Vibrational-Rotational Spectra Simultaneous Generation of HCI, DCI, HBr, and DBr N. ~ana~athisubramanian' Wake Forest University, WinstonSalem, NC 271 09
Among the few undergraduate physical chemistry experiments that are based on the principles of quantum chemistry, the analysis of the vibrational-rotational spectrum of HC1 (in fact H3'C1 and H37C1)is an excellent experiment (131 in which data of high quality can be obtained quickly. Typically, the experimental procedure involves the generation of HCl gas, filling an infrared gas cell with the generated HC1 gas, and obtaining a reasonably resolved infrared s p e c t m . Recently, Rieck et al. (4) have reported a novel variation of this classic experiment, in which DBr is the gas of interest and is generated by the reaction of liquid Br2on D&luene in the presence of a Lewis acid cataly~t.~ Rieck et al. observed that, although the DKtoluene they used was 99% pure (D atom). sufficient HBr was eenerated alone with DBr. so that the vibrational rotational ipectrum conteked due to HBr, the intensities of which were slightlygreater than those of DBr! This exercise has the additional purpose of demonstrating the influence of isotopes (and their masses)on molecular parameters (of a diatomic molecule) such as bond length, the fundamental vibrational freauenw . - at eauilibrium seuaration, and the force constant. In our attempts to implement this experiment in our undergraduate physical chemistry laboratory curriculum, we have discovered inadvertently that by using anhydrous FeC13 as the Lewis acid catalyst, all four of the gases, namelv. .. HC1. DC1. HBr. and DBr can be generated in a single experiment, in quantltles adcquaw for obtaining their infrared sDectra. Fortunatelv. the vibrational-rotational spectra of'these four gases a& well separated from each other. The spectrum (actually four spectra in one) obtained is well resolved and is easy to anal ze In fact, the splitting of the HC1 and DC1 lines due to 'Ci and 37C1isotopes is also well resolved. In this note, details of the experimental procedure along with a brief discussion of the obtained results are presented.
-
Experimental Procedure Eaui~ments and materials rewired for this exoeriment . . are an mfrared spectrometer with reasonable resolution rwe used a Mattson model 4020 FTlR instrument,. a 10-cm long infrared gas cell (about 2 a n d i m . ) fitted with NaCl windows, a gas inlet and an outlet, magnetic stirrer and stir bar, 50-mL round-bottomed flask, 50-mL liquid addition funnel, cold water condenser, a trap (bubbler) of 100 mL volume containing 50 mL mineral oil, glass and polythene tubings of appropriate sizes, 2 mL D8-toluene (Aldrich, 99 + atom % D), l mL liquid Br2(Aldrich), and 0.1 g 'Current address: Depaltment of Chemistry, Yale University, New Haven, CT 06511. 'We learned from Rieck thatthe catalyst they used was ferricoxide in the form of a rustv tack. which leads to DBr and HBr. Usina FeCI, instead. affords the Himu~taneousaeneration of all the four o&es. "
anhydrous FeC13(Fisher). Before starting the reaction, the reaction flask should be connected (either through the ownim in the condenser or through a third neck) to the t;ap, which in turn should be conne'&d to the infrared gas cell. Caution: The experiment should be carried out i n a fume hood. A& t h e experiment, wastes should b e treated prudently. To the D8-toluene taken in the mund-bottomed flask, h t FeC13is added. Then, while stirring the mixture, liquid Brz is added slowly from the addition funneL The ensuing reaction (which is an eleetrophilic aromatic substitution)is quite vigorous. It produces a mixturr of gases that are mainly DBr and DC1. The gases bubble throughthe mineral oil inthe trap (it may be necessarv to adiust the heieht of the column of mineral ~ - oil ~~ thmugh whkh thggases are b;bbling), and begin to 6ll the infrared cell. The addition of Br2 is continued until fumes are observed at the exit of the infrared cell. Then the inlet and the outlet of the infrared cell are closed, and the IR spectrumof the gas sample is obtained3in the wavenumber range 1700-3100 an-'. Commercial infrared spedrometem available in mast undergraduateinstitutions--be used to obtain the IR spectrum However, it is generally not possible to analyze the spectrum satisfactorily,unless its resolution is of the order of 0.01 a+. Comments It is obvious that the source of C1 in HCl and DCl in the gas mixture is FeCb. The mineral oil traps any Br2 and toluene vapors that escape unreacted. We believe that the source of H in HCl and HBr is not the residual protonated toluene in Drtoluene. If the residual pmtonated toluene is the source of H. it would i m ~ l vthat the rate of the reaction of protonated toluene with B& is a t least 10' times as fast as that of deuterated toluene. Such an isoto~eeffect has no precedence in the literature. We believe that the formation of HCI and HHr in the ecneratcd eas mixture is due to the exchange of H for D bythe moist&e in the ambient atmosphere and in FeC13.As an additional surprise, the isotopic splitting due to W 1 and 37C1shows up clearly; however, the resolution is much better in the case of DC1 than in the case of HC1. Although Br2 is also available in two major isotopes P B r and "Br in almost equal abundances), there is no splitting of the lines in the DBr and HBr spectra. These splitting features are in accordance with what would be expected based on the ratios of reduced masses.' The ratios w r~''Hr, (HnBr, and w rDb'Br 1 c I I)"Rr, are so close to one that ultra high resolution is required to observe the splitting due to the Br isotopes. Acknowledgment NG thanks Wake Forest University for financial support of this work. Literature 1.Sim8.R.J.Physimi Ckamisf'y Mdhods ~chnipuosEzperimenfs:Saunders:Philadelphia, 1990, pp 54-87. 2. Shoemaker,D.P.;Garland, C. W.,Nibler,J. W.ElperimontsinPhysicolChemistry;5th d.M f f i r a M i l l : New Ymk. 1989, pp461488. 3. Halpem, A. M.:Rewes, J. H.Er~rimenfol Physiml Ckmismi. Smn.Foreaman and Company: Glenuiev, 1988,pp 367-378. 4. Rieek, D. E;Kundell, F A, Clernents, P J.J. Chm.Educ 1889,66, 682. Volume 70 Number 12 December 1993
1035
~
-
