The Modern Student Ioborotory: Spectroscopy Synthesis, Oxidation and UVllR Spectroscopy Illustrated An Integrated Freshman Lab Session Uri Zoller, Aviva Lubezky, and Miriam Danot Division of Chemical Studies, Haifa University-Oranim, The School of Education of the Kibbutz Movement, P.O. Kiryat Tibon 36910, Israel In recent years, the trend in teaching the introductory courses has been to incorporate into the lecture increasingly more theory as well as selected topics of advanced descriptive chemistry. This has caused increased problems in providing a coherent laboratory program to accompany the lecture course. Specificallydifficulties arise in (1) correlating laboratory experiment with the lectddismssion part of the course, (2) including the new topics and experimental work within the limited time allotted to lab sessions; and (3 linding effrctive teaching strategies to design, for inexperienced students, up-to-dotr,integrated, meaningfullabsessions rrflecring the most modern lecture content and texts.
This paper describes a specially designed, and successfully implemented, lab-session, for the first-year college general chemistry course. The essence of this lab illustrates the electronic distribution in chemical bonding, using simultaneously the W and IR spectroscopy of dimethyl sulfoxide (DMSO), its specially prepared metal-ion-bonded complexes, and oxidation product dimethyl sulfone (DMSOZ).Thus, this lab session correlates with the theoretical topics of chemical bonding and the corresponding electronic configurationklistribution, oxidation, and spectroscopy (i.e., W and IR). In additionit adds to experimental competencies by deveopling skill in preparative synthesis and spectral analysis. Related Theory Both the valence-bond and hybrid-orbital descriptiuons of chemical bonding and the molecular-orbital theory of electronic structures have been shown to be appropriate in discussing ground-state and excited-state properties of molecules ( I ) . Spectroscopic methods are currently used in the solution of manv vroblems in modern chemistry. All of the common spectri&opic method* used to measure bond properties involve impimcment of light on the molecule under study: M+~v+M~. Ultraviolet and infrared spectroscopy are among the more important techniques that are routinely used by chemists to gain information about the particular structural features and bonding properties of molecules ( 2 4 ) . Compounds that contain nonbonding electrons on oxygen, nitrogen, sulfur and other heteroatoms are capable of showing absorptions owing to n + o* transitions in the "ordinary" W region, i.e., 18&260 mp. In the IR spectrum, the region 1000-4000 em-' is of particular importance for determination of functional groups and the shifts in ahsorption frequencies of these groups when bonded or complexed. A274
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The chemistry of sulfoxides and sulfones, including the electronic distribution in these functional gmups are wellestablished (5).The oxidation of sulfoxides to sulfones is a straightforward transformation in which the ultimate result is an increase in the oxidation number of the sulfur atom (being oxidized by losing electrons), and decrease in the oxidation number of the added oxygen atom (being reduced by accepting electrons) as illustrated below for DMSO:
In conducting the above oxidation and the actual isolation of the purified product, the W and IR spectra of both DMSO and DMSOz can be taken, and the respective ahsorption energies (of the SO and SOzgroups) can be determined and interpreted in terms of the sulfur-oxygen bond strength and charge distribution. Similarly, the synthesis of the metal-ion-bondedDMSO complexes (6)facilitates the undertaking of their IR spectra, followed by the determination of the structural features of the metal-bonded SO group (through its oxygen or sulfur) via the direction of the shiR in the frequency of the absorbed energy (7). Analysis of (1)the electronic excitation involved in the primary photochemical reactions of DMSO (8);(2) the Bond Overlap Population (BOP), which increases in going from the sulfoxide to the sulfone group (9);(3) the role of d orbitals in reducing the overall sulfur-to-oxygen electron donation (9); and (4) vibrational analysis (lo),indicates that the excitation energies required for both W and IR absorption are expected to increase in going from the sulfoxides to the corresponding sulfones. The bond streneth and the masses at either end of the bond affect theexcitation vibrational energy and, therefore, the freauenw of ribration of bonded atoms recorded by the y bond strength results in a higher IR s p e ~ r o s c ~ pAhigher frequency absorption. Since the bonded atoms in the SO and SO2groups are the same and so are the two substituents (the methyl groups), a shift towards a higher frequency is expected when the sulfoxide is oxidized to the sulfone group. The IR absorption of the sulfoxide group in DMSO is 1050 cm-' whereas the symmetric stretch absorption of the oxygen bond in the sulfone gmu of DMSOz is 1,160 (Continued onpageAh3)
Fioure 1. The ultraviolet soectrum of DMSO. DMSO,. and rnetal-ic w h i d DMSO Complexes (in E I O ~95%) I a s recdraed on Perki' Elme! 124 Do~oleBeam spenropnorometer (C 6.4 * 1 0 .ana ~ 1 x 10.' rnoVL, respectively) -
7
~
~
~
-
-
ad.I n its metal ion-bonded complexes t h e S O absorptio occur either at frequencies below that of pure DMS( implying a weakening of t h e sulfur-oxygen bond throug an electron shift from t h e S-0 environment into t h e new1 formed O-metal bond (e.g., 3),o r at frequencies above t h r of pure DMSO, implying strengthening of t h e origin;. sulfur+xygen bond explained by metal bond formation through sulfur (i.e., 4) (7):
FigJre 2. The nfrared spectrLrn of dimethy SJlfoxide (thin qJia f rn) -, CuCI,.2DMSO SnC14.2DMS0... . . ..and PdC ,.2DMSO -.-.-.--(in Nujol) as recorded on Perkin-Elmer 1 5 7 ~Grating Infrared spectrophotometer. of their samples or will just hand them to the TAs for recording. The lab session should, therefore, he designed in accordance with the local constraints.' The preparation of the following three DMSO complexes (i.e. 8-10, eq 2) has been previously described in detail (7):
T h e UV a n d IR spectra of DMSO, DMSO,, a n d t h e metalion-bonded complexes of t h e former are given above (Fig. 1 a n d Fig. 2, respectively). The UV effect i n these complexes is apparent.
Organization and Experimental Procedures The purpose of this lab session is three-fold: ( 1 ) to prepare metal ion-bonded DMSO complexes and DMSO, from DMSO; (2) to record the W and IR spectra of both reactant and products; and (2) to compare (and interpret) the spectroscopicdataobtained with theoretical predictions in terms of sulfur oxidation states, sulfur 4xygen bond strengths, and electron (charge) distribution in the SO and SOz groups of DMSO, eomplexed DMSO, and DMSO,, respectively The followingexperimental activities are to be eamedout by the students suooorted bv the TA's and instructors narticularlv during the fourth &vity: 1. Preparation of the metal-ion-bonded DMSO complexes. 2. Synthesis of DMSOl via oxidation of DMSO. "
4. Recording the spectra. The order of activities for each student team and the particular organizational details are dependent on (1)type andnumber ofUV and IR soectrometers a t students disoosal): (2) the number of students in ihe lah; (31rhr method used for the IR recording k., samples as thin tilrn, or in Nujot, KBr pellet, or a solvent ; 14, the pre-lab availability (ta *tudm~q.s, of the U V and I R spectra of DMSO: and 5 , whetherthestudents actually will take thespectra
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'The acldal organizaton an0 proced~resappl ea in our lab can be Ootainea from the a~thorson request.
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-The Oxidation of DMSO to DMS02 This work should b e done i n t h e hood and t h e u s e of plastic o r rubber gloves is mandatory. Any direct contact with DMSO should b e avoided. To a stirred solution of DMSO (1.6 mL, 0.0225 mole), distilled water (4 mL) and glacial acetic acid (6 mL) in a 50-mL beaker, hydrogen peroxide 15%(10 mL) is added dmpwisevery slowly. At the end of the addition period the reaction mixture is gradually heated to 90 "C, maintained for seven minutes at this temperature and fallowed by continuingthe evaporation at 7 0 4 0 OC until about onefourth of the original volume remains. Cooling of this solution in a water-ice bath results in crystallization of DMSO* as white needles (after about 5 min) which are filtered on a Buchner funnel, washed with cold ether, and air dried. The yield is almost quantitative.
Selected PreparationQuestions (For Students): 1. Completethe fallowing chemical equation related to the above transformation and explain which compound/atom was oxidized and which was reduced?,. .. . . . ... , ...,. .., , ... .... . (CH;)~SO + 'H,o; " " 2. Why, in your opinion, should the volume of the reachon mixture be reduced to about one fourth of its original one?
.,,, .
'
UV and IR Absorptions of DMSO, Metal-lon-Bonded DMSO Complexes, and DMSOz
Compound DMSO CuCIr2DMSO SnCIq2DMSO PdClr2DMSO DMS02 'In EtOH (95%) Thin liquid film.
eln CHC13.
UV:hrnax IR:S-0 Literature ( r n ~ ) ~ band (mi') 1,050' IR: 1,050 228 209 9 ~ 5R 9~2 3 221 9 ~ 5IR:~920,905 208,241 1 , 1 1 5 ~ IR: 1,120 1,140b, UV: 180 210 1 310"' IR: 1130.1280~
(7)
(7) (7) (7)
siderations/calculation. I n contrast, the weakening of the sulfur-oxygen bond in the copper and tin complexes of DMSO results i n the obsemed absorption shiRs to lower frequencies in the IR spectrum. The palladium-DMSO complex, however, is a n exception: its IR absorption i s higher than that of the original sulfoxide, pointing to the strengthening of the original sulfur-oxygen bond. this bond strengtheningis explained in terms of palladium-sulfoxide bond formation through sulfur (i.e., 11).
Y
(11)
-S
(12)
?he asym. S = 0 'in Nujol. ?he sym. S = 0.
3. Which direction would you expert the LW and the IH absorptions of L)hlS02 to shift each i . e , towards lnnger or shorter wavelenkqhsr compared to those of DMSO? Explam and compare
later with the actual experimental results, Spectral Results and Interpretations The & and the location of the sulfur-oxygen hand in the UV and IR spectra of DMSO, metal-ion-bonded DMSO complexes, and DMSOz are given i n the table. Based on both the relevant theoretical considerations and the higher electron population i n the SO bonding refion inDMS0, c o m ~ a r e dwith that i n DMSO, the excitatiok energies in both lh' and IR are expected toincrease in the latter. Indeed, the frequencies of the UV and IR absorptions of DMS02 (210 and 1140) are considerably higher than those of DMSO (228 and 1050, respectively). The experimental results thus corroborate the theoretical eon-
11
.
The caoacitv " and usabilitv of both UV and IR soectroscopy for molecular structure determination a s well a s for identification of internal electronic effects and charee distribution are thus vividly demonstrated within one integrated freshman lab session. Literature - Cited ~~
1. DeKock, R.L..;Gray, H.B. ch2mim1SLluetu.ondBondig: Benjamvxamv~mmin*: MenloPark 1980. 2. Dyer, J. R.Applieations ofAbsorptbn SmtmrmpyofOrgonie Compounds;Prentiee
Hall: Englewood Cliffs. NJ,1965. 8. Williams, D. H.: Fleming, I. Spciiimpie Methoda in O g o n b Chemidly; Mdj,awHill: Landon, 1973. 4. Siluersteb, R. M.;Bassler, G. C.; Momill. T. C. Speztrmfrh Idenfificolbn of
Orgonie Compounds: Wiley: New York, 1981.
Eds.
ondSul,hzidpa:
5. Patai, S.;Rappoprt, 2.; Stirling, C., Tke ChemiatlyofSul/am Wiloy: Chicheater, 1988. 6. Reynolds, W. L. Dimdhyl Sulfonde in Inorganic Chemistry; Interscienee:New York,
197QVol. 12, pp 1-99. 7. Borhmann. E. J. Chem. Edue 1953,€0,413-414, 8. Still, I.W.J. In Ref 5, Chapter 18. 9. Gavezzdd, A In Ref. 5, Chapter 1. 10. Diesler, G.;Hanschmam. G. J.Mol Struct. 1971,8.293. 11. Neckers,D.C.Mechon~tieOgonicPhofockemisfni;VsnNostrand:NewYork,1967. 12. Pouehert, C. J. The Aldrieh Libmryoflnfm RedSpctm; 2nd Ed., X 470A
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