In the Laboratory
Ozonolysis Experiments Using Gas Chromatography– Mass Spectrometry An Undergraduate Organic Chemistry Laboratory Experiment Charlene M. Rhoads, George R. Farquar, and William F. Wood* Department of Chemistry, Humboldt State University, Arcata, CA 95521 Reaction of ozone with organic compounds was first described by Schönbein in 1855 from a reaction of ethylene with ozone to produce carbonic acid, formic acid, and formaldehyde (1). Ozonolysis has been used in the synthesis of aldehydes, ketones, and carboxylic acids (2), but its most important application has been in determining the position of carbon–carbon double bonds in a compound (3). This technique has rarely been introduced into the organic chemistry laboratory curriculum due to the specialized apparatus needed to generate ozone and to identify the products. We developed several ozonolysis experiments using an easily constructed ozone generator and analysis with a gas chromatograph–mass spectrometer (GC-MS). Ozone reacts with carbon–carbon double bonds by a 1,3-dipolar addition mechanism (4) to give a 1,2,3-trioxolane or molozonide product (eq 1). This trioxolane is thermally unstable and quickly rearranges to a 1,2,4-trioxolane,
generator was reported to produce about 0.5 mg of O3 per minute (6), but our apparatus produced about half this value. Frequently ozonolysis is done at dry-ice temperature to minimize reaction at sites other than the double bond; however, in our experiments we found few side products at room temperature. Many reagents reduce the ozonide; we found triphenylphosphine to be efficient and not to interfere with the GC-MS analysis. Ozonolysis products were identified by comparison of retention times and mass spectra with those of known compounds. Methyl oleate was used to illustrate this technique on a straight-chain compound. A solution of this compound was analyzed by GC-MS as a reference. After ozonolysis, the compound in the initial analysis was absent and two new compounds were observed. Comparison of the retention time and mass spectra of these with a homologous series of
O O R
H C
O3
C
R
R
R
O C
C
R
(1)
H
R
better known as an ozonide (eq 2). Because ozonides frequently detonate on purification attempts, they are rarely isolated. Instead they are usually reduced to aldehydes O O R
O
O C
C
H C
H
C O
R R
O
R
(2)
R
R
and/or ketones with a variety of reagents (eq 3). Examination of these fragments can be used to locate the position of a carbon–carbon double bond in the original molecule. The new carbonyl groups indicate where the –C=C– bond existed in the original molecule. O
H C
R
[H]
C O
H
R
O
R
C R
O
+
O
C R
(3)
R
Discussion of Chemistry Ozone was produced in an apparatus described by Beroza and Bierl (5), which can be constructed from material found in most chemistry laboratories (Fig. 1). This
*Corresponding author.
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Figure 1. Ozone generator. A: Tesla coil (vacuum leak detector); B: stainless steel needle; C: injectable septum; D: test tube; E: solution of compound; F: moistened KI-starch test paper; G: Teflon tubing; H: rubber stopper (also GC septum or Teflon plug): I, glass tubing; J: aluminum foil; K: rubber tubing; L: ground; M: copper wire.
Journal of Chemical Education • Vol. 74 No. 10 October 1997
In the Laboratory aldehydes (heptanal to dodecanal) showed one to be identical with nonanal. This indicates that the double bond in the original compound was 9 carbon atoms from the end of the chain. Because no molecular ion was observed and a reference compound was not available, the other product of this ozonolysis was tentatively identified as methyl 9-oxononanoate (eq 4). CH 3(CH 2)7CH
CH(CH 2)7CO 2CH 3
1. O 3 2. Ph3P
O
O HC(CH 2)7CO 2CH 3
+
CH 3(CH 2)7CH
(4)
Unknown aromatic compounds with noncyclic double bonds were ideal substances for student analysis. Stilbene (M+, m/z = 180) gives a single product on ozonolysis, indicating it may be a symmetrical molecule. Analysis of the mass spectrum of this product showed it to be benzaldehyde (eq 5). CHO
CH
CH
1. O 3
(5)
2. Ph3P
This information, with the molecular weight of the original compound, leads to the determination of the structure. Other similar aromatic compounds (and products identified) given to students as unknowns were α-methylstilbene (benzaldehyde and acetophenone), α-methylstyrene (acetophenone), and 4-methylstyrene (4-methylbenzaldehyde). Experimental Procedure CAUTION: the ozonolysis should be done in a fume hood. Flammable solvents and oxygen form an explosive mixture that can ignite explosively from the electric discharge and so must be avoided.
Ozonolysis Apparatus The ozone generator (Fig. 1) was made as follows. A long piece of 21-gauge or similar size stainless steel needle or tubing was pushed through a rubber stopper (a gas chromatographic injection port septum or Teflon® plug can be used) in a 10-cm length of glass tubing (6 mm i.d., 8 mm o.d.). After wrapping the glass tube with several layers of aluminium foil, a length of copper wire was added so that it extended beyond the top. The apparatus was then inserted into an 8-cm length of thick-walled rubber tubing to give a snug fit. Tygon tubing was attached to the top of the glass tube with a 3-way stopcock so oxygen or nitrogen could be added alternately. For ozone generation, about 10 mL per minute of O2 was passed through the tube. The copper wire from the top of the apparatus was grounded and an electric discharge from a hand-held Tesla coil (Vacuum Leak Detector, Fisher 15-340-75) was applied to the stainless steel needle or tubing. The ozone-containing oxygen stream was bubbled through a test tube containing a dichloromethane solution of the test compound. Effluent gas was directed to a test tube containing a piece of moistened KI–starch indicator paper to detect ozone.
Ozonolysis Procedure Two milliliters of a stock solution containing 35 mg of compound in 50 mL of dichloromethane was treated with O3 . After about 2 min, the moistened KI–starch indicator paper turned blue. To ensure complete reaction, ozonolysis was continued for a total of 5 min. After the solution was purged with N2 for 1 min to remove excess O 3, triphenylphosphine (2 mg) was added and the solution was gently shaken for a few seconds. A 2-µL sample of this solution was analyzed by GC-MS.
GC-MS Analysis All samples were examined by GC-MS before and after ozonolysis. GC-MS analysis was carried out using a Hewlett–Packard gas chromatograph (Model 5890) fitted with a mass selective detector (Model 5970) using a 12-m cross-linked methyl silicone capillary column (HP-1). The gas chromatograph was programmed so the oven temperature was kept at 40 °C for the first 4 min and then increased to a final temperature of 250 °C at a rate of 30 °C/min. The gas chromatograph was kept at this final temperature for 4 min. The initial compound and the resulting ozonolysis products for these experiments had the following mass spectra and retention times (RT): Methyl oleate: m/z = 296(M+, 0.1), 97(23), 96(21), 87(24), 83(28), 74(41), 69(44), 67(30), 55(86), 43(72) and 41(100); RT = 12.1 min. Nonanal: m/z = 98(25), 82(23), 70(28), 69(22), 57(91), 56(55), 55(48), 44(60), 43(69) and 41(100); RT = 7.4 min. Methyl 9oxononanoate: m/z = 143(218), 111(29), 87(59), 83(37), 74(90), 59(51), 55(83), 543(69), 41(100) and 39(52); RT = 9.1 min. Stilbene: m/z = 181(M+1, 12), 180(M+ , 100), 179(92), 178(54), 176(11), 165(42), 152(13), 151(8), 89(19), and 76(16); RT = 10.5 min. Benzaldehyde: m/z = 107(M+1, 6), 106(M+, 82), 105(79), 78(17), 77(100), 74(10), 52(16), 51(71), 50(44), and 39(14); RT = 6.2 min. a-Methylstilbene: m/z = 194(M +, 65), 193(14), 180(16), 179(100), 178(52), 165(16), 115(59), 103(27), 91(29), and 89(31); RT = 10.5 min. Benzaldehyde: m/z = 107(M+1, 6), 106(M +, 82), 105(79), 78(17), 77(100), 74(10), 52(16), 51(71), 50(44), and 39(14); RT = 6.2 min. Acetophenone: m/z = 120(M+, 36), 106(8), 105(99), 78(10), 77(100), 74(8), 51(49), 50(28), 43(32), and 39(13); RT = 7.1 min. 4-Methylstyrene: m/z = 118(M+ , 82), 117(100), 115(35), 103(12), 91(35), 65(19), 63(19), 57(18), 51(22), and 39(32); RT = 6.7 min. 4-Methylbenzaldehyde: m/z = 120(M+, 68), 119(84), 92(10), 91(100), 89(13), 65(36), 63(22), 51(19), 50(16), and 39(42); RT = 7.2 min. a-Methylstyrene: m/z = 119(M+1, 10), 118(M+, 100), 117(93), 115(35), 103(59), 91(23), 78(38), 77(31), 51(33), and 39(27); RT = 6.5 min. Acetophenone: m/z = 120(M+ , 36), 106(8), 105(99), 78(10), 77(100), 74(8), 51(49), 50(28), 43(32), and 39(13); RT = 7.1 min.
Literature Cited 1. 2. 3. 4. 5. 6.
Schönbein, C. F. J. prakt. Chem. 1855, 66, 282. Henne, A. L.; Hill, P. J. Am. Chem. Soc. 1943, 65, 752. Long, L., Jr. Chem. Rev. 1940, 27, 437. Criegee, R. Angew. Chem. Int. Ed. Engl. 1975, 14, 745. Beroza, M.; Bierl, B. A. Anal. Chem. 1967, 39, 1131. Beroza, M.; Bierl, B. A. Anal. Chem. 1966, 38, 1976.
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