In the Laboratory
The Microscale Laboratory
Reaction of Dibenzoylethylene with Hydriodic Acid
W
Fred H. Greenberg Department of Chemistry, SUNY College at Buffalo, Buffalo, NY 14222;
[email protected] The reaction of the title compound in acid solution with iodide ion provides an experiment that exemplifies fundamental organic pathways and the use of NMR and IR spectra in discovering the identity of the product 1,2-dibenzoylethane. Ph
CO
CH
CH
CO
Ph
Ph
CO
+
25 °C
2 HI
acetone
1
CH2
CH2
CO
Ph
+
I2
Experimental Procedure1
2
A proposed mechanism (1), outlined below, involves carbonyl protonation, addition of HI to the carbon–carbon double bond, and nucleophilic substitution by iodide ion on the carbon-bound iodine to form molecular iodine with the displacement of an enol of 2, followed by ketonization. H
+
O
+
1
H+
Ph Ph O
H
H
+
Ph Ph
+
Ph
HI
Ph I
O H
O
H
+
O
O Ph
Ph Ph
Ph I
I−
+
O
O
O
+
by hydrogen effected by hydriodic acid, called reductive dehalogenation (3), and recognized as the reverse of acid-catalyzed α-halogenation of carbonyl compounds (4). Hydrogen halide additions to 1, with halide ions less nucleophilic than iodide ion, allow isolation of the corresponding dibenzoylhaloethane. Other enediones reduced by this method are 3,6-dioxo4-cholestene, heptadec-3-ene-2,5-dione, 4-oxo-heptadec-2enal (1), and 1,2-dibenzoylacenaphthylene (author’s unpublished results).
2
In a 25-mL Erlenmeyer flask place 0.5 mmol of dibenzoylethylene. Dissolve the solid in 3 mL of acetone and add 325 mg of powered anhydrous sodium iodide. If needed, use a stirring rod to completely dissolve the salt. Add to this mixture 0.2 mL of concd HCl. Swirl the flask and note the changes that occur. To recognize the color that forms, add a drop of the mixture to a dime-sized portion of wet starch on a paper towel. To remove the color, add to the mixture, with swirling, a volume of an aqueous solution of 0.1 M sodium bisulfite so that the millimolar ratio of sodium bisulfite to dibenzoylethylene is two to one. After chilling in an ice-water bath, collect the solid at reduced pressure on a Hirsch funnel. Wash the solid with three 5-mL portions of distilled water to remove sodium salts. Recrystallize the wet product from a minimum, ca. 3–4 mL, of ethanol. Slow cooling gives white needles: mp 145–146 °C; IR (KBr) 1669 (C=O) cm᎑1; 1H NMR, δ 8.06–7.35 (10 H; m), 3.49 (4 H; s); 13C NMR, δ 198.4, 137.0, 133.0, 128.7, 128.2, 32.8.2 WSupplemental
Material The C NMR spectra of dibenzoylethane and dibenzoylethylene are available in this issue of JCE Online. 13
O
I2
After being given this mechanism with the three intermediates replaced by molecular formulas, students were asked to draw the corresponding structures and the structure of 2. Identification of 2 is confirmed by comparing the spectral features of 1 and 2, in which the 13C NMR spectrum of 2 shows a saturated carbon at δ 32.8 and the 1H NMR displays four methylene protons at δ 3.49. Also, the IR spectrum of 2 shows the less conjugated carbonyl absorption at higher frequency by 12 cm᎑1. The reaction is reported to be nearly quantitative (1), although student yields average 54% (64 mg), presumably owing to losses during purification. The iodine color is seen immediately and 2 has been isolated after a reaction time of one minute While hydrogen chloride and hydrogen bromide add to 1 to give dibenzoylchloroethane and dibenzoylbromoethane, respectively, hydrogen iodide gives only reduction to 2 (2). Presumably the intermediate dibenzoyliodoethane, as an α-haloketone, undergoes the characteristic replacement of halogen
Notes 1. A variation of this procedure is the direct addition of commercial hydriodic acid (5.5 M; 0.6 mL) to a solution of 100 mg of 1 in 2–3 mL of acetone. 2. Reproductions of spectra are available on request. Spectral features of 1: IR (KBr) 1657 (C=O) cm᎑1; 1H NMR: δ 8.18–7.30 (10H; m), 8.02 (2H s); 13C NMR: δ 189.6, 136.8, 135.0, 133.8, 128.8.
Literature Cited 1. D’Auria, M.; Piancatelli, G.; Scettri, A. Synthesis 1980, 245–248. 2. Paal, C.; Schulze, H. Chem. Ber. 1902, 35, 168–176. 3. Penso, M.; Mottadelli, S.; Albanese, D. Synth. Commun. 1993, 23, 1385–1391. Mandal, A. K.; Nijasure, A. M. Synlett 1990, 554. Ono, A.; Fujimoto, E.; Ueno, M. Synthesis 1986, 570– 571. Gemal, A. L.; Luche, J. L. Tetrahedron Lett. 1980, 21, 3195–3198. 4. Zimmerman, H. E. Acc. Chem. Res. 1987, 20, 263–268.
JChemEd.chem.wisc.edu • Vol. 77 No. 4 April 2000 • Journal of Chemical Education
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