Som E K Inetic and Equilibrium Isotope E Ffects in The
Total Page:16
File Type:pdf, Size:1020Kb
Some Kinetic and Equilibrium Isotope Effects in the Isobutylphenone Enol and Enolate Ion System Y. Chiang, A. J. Kresge, and P. A. Walsh Department of Chemistry, University of Toronto, Toronto, Ontario M5S 1A1, Canada Z. Naturforsch. 44a, 406-412 (1989); received November 18, 1988 This paper is dedicated to Professor Jacob Bigeleisen on the occasion of his 70th birthday The following kinetic isotope effects were determined for acid-catalyzed ketonization of isobutyro- phenone enol and enolate ion through rate-determining hydron transfer from catalyst to substrate: enol, kH/kD = 3.30±0.07 (hydronium ion catalysis), /cH//cD = 4.0 + 2.8 (acetic acid catalysis); enolate ion, kH/kD= 1.00 + 0.21 (hydronium ion catalysis), kH/kD = 3A \ +0.20 (acetic acid catalysis), kH /kD = 7.48 + 0.23 (water catalysis). The magnitude of these isotope effects, when assessed in terms of the free energies of reaction for the processes in which they occur, are consistent with Melander-West- heimer-Bigeleisen theory. An equilibrium isotope effect of KH/KD = 5.88 + 0.32 was also determined for the ionization of isobutyrophenone enol as an oxygen acid. There has been a renaissance of interest in the Experimental Section chemistry of simple enols lately, fueled by the inven tion of methods for generating these unstable species Materials. Isobutyrophenone enol was generated in in solution in greater than equilibrium amounts under aqueous solution by hydrolysis of its potassium or conditions where they can be observed directly. This lithium enolate, as described [2 b]. Isobutyrophenone has allowed accurate measurement of rate and equi enol ether, 2, was prepared by acid-catalyzed elimina librium constants for their reactions, and that has led tion of methanol from isobutyrophenone dimethyl to much new enol chemistry [1], In this paper we add ketal, which in turn was made from isobutyrophenone some kinetic and equilibrium isotope effects to this and methanol using a standard method [3]. The elim store of information. In particular, we compare iso ination was conducted by heating the ketal with a tope effects on the same reaction of an enol and its catalytic amount of p-toluenesulfonic acid at 120° for enolate ion; this is of special interest because enolate 2 hours, under slightly reduced pressure, and remov ions are much more reactive than enols [2], and the ing methanol as it formed. The product enol ether was comparison therefore involves substrates of closely separated from the catalyst by distillation, and final similar structure but grossly different reactivity. purification of a sample for kinetic measurements was We have chosen the enol of isobutyrophenone, 1, for performed by gas chromatography; the material had this study because there is a background of informa a proton NMR spectrum consistent with its structure: tion on this keto-enol system [2 b] which allows com- (CDCI3/TMS) : — ö 7.30 (5 H, s), 3.30 (3 H. s), 1.83 (3 H, s), 1.66 (3 H. s), and a high resolution mass OH OMe spectrum consistent with its elemental composition: p h A / P h A f M/s = 162.1039, expected for Cn H 140, 162.1041. 1 2 All other materials were best available commercial grades. Solutions were prepared using deionized H20, parison of isotope effects with other reaction param purified further by distillation, or D20 (MSD, 99.8 eters, notably free energies of reaction. We have also atm% D) as received. carried out some studies on the related methyl enol Kinetics. Rates of ketonization of isobutyrophe ether. 2. for comparison. none enol were measured spectroscopically by moni toring the increase of ketone absorbance at /. = 245 nm, Reprint requests to Dr. A. J. Kresge, Department of Chemis try, University of Toronto, Toronto. Ontario M5S 1A1. as described [2 b]. Rates of hydrolysis of isobutyrophe Canada. none enol methyl ether were also measured spectro- 0932-0784 / 89 / 0500-0406 $ 01.30/0. - Please order a reprint rather than making your own copy. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung This work has been digitalized and published in 2013 by Verlag Zeitschrift in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der für Naturforschung in cooperation with the Max Planck Society for the Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Advancement of Science under a Creative Commons Attribution Creative Commons Namensnennung 4.0 Lizenz. 4.0 International License. Y. Chiang et al. ■ Kinetic and Equilibrium Isotope Effects 407 scopically by monitoring either the increase of ketone Table S 1. Rate data for the ketonization of isobutyrophenone absorbance at /. = 245 nm or the decrease of enol ether enol in D,0 solutions of hydrochloric and perchloric acids at 25 C. ' absorbance at /. = 217 nm; similar rate constants were obtained by the two methods. [Acid]/10 2 M KU 10 -2 s DC1a 0.78 0.498, 0.477 Results 1.17 0.732, 0.769 1.56 1.04, 1.03 Ketonization. Rates of ketonization of isobutyrophe- 2.60 1.68, 1.71, 1.71 3.25 2.07, 2.12 none enol were measured in D20 solutions of DC1 and DCIO4. The data conformed to the first-order k0Js ~ l = -(0.58 ±1.82) x 10"4 + (6.52 + 0.09)x 10"1 [DC1] rate law precisely, and observed first-order rate con d cio 4 stants were accurately proportional to acid concentra 0.72 0.550, 0.548 tion over the range employed (0.008 — 0.03 M for DC1 1.40 0.957, 0.966, 0.944, 0.975 2.20 1.53, 1.58, 1.56 and 0.008-0.04 M for DC104). These results are sum 2.77 1.94, 1.88, 1.79 marized in Table S 1. 2.87 1.94 Linear least squares analysis gives the hydronium 3.65 2.40, 2.41 ion catalytic coefficients kD+ =0.652±0.009 M~1 s~1 kaJs ~ 1 =(8.75±2.99)x 10"4+(6.43±0.13)x 10"1 [DCIOJ for the DC1 solutions and kD + = 0.643 ± 0.013 M " 1 s " 1 Ionic strength = [DC1]. b Ionic strength = 0.10 M (NaCl) for the DC104 solutions. The measurements in DC1 solutions were done at ionic strengths equal to the DC1 concentration, while those in DC104 solutions Table S 2. Rate data for the ketonization of isobutyrophenone were performed at a constant ionic strength, ^ = enol in d 20 solutions of sodium hydroxide at 25.0 °c a. 0.10 M, maintained by adding NaCl. The fact that the [NaOD]/ ^Obs/S" catalytic coefficients obtained by these two methods 10"2 M are closely similar shows that there is no salt effect on 0.50 3.54, 3.66, 3.73, 3.67, 3.80, 3.60, 3.70, 3.67, 3.66 the ketonization reaction over the concentration 0.66 4.29, 4.44, 4.11, 4.48, 4.24, 4.89, 4.10, 4.80, 4.09 range studied. The weighted average of the two values 0.80 5.43, 4.84, 5.57, 4.66, 4.48, 4.07, 4.48, 4.18, 4.83 is kD+ =0.648 + 0.007, and combination of that with 0.82 4.27, 4.35, 4.32, 4.71, 4.84, 4.55, 4.85, 4.77, 4.83 0.92 5.53, 5.20, 5.55, 5.82, 4.80, 5.26, 5.75 the protio rate constant determined before [2 b] gives 1.10 5.58, 5.11, 5.00, 5.15, 5.09, 5.48, 5.24, 5.51 the isotope effect kH+/kD+ = 3.30 + 0.07. 1.53 6.74, 6.87, 6.28, 5.85, 6.86, 6.03, 6.42, 6.77 2.91 6.98. 6.66, 6.68, 7.13, 6.91 Rates of ketonization of isobutyrophenone enol 3.93 7.59, 7.96, 7.44 were also measured in D20 solutions of NaOD. Once 6.01 8.17, 8.14, 8.27, 8.26 again the data conformed to the first-order rate law 7.91 8.36, 9.27, 9.17, 8.92, 8.74, 9.23, 9.21 accurately. Determinations were made over the con 9.91 8.97, 8.61, 8.80, 8.97, 8.72, 8.97. 8.73, 8.76 centration range [NaOD] = 0.005 —0.1 M at constant A;bsVs = (1.08±0.03)x 10"'+(3.82 + 0.16) x 1011 [d + ] ionic strength (0.10 M). The results are summarized in a Ionic strength = 0.10 M (NaCl). TableS 2. These observed first-order rate constants obtained in NaOD solutions were proportional to hydroxide from water, ion concentration at low [NaOD], but, as the concen w od + d o -£ x ^ D-,0 (1) tration increased, the hydroxide-ion catalysis of the ^ S h h rh ketonization reaction diminished and rate constants Eventually, however, at sufficiently high hydroxide ion eventually reached a limiting value, Figure 1. Such concentrations, the equilibrium preceeding the rate- behavior is characterized of the ketonization of simple determining step shifts completely to the side of eno enols in basic solutions [2], It is due to the fact that the late; further increases in hydroxide ion concentration hydroxide-ion catalyzed reaction occurs through con then have no effect, and catalysis becomes saturated. version of the enol to the much more reactive enolate The rate law which applies to this reaction scheme is ion followed by rate-determining acid-catalyzed ke tonization of the enolate ion by hydron transfer kobs = k'D20K[DO~]/(K [DO-] + l). (2) 408 Y. Chiang et al.