Undergraduate Infrared Spectroscopy Experiments Patrick MacCarthyl and Susan J. Bowman Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO 80401 Students are generally introduced to the practical aspects of infrared spectroscopy in an undergraduate instrumental analysis course, or by measuring spectra of samples prepared in an organic or inorganic synthesis course. Solid samples are usually prepared by the KBr disc method or by the mull technique. The student then records the spectrum and attempts to interpret it. The purpose of this note is to present a set of experiments, suitable for an undergraduate laboratory course, which provides experience in deuteration and derivatization procedures as applied to infrared spectroscopy. Basic skills in vacuum-line technique are also taught. The experiments have been designed specifically to suit an undergraduate lahoratory in terms of educational value, duration, cost of chemicals and equipment, and safety factors. The basic experiments have been tested thoroughly, and possible variations, to he considered at the discretion of the instructor, are suggested. Chemicals Required In addition to the com~oundschosen for deuteration or derivatiration, the following chemicals and materials are required: 1)?0 199.8atom %), CHzOD (99.5 atom %), CH30H, mineral oil and/or perfluorohydrocnrbonoil, toluene, and dry ice. Equipment Required The apparatus for both experiments is illustrated and described in Figure 1.This apparatus is readily construded from conventional lahoratory equipment. The following items are also required: safety goggles andlor safety screen; syringes (3 ml capacity) and needles (22 gauge); rubber policeman; ball bearings or Pyrex" glass heads (6 mm diameter); NaCl windows and infrared cell holder; lens tissue; a hot air gun, if available, is useful for facilitating the evaporation of the D20, CH30D or CH30H; mortar and pestle or Wig'l'huk vibrator, and grinding vials. All spectra were recorded on a PerkinElmer, model 700, spectrophotometer. Experiment 1. Measuring Infrared Spectra of Deuteraied Solid Samples The purpose of this experiment is to acquaint the student, in a hands-on fashion, with the value of isotope exchange as
an aid to spectral interpretation. Experience with handling moisture-sensitive samples is also acquired. The isotope exchange involves deuteration of solid acid and alcohol compounds a t the site of the exchangeable hydrogens. This avoids the need for carrying out more lengthy syntheses to incorporate isotopes a t inert sites of molecules, while still retaining the pedagogic value of the experiment. The mull technique is used in these experiments.A description of the conventional mull technique is given in references (1-3).
' Author to whom correspondence should be addressed.
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Figure 1. Apparms far Experiments 1 and2. A is a round-bottomed flask w l h side tube ( K i m No. 25323. 250 mi. 19/22 neck). with rubber septum B and h i 1 bearings C, resting on a ring suppat D. E is a 19/22 gound glass mnnenor. and F isa sectimof heavy-dutyTygon"tubing of sMicienI lenmtoallow swirling of flask A when it is removed from support D during evacuation. G and H are vapor traps placed in Dewar flasks J and K containing dry ice. L i s a t h r e w a y valve connecting the reaction flask to the vacuum pump, P. w the atmmphere. The complete apparahls is mounted on a support frame.
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Procedure Approximately 100 mg of compound is ground to a very fine consistencv using a mortar and pestle or Wie'l'huP vibrator. Using a syringe, add approximately 0.2 ml of mineral oil or nertlnorohvdrocarbon to the round-bottomed flask containing the grounh compound (100 mg) and five hall hearings Pvrexm elass heads. The ontimum ratio of sample to oil mav &ry somewhat from com&,und to compound. swirl the tlask -aentlv: .. the rollinr ball bearinas cause effwtive mixingof the compound with the oil. wheia satisfactory mull is prepared a portion is removed by means of a ruhher policeman and applied to the sodium chloride windows and its infrared spectrum recorded. The round-bottomed flask containing the mull and attached septum is then connected t o the vacuum line. The water soluhle compounds are deurerated using 1)*0and the methanol soluble compounds are deuwrated usinr! CH.,OD. A comhination of these two solvents is suitahle for wo;king with a wide variety of organic acids and alcohols. Approximately 0.8 ml of DzO or CHxOD is injected through the septum by means of a syringe. The mixture is again shaken in a gentle rotary fashion. At this stage the compound generally dissolves in the DzO or CHBOD,resulting in two clear immiscible ~ h a s e sVacuum . is then annlied while continuing to swirl the'flask, causing evaporation of the D ~ O or CH,OD. During evaporation of the solvent the flask becomes coal. The student may wish to expedite the evaporation under vacuum hv. eentlv " - heatine the flask hv means of a hot-air eun. However, one must hecareful not cause sublimation 07the more volatile com~oundsthroueh excessive heatine. The contents of the flask change in appekance, becoming cliudy as the mull is reformed. The vacuum is then slowly released by means of the three-way valve; any atmospheric moisture entering the system would be condensed in the two traps. A calcium chloride drying tuhe could he added as an extra precaution at the three-way valve, but we did not find this necessary. The complete procedure is repeated for two more aliquots of D 2 0 or CHnOD, with the flask constantlv attached to the vacuum line. he vacuum is then released &d a portion of the mull is again removed with a rubber policeman and applied to sodium chloride windows. After recording each spedrum, the infrared windows are cleaned using toluene and lens tissue. I t is good practice to clean out the traps at the end of each experi&ntal run, particularly when deuteromethanol is used, since this solvent does not freeze at the temperature of dry ice. The ruhher septum is also replaced by a new one at the end of each experimental run.
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Safety Precautions As with all chemistry experiments the student should he encouraged to check the precautions appropriate to the specific chemicals involved in the experiment ( 4 , 5 ) . Protective aoaales - -- should be worn a t a l l times when working- with t h e vacuum line. In order to avoid cracking of the roundbottomed flask, the hall hearings should he rolled around the sides of the flask by a gentle rotary motion of the wrist; a shaking o r vibrating motion should be avoided. T h e flask should be constantly inspected for cracks caused by the hall hearings hitting the sides. If a crack is observed, the vacuum should he released and the experiment restarted with a new flask. We have carried out 147 experimental runs on this system without experiencing a single implosion. However, on five separate occasions a small crack was detected in the flask; in these cases the flask was discarded immediately. In efforts to further improve the safely of mese experiments for an undergraduate laboratory we placed the round-bottomed flask inside a tightly fitting transparent plastic bag (thickness: 0.05 mm) which was taped around the neck of the flask, and intentionally smashed the evacuated flask behind a protective barrier. The plastic bag was very effective in containing the shanered glass fragments. The transparent bag allows me contents of the flask to be seen readily during the course of the experiment.
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Figure 2. Infrared spectra of Bhydroxybenzyl alcohok vndeuterated (a) in perfluorahydrocarban(b) in mineral oil: deuterated (c) in perfiuaohydnxarbon and (d) in mineral oil. Daterating solvent: CH30D. Absorption bands due to the mineral oil in spectrum (d) are indicated with h e letter M. me wavenumber axis is linear between 800 and 2000 cm-'; his also linear between 2000 cm-' and 4000 cm-'. but with a different scale.
Discussion of Experiment 1
When a sample is deuterated in the ahove manner, it can he exposed to the atmosphere for an extended period without deteriorating significantly. We have had success using this technique with all of the following compounds: 2-hydroxybenzyl alcohol, p-hydroxybenzaldehyde, dimethylglyoxime, p-toluic acid, p-aminohenzoic acid, pyrogallol, sorhitol, 3,5-dihydroxyhenzoic acid and p-nitrophenylhydrazine. The instructor may wish to suggest other compounds not included in the ahove list. Figure 2 illustrates the results obtained using 2-hydroxyhenzvl alcohol. Perfluorohvdrocarhon is infrared transnarent in thk 4000-1350 cm-' region, while mineral oil is transparent over the 4000450 em-' range hut has absorption hands a t ~2900,1460,and 1380 cn-'. Thus, a comhination of these two mulling media provides an infrared window over the range 4000-650 cm-'. The results clearly illustrate the changes which occur in the infrared spectrum following isotope suhstitution and how these changes can facilitate the interpretation of complex spectra. For example, the two 0 - H stretching hands at ~ 3 4 2 cm-' 0 and ~ 3 1 4 cm-1 0 are replaced by 0 - D stretching hands a t -2570 cm-' and 2370 cm-', respectively (Fig. 2(a) and (c)). The validity of the ahove assignments is substantiated hy calculating the wavenumher for the ahove pairs of hands, gkingvalues of ratios, F~JH~~OD, 1.33 and 1.32, respectively; these values are m satisfactory agreement with the ideal value of 1.41. The aliphatic CH stretching hands, which are virtually concealed beneath the broad 3140 cm-' OH hand in the undeuterated compound (Fig. 2(a)), are clearly visible a t 2900 cm-' and 2950 cm-1 in the deuterated comoound (Fie. 2(c)). The aromatic CH stretching bands arealso revkal;?d a t 3050 cm-' in the spectrum of the deuterated compound (Fig. 2(c)). Other changes which occur upon deuteration are the disappearance of hands a t a ~ ~ r o x i m a t e1610 l v em-'. 1415 cm-'. 1105 cm-l. 775 cm-' and-fl0 cm-', &d the a p p e k n c e of hinds a t approximately 1500 cm-' and in the region of 1000 cm-'. This experiment also teaches basic skills of vacuum-line technique and avoids the more expensive and time-consuming
drybox procedures, thus making the experiment more amenable to the typical undergraduate laboratory. Assuming that the basic vacuum-line set-un is available already, rhe experimenr can be mmpleted ~ ~ m f o r t a h liny atnut 1 hr bv an inexwrienced student. The ex~erimenttakes cousiderab$ less time when CH30D is used in place of DzO, because the former evaoorates much more raoidlv. Evaooration of each aliquot of 'D20is complete after a'huu; 7-8 kin; a significantlv shorter period is required for evanoration of deuieromethkol. I t should be pointed out that there are simple published procedures for deuterating compounds which are soluble in an iufrared-transparent, water-immiscible solvent such as carbon tetrachloride (6, 7) or cyclohexane for far-infrared studies (8).Those procedures could also be readily incorporated into an undergraduate experiment. Howeve;, the G o cedure described in this paper is more generally applicable to a wide variety of compounds.
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Experiment 2. Derlvatization a s an Aid to Spectral Interpretation The obiective of this ex~erimentis to demonstrate the value of derivarizatim as an aid to incerpreting infrared spectra. Many derivatization reacriuns involve refluxing, followed hy distillation or recrystallization steps before the final product is obtained, thu3excluding them from thedesired time-frame ulthe present experiments. It isdesirable to choose derivatizatim reactions which proceed smoothly and efficiently and where rhe reaction hy-producu and rwdual reagents can he readil\f removed frmn the reaction flask. The methvlation of phenilboric acid satisfies these requirements. Procedure The methylation of phenylhorie acid can be carried out readily using the apparatus of Figure 1, and the procedure described for Exoeriment 1 above. The mull of ohenvlhoric acid is meoared in the flask and its soeetrum reeorddkhe brocedure des&i&d for deutrrarion is then idlcwrd, except that &OH is subsrltuted for the D1Our CH,OD. Of ruurse, CH@D muld aliv be u w l I'm the methylation, but this would be wasteful. The safety precautions for Experiment 1apply here also. ~~
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Discussion of Experiment 2
The results of the methylation experiment are shown in Figure 3, clearly demonstrating the changes in the infrared spectrum resulting from derivatization. For example, the disappearance of the OH stretching band a t 3250 cm-I, and the appearance of the CH stretching bands a t 2860 em-' and 2950 cm-1 due to the introduced methyl groups, are evident. Other changes which occur upon methylation are the disappearance of bands at e l 5 0 0 cm-', at e l 0 0 0 cm-I and 800 cm-' and the appearance of an absorption band a t "1470 cm-I. The shoulder a t "3050 cm-' is due to aromatic C-H stretching vibrations. The ~edaeoeicvalue of the ex~erimentis not diminished in any W R Y d; ;he fact that a fnriie reaction has been chosen which dramaticallv shortens the time and effort involved in the derivatization-per se. The metbylated product is automatically incorporated into the mull, and the H20 resulting from the reaction, and the residual CHBOH,are readily removed from the system, thereby streamlining the complete operation. This experiment can be carried out easily within
Figure 3. Infrared specha ot: pnenylboric acid in (a) pertluorohydracarbon.(b) m~neraloil, memylaled phenylooric acid in id Dsrfl~arohydrocaroon.an0 (dl mineral o f . The posll onsol the minera.011D a d s In spenr,rn(b)arehdcarsd by M. The wavenumber axis is same as that in Figure 2
a l-hr period; thus, both Experiment 1and Experiment 2 can be accommodated within a single laboratory session. The instructor may wish to suggest other facile derivatization reactions for the student to investigate using the above procedure. [Possible reactions to be considered in this regard are reactions of acid halides with alcohols, amines or water; or the aqueous hydrolysis or triphenylmethyl chloride. However, the authors have not tested these latter reactions in this experimental set-up.] Acknowledgment Support of this work by an L. J. Beckham grant from the Colorado School of Mines is gratefully acknowledged. We thank Dr. G. Lucas of the Colorado School of Mines for suggesting other possible derivatization reactions for Experiment 2. Literature Cited
(61 Fslen, H. M. and Robsrtaortao, A. V. TetrahedmnIaffa.,No. 3,111, (1962). (71 WorIw,J D. and Garteiz, D. A,, J. CHBM.EnUC.,46,608, (1969). (8) Carlaon, 0.L.. Fstpley, W. G. and Bentley, F. F.,Speetmrhim. Aelo, 28A. 177,
(19721.
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