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A Microscale GGMS Experiment: Identification of an Acyclic Saturated Ketone Alex T. Rowland Gettyso~rgCo ege Gettyso~rg.PA 17325 Mass spectrometry has found a niche in the undergraduate organic laboratory (1-7) due in part to improved instrumentation and increased textbook coveraw. The eeneration of mass spectra is especially attractive inihe mi&scale laboratorv because onlv minute amounts of liauids are needed to o b k importan< data about molecular structure. The experiment described here, which involves the identification of a liquid acyclic ketone (C,H2,0 where n varies from 5 to 91, provides a sound introduction to mass spectrometry for students in our introductory organic course.' We focus on ketones due to the importance of the carbonyl group in organic compounds and because the fragmentation pathways are fairly predictable. Avital part of the experiment involves a base-catalyzed exchange of alpha hydrogens in the ketone, giving a deuterated ketone whose mass increases by the number of exchanged hydrogens. The deuterium exchange verifies textbook statements
Figure 1. Mass spectrum of 5-methyl-3-heptanone.
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about the acidity of alpha hydrogens i n carbonyl compounds (e.g., Aldol condensations and racemization) and provides important evidence for the identification of the ketone. Emphasis is placed on the depiction of single electron shifts to rationalize fragmentation pathways (81, for example, alpha cleavage of alkyl groups from MI, loss of CO from acylium ions, and the McLafferty rearrangement involving Mi or a n acylium ion.
'The theory of MS and the fragmentation behavior of all major classes of oraanic comoounds are covered in detail in a seoarate second-semester tours; Chemical Aoollcations of ~oectros.--- - ~-~ - - -~ entitled ~ copy. Tne exper ment oescr oed in rn s paper isoes gned to reach the wooer a~d~ence of at organ c srments, so tne OacngroJnd necessary for the nterprerat on of resJlts s covereo in mtroo.clory noles an0 n class discussions. 'Mass spectra were generated by injection of 1 yL of a solution oreoared from one droo of the unknown ketone in about 3 mL of an, nyoroJs ethy etner ~ p k c s awere ran on a rlew en Pac6ard 5890 GC mated ro a 5970 mass-select ve oetector JS ng a rnelhy s~l~cone cob Jmn ne lum carr er gas a1 50 ps , remperar-re ramp.ng of 10 C mln. and a 0.5-min delay. Mass fragments were recorded for 30 and up. Elution times ranged from 0.5 to 3.5 min for the ketones used. ~~
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Figure 2. Mass spectrum of 5-methyl-3-heptanone-2,2,4,4-d, Identification Procedure Although an IR spectrum can confirm the presence of the carbonyl function, it does not permit identification of the alkyl substituents because no spectrum "matching"is done. On the other hand.. the parent peak (Mi) in the mass spectrum imme. diately givtrs the molechar fi~rmulaof rht! unknown. Adeuterium cxchanrre pnredure is ~mducwdwith KODIMeOD and the M+ of t h e u h o w n - d , can be used to lind the number of alpha hydrogens in the unknown. Consideration of the usual fragmentation pathways exhibited by ketones identifies the unknown, either in part or conclusively, depending upon its complexity. Amajorpoint oftbis experiment is to help students understand that a definitive answer is not always possible from the interpretation of the data at hand. Additional information may be given by the instructor to aid in a complete identification. Helpful hints include the tendency of compounds with long chain alkyl groups to show peaks in the mass spectra a t intervals of 14 mass units due to random chain cleavage (8) and the number of carbon signals that would be observed in the 13C NMR spectrum of the unknown. We find that students can identify most unknowns from the experimental data and the paper clues.
A Typical Example The MS results of dl-5-methyl-3-heptanone (1)and its 2,2,4,Ptetradeuterio derivative (1-d,) serve as examples
of this procedure. In Figure 1the M+ peak a t 128 identifies 1as a ketone with the molecular formula CsH160.The major fragments from 1appear a t acylium ion: loss of ethyl radical by alpha cleavage loss of C4H8by a MeLafferty rearrangement loss of CO from the 99 fragment acylium ion: loss of CsHll radical from M+ probably a protonated ketene from a McLaffertytype rearrangement of the 99 fragment 99 72 71 57 43
The mass spectrum of I-& is shown in figure 2. The M' of 132 indicates the vresence of four alpha hvdromns. - - The fraaments a t 101 76 73 59 31
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loss of CH,CDc d4MeLafferty fragment C,H,D2+ (dz aeylium ion) CH,CD2+
correlate with the peaks noted in Figure 1leading to the constitution of 1as 7
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0 The CdH, group may be deduced using the hints noted above. The experiment is conducted by students working in pairs. One member of each pair obtains the IR spectrum of
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the microscale laboratory the unknown as a thin film, then generates the mass spectrum.? The other student conducts the deuterium exchange experiment and obtains the mass spectrum of the exchange product. Up to 14 students can complete the procedure within 2.5-3 h. Experiments such as the one described here undoubtedly will become common as instruments capable of autosampling (9)become more available to undergraduate institutions.
t o meter
Experimental C a u t i o n : All manipulations should he conducted in an effi-
cient h a d and care should be exermed in handlmg MeOD and the KOD/D20mlution.
Chemicals (Aldrich) were used as received. Amino (8-mm x 1.5-mm)stirring bar is placed into a dry, 1-dram (15-mm x 45-mm)vial, and the vial is marked &7 mm from the bottom. Methanol-dl (about 0.5 mL) is added to the mark, and one drop of the unknown ketone is added. The vial is clamped above a magnetic stirrer. One drop of a 40% KODm0 solution is added, and the mixture is stirred for 20 min. About 1 mLof D20and 1.5mLof ethyl ether are added to thevial, and the contents are mixed well using a Pasteur pipet. The top (ether) layer is transferred to a clean 1-dram vial, and the aqueous phase is extracted with an additional 1mL of ether. The combined ether extracts are washed with 0.5 mL ofD20, and the bottom phase is removed. The ether is dried with anhydrous NazSO4for a few minutes and the solution is filtered through a small cotton plug mto another d r y mal. 'l'he filtrate ~ s d ~ l u t etodabout 3 mL with anhvdrous ether. This solution ( 1 uL) 1s iniected into the inst&ment for a n a l y s i ~ . ~ Literature Cited 1. Novak, M.; Heinrich, J. J C h . Educ 1985,7O,A15&Al54. 2. Novak, M.; Hekich. T;Martin, K A; Green,J.; Lytle, S. J. C h . Edw. 1985, 70, A10>A110. 3. Holdsworth, D.;C k , 0. S.; bul Hj Abd Hamid, M. J. J. Chem. E d w . 1992, 69, 856858.
4. E i h t a d t , K E. J. C k m . E d w . 1992,69,4&61. 6. Hamann, C . S.; Myers, D.P;Rittle, K J.;Wlrth, E. F;Mae,Jr,O.A. J C h m . Educ
1891.68. 43-42, 6. Mabbon, G. A J. C k m . Edue 1890.67.441-445, 7 . E l l , D. W.; M c S h w B.T.; lhupek, L. S. J.Cham Edue. 19W. 65,907410. 8. Silvemtain,R. M.; Bassler.0. C.;MomU,T.C.Spectmmetric I&ntlfieationo,fOrgonic Compounds. 5th ed.; Wiley: New York. 1991:Chapter 1. 9. Aaleson,G. L.; Uoig. M. T;Heldtieh. F J. J C b m . Edm. 1985.70,A29a-A294.
Small-Scale Potentiometry and Silver One-Pot Reactions David W. Brooks University of Nebraska Lincoln, NE 68588 Dianne Epp Lincoln East High School Lincoln, NE 68510 Helen B. Brooks Synaps Lincoln, NE 68510 Shakhashiri suggests a demonstration in which the potential of the reaction cell for a one-pot reaction is displayed as the reactions are completed ( 1 , 2 ) ,and this was demonstrated for silver one-pot reactions (3).We recently suggested a strategy for constructing small-scalereference electrodes for potentiometric titrations (4). Here we report A162
Journal of Chemical Education
Schematic of reference electrode and reaction cell. the construction of a simple silver reference electrode, and its use in silver one-uot reactions. Usine small-scale electrodes, what has been used by teachers as a large-scale demonstration can be conducted as an experiment. Scores of students use smaller total amounts of chemicals than are consumed during a typical conventional scale demonstration. The strategy in making the reference electrode is to create a conductive, ~ o r o uds u p in the stem of a standard size plastic transfer pipet b k b by allowing a dry "crystal" of Soil Moist to swell in a solution of electrolyte. For the purpose of this experiment, 0.1 M KN03is an effective electrolyte. Cut the stem of a plastic pipet to a length of about 3 cm beyond the bulb. Insert a crystal of Soil Moist that fits snugly in the stem, and use a toothpick or open paper clip to push the crystal about half-way into the stem (1.5 cm from the tip). Carefully squeeze the bulb to expel some air and slowly draw 0.1 M KN03 into the bulb. Set this aside with the stem dipped in 0.1 M KN03 until the gel swells (about 5 min). Use a scissors to cut the top from the bulb. Pour out any 0.1 M KN03 and replace with about 1.5 mL of 0.1 M AgN03. Take a 10-cm length of 22 mesh silver wire, form a coil at one end, and insert the coil into the 0.1 M &NO3 in the opened bulb. Bend a small length (4 cm) of 22-mesh silver wire over the side of a corner well in a 24-well plastic plate. Use standard plastic pipets to make additions to this cell. Add 15 drops of 0.1 M &NO3 to the reaction cell. Clip the leads of a high impedance voltmeter, one to silver metal in the reaction pot, and the other to the silver metal in the reference electrode (see the figure). The meter reading will be close to 0 millivolts. Although we recommended using the reference electrode as a stirring device in our earlier work with homogenous solutions (4),this was not satisfactory in this case where precipitates are involved in five of the seven reactions studied. The most effective way to accomplish mixing was to use a separate, dedicated plastic pipet to withdraw the cell contents and then expel the contents from the pipet back into the cell. The meter reading is related to the difference in [Ag+l between the two solutions. The cell voltage is given as: (Continued on page A164)
