantitative analvtical test for unsaturation. Since bromide and tribromide-ions are involved, electrochemical methods seemed to us t o be most awwrowriate to .. . determine the amount of added bromine. Experimental T h e titrations were carried out in a thermostated (22 "C) e l w t r d y s i s r ~ l with l mngnrticstirring, under an argon atmusphere. A rutating IXK-rpm)platinum mirruelectn~dr'(Tncu.;sel EM-El)[ I't I),! was used a s thew or kin^ electrode: the auxiliary electrode was a platinum grid. We used a n Ag/10-2 M Agf-acetonitrile reference electrode. T h e potential of the working electrode was imposed hy a P.A.R. 173 potentiostat coupled with a P.A.R. 175 universal programmer. T h e voltammograms were recorded with a n Ifelec (IF 3802) plotter.
Figure 1. Valtammehy of Br- and Br3- ions In CH&N (0.1 M BU,NCI04 supporting electrolyte)at 22 ' C : potential scan rate. 20 mV/s: (a) supporting electrolyteonly:(b) bromide ion (2.3X TOW3 4:(c)tribromideion (1.5 X M).
Materials BulNBr, Bu4NBr3, and CH&N (99+% spectrophotornetric grade) were obtained from Janssen Chimica. BurNCl01 (purity > 98%) was purchased from Fluka A. G. We used 0.1 M solutions of this electrolyte. Bu4NBr3was dissolved in dry chloroform and purified by washing, first with an aqueous NaHC03 (10%)solution, then with water until pH equaled 7. The chloroform solution was then dried over Na&04 and finally evaporated. Pure BudNBradisplayed a melting point of 84 "C.
Electrochemical Titration of B T and BrsTwenty-five milliliters of CHBCNcontaining 0.1 MBurNC104 as a supporting electrolyte are placed into the electrolysis cell under an argon atmosphere. Before adding any reagent, the i-Ecurve of the medium is recorded (see Fig. 1,curve a) for control. Then 18.5 mg of BurNBr is dissolved to obtain a Brr concentration of 2.3 X 10F' M, convenient for electrochemicalstudies of this type. Author to whom CorresDondence should be addressed. Berthelot, J ; Fournier. M J Chem. Educ.. in press The PI disk was polished witha dlamond paste (Strlrers 1 p m ) and rmsed in acetone with ultrasonic stirrmg oefore use I
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Figure 2. Diffusioncurrents versus concentration: b = f[c]
Figure 3. Voltammetric curves obtained during styrene bromination by TBAEJr3: (a) supporting electrolyte only: (b) t = 0; (c) t = 5 min; Id) t = 20 min.
Figure 4. Bromide 10)and tribmmide ( 0 )ions concentrations In ma solution during the bromination of styrene by TBABr3.
Figure 1, curve b represents the voltammetric behavior of Brr. Bromide ions are oxidized in two successive steps corresponding to two waves (located, respectively, at +0.41 and +0.75 V versus Agl kt). If Bu4NBra (18 mg, 1.5 X 1W3M) is used instead of BuaNBr, the voltammogram displays a reduction wave at -0.1 V and an oxidation wave at +0.67 V (versus Ag/Agi), (see Fig. 1, curve c). All of these processes correspond to slow electronic transfers. The intensities of the im,im,iov, and ior diffusion currents are proportional to the concentrations of B r or Br3- (see Fig. 2). The ratio of slopes of the straight lines I and I1 is equal to 2.0 in the case of Br- and to 1.8 in the case of Br3-. These values are in agreement with the results of Popov and Geske,', who proposed the following electrochemical reactions: 6 B r - 4e = 2Br,I
2Br,- - 2e
*
3 Br,
11.11'
T h c results show that voltammetry allows t h e drtermination of Hr and Br; c o n c r n t r a t i u n in t h e same sulutiun. It is t h u s possible t o m o n i t o r a n alkene bromination reaction by TBABr:3 with this technique. In t h e course of such a reaction, t h e I' reduction wave (reduction of Bra- into Br-) will decrease d u e to t h e consumption of Bra-. Conversely t h e formation of Br- ions will result in t h e appearance of a n increasing oxidation wave (wave I, oxidation of Br- i n t o Brg-. Vonamrnetric Monitoring of Styrene Bromhation by TBABr3 First we verified that stvrene eives no oxidation or reduction wave
and styrene were thus equal to 0.1 M.) The evolution of the reaction was followed by titration of 250 pL
concentrations of the analyzed ions were of the orderbf 10WM.'~he different voltammetric curves thus obtained represent the variations of Brr and B r 3 concentrations as a function of time, (see Fig. 3). The oxidation wave located approximately at +0.7 V versus Agl Agt is the result of the presence of both Brr and Brs- ions in the solution. The observed t,,? for this reaction was about 1 mi"; the bromination was completed in about 20 min (see Fig. 4). A satisfadory agreement is obtained when the experimental values of the diffusion currents are compared to the calculated values.
+
Figure 5. Comparison of the values b (I1 11') during the styrene bromination. ; values (0). Experimental values ( 0 ) caiculated
Calculated values from calibration curves (see Fig. 2) using Br3- and B r concentrations were deduced for waves I'and I (see Fig. 5). Styrene bromination by TBABrz may also be run at room temperature in other solvents (CH2CI2,CClc CH3CN) and monitored with the same technique. The reaction is slower in these solvents than in CHCIa. In acetonitrile for instance the half reaction time is close to 5 min and the bromination is completed after 120 min. Conclusion Bromination of alkenes is a reaction of considerable interest both in synthesis a n d analysis (characterization of double bonds). TBABr:%is stable a n d nontoxic a n d is therefore easily handled, unlike bromine. Furthermore, t h e voltammetric technique described herein provides easy means of determining t h e extent of bromination and monitoring t h e reactions.
' Fwov, A. I.: Geska, D. M. J Amer. Chern. Soc. 1958, 80, 5348. Volume 64
Number 1
January 1987
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