119 system at temperatures between 180 and 230”. Complete highresolution mass spectra were measured on the CEC 21-110 mass
spectrometer. The low electron voltage spectra also were obtained on the model 21-1 10B mass spectrometer. Metastable ion measurements reported in Table I1 were made on an Associated Electrical Industries MS-12 mass spectrometer with a glass inlet system at 200”. Deuterium Exchange. The appropriate compound was subjected to exchange on a deuterium oxide treated potassium hydroxide-Carbowax vapor phase chromatography column.8 4 (34) M. Senn, W. J. Richter, and A. L. Burlingame, J . Amer. Chem. SOC.,87, 680 (1965).
z
4,4-Dimethyl-2-cyclohexenone yielded 76 % 6,6-d2and 24 6-d2; 6,6-dimethylbicyclo[3.l.0]hexan-2-oneyielded 75 % 3,3-d2and 18 3 4 ; 4,4,6-trimethyl-2-cyclohexenoneyielded 79 % 6-dl; and 4methyl-2cyclohexenone yielded 58 Z,2,4,6,6-d4 and 34 % da. 5-Methylbicyclo[3.1.O]hexenone was exchanged to the 3,3-d~derivative of greater than 99% isotopic purity by stirring in deuterium
z
oxide and methanol-OD with sodium methoxide. Synthesis of the Ketones. The syntheses of the ketones used in this study have been described in a separate publication.10 Acknowledgment. The authors express their appreciation to Professor A. L. Burlingame for his interest and assistance in this work.
The Two Mechanisms for the Acid-Catalyzed Hydrolysis of Enol Acetates’ Donald S. Noyce and Ralph M. Pollack Contribution from the Department of Chemistry, University of California, Berkeley, California 94720. Received August 2, 1968 Abstract: Hydrolysis of a-acetoxy-p-nitrostyrene (6) in 1 M sulfuric acid shows the characteristics of normal ester hydrolysis. In DzO,the solvent isotope effect, kHtO/kD,O, is 0.75; the rate of hydrolysis is very similar to the
rate of hydrolysis of isopropyl acetate. Increasing the concentration of sulfuric acid causes an increase in rate but which closely parallels the increasing rate of hydrolysis of isowhich is nonlinear with the acidity function Ho, propyl acetate. In sulfuric acid concentrations greater than 55 %, the rate of hydrolysis of 6 increases very rapidly. In 69z sulfuric acid, the solvent isotope effect, kHzO/kD,O, is now 3.25. Thus, the mechanism has changed to one involving initial, and rate-determining, olefin protonation. The effect of substituents in substituted a-acetoxystyrenes further serves to substantiate the two mechanisms, and to delineate the circumstances under which each of the two mechanisms will be dominant.
R
ecently there has been a good deal of interest in the acid-catalyzed hydrolysis of enol derivatives. 2-4 Studies with enol ethers have shown that these reactions involve rate-determining protonation at carbon and that they show general acid catalysis and a high order of reactivity. Studies of the hydrolysis of enol acetates have been somewhat less extensive. DePuy has studied the alkaline hydrolysis of several enol acetate^.^ The hydrolysis of vinyl acetate in relatively concentrated hydrochloric acid has been examined very carefully by Yrjana,6 and he concludes that normal ester hydrolysis is occurring. Though Kiprianova and Rekasheva’ and Landgrebes propose mechanisms involving initial protonation on carbon, Yrjana6 gives very compelling reasons for preferring a mechanism for the hydrolysis which is normal ester hydrolysis, including a consideration of the relative rate of reaction, the entropy of activation and the solvent isotope effect. (1) Supported in part by a grant from the National Science Foundation, GP-6133X. (2) A. J. Kresge and Y . Chiang, J . Chem. Soc., B, 53, 58 (1967). (3) D. M. Jones and N. F. Wood, ibid., 5400 (1964). (4) E. J. Stamhuis, W. Drenth, and H. van den Berg, Rec. Trav. Chim., 83,167 (1964). ( 5 ) C. H. DePuy and R . E. Mahoney, J . Amer. Chem. SOC.,86,2653 (1964). (6)T. Yrjana, Soumen Kemisrilehfi, B, 39, 81 (1966). (7) L. A. Kiprianova and A. F.Rekasheva, Dokl. Akad. Nauk, SSSR, 144, 386 (1962); Proc. Acad. Sci. USSR,Phys. Chem. Sect., 144, 393 (1962). (8) J. A. Landgrebe, J . Org. Chem., 30, 2997 (1965).
It was noted by Hammettg that in acid hydrolysis of esters the effect of structure in the alcohol component is very small in marked contrast to the large effect observed in alkaline hydrolysis. Variation from t-butyl to benzyl or phenyl caused a change of less than a factor of 2 in rate. The acid hydrolysis of benzyl acetates’O has a p of only -0.05. We have examined the rates for the acid-catalyzed hydrolysis of a series of ring-substituted a-acetoxystyrenes and we find that enol acetates may hydrolyze through two different pathways. The first is that proposed byYrjana,6 which is the same mechanism by which most saturated esters hydrolyze. The second involves protonation of the carbon-carbon double bond. Two different factors determine which mechanism is to be active in any given situation. The first is the acidity and the second is the stability of the carbonium ion formed by the protonation of the carbon-carbon double bond. Experimental Section11 Preparation of Materials. The following general procedure was used to prepare the substituted a-acetoxystyrenes used in this study. (9) L. P. Hammett, “Physical Organic Chemistry,” McGraw-Hill Book Co., Inc., New York, N . Y . , 1940, p 213. (10) E. Tomilla and C. N . Hinshelwood, J. Chem. Soc., 1801 (1938). (1 1) Analyses are by the Microanalytical Laboratory, University of California, Berkeley, Calif. Melting points and boiling points are uncorrected.
Noyce, Pollack / Hydrorysis of Enol Acetates
120 Table I Compound
p-Methoxy-a-acetoxystyrene (1) p-Methyl-a-acetoxystyrene (2) a-Acetoxystyrene (3) p-Chloro-a-acetoxystyrene (4) m-Chloro-a-acetoxystyrene (5) p-Nitro-a-acetoxystyrene ( 6 )
Bp, 'C(mm)
Mp, O C
C
108-1 10 (1.1) 106-108 (5.0) 92-95 (4.5) 116-118 (4.1) 114-116 (4.4) 120-121 (0.12)
77-78 28.5-30.5
68.73 74.97 74.05 61.08 61.08 59.97
...
... 52-53'
The appropriate acetophenone (20 g) was mixed with a two- to fourfold molar excess of isopropenyl acetate and about 200 mg of p-toluenesulfonic acid as catalyst. The solution was refluxed and acetone was collected by distillation as it formed. The amount of acetone collected did not accurately reflect the extent of reaction, as approximately twice as much was formed as would have been predicted. This was presumably due to decomposition of isopropenyl acetate to acetone and ketene. However, the reaction could be monitored quite conveniently by gas chromatography on a 5 ft X 0.25 in. SE-30 column at about 140". Since the half-life of all of the reactions was about 2 days, they were stopped at 5 G 7 5 x completion. Ether was added to the cooled solution to render it homogeneous. The solution was washed with aqueous sodium bicarbonate, dried over magnesium sulfate, concentrated on a rotary evaporator, and distilled through a spinning-band column under reduced pressure. Yields were between 40 and 65 %. The properties of the compounds prepared are given in Table I. Compounds 2-5 were used directly in the kinetics. 1 was recrystallized from ethanol and 6 was purified by gas chromatography before they were used in the kinetic studies. Nmr and infrared spectra were distinctive and in accord with the structural assignments. The preparation of sulfuric acid-h has been described previously. 1 2 Kinetic Methods. Rates were measured by following the change in absorbance at a wavelength between 280 and 310 mk at concenM . Since the trations varying between 5 X 10-5 and 5 x spectra of the starting material and products were similar for compounds 2-5 measurements had to be made on the high-wavelength n - ~ *band of the ketone and this necessitated the use of concentrations approaching 5 X 10W M . The spectra of starting material and products were sufficiently different in the case of 1 and 6 so that lower concentrations could be used and kinetic measurements could be made by following the change in absorbance at the main bands. To 3.00 ml of sulfuric acid of the requisite strength which had been temperature equilibrated in a 1-cm ultraviolet cell 25 ~1 of an ethanol stock solution of the organic substrate was added to initiate the reaction. Absorbance measurements were generally made to beyond 95 reaction. To determine the final sulfuric acid concentration weighed aliquots here titrated in duplicate with sodium hydroxide. Havalues were taken from the data of Bascombe and Bel1,la and Jorgenson and Hartter.I4 The small amount of ethanol was ignored in determining the HOvalues. Solvent isotope effects were calculated at the same mole fraction of acid. Justification for use of mole fraction as a basis for comparison has previously been given. 15 All compounds gave excellent pseudo-first-order kinetics beyond 95 reaction. Rate constants were calculated by a least-squares computer program. The standard deviations of the rate constants as calculated by the computer showed the precision of the measurements to be good, the error limits obtained generally being less of the observed rate constant. than 11
Results The acid-catalyzed hydrolysis of substituted aacetoxystyrenes may be conveniently followed kinetically by observing the appearance of the cor(12) D. S. Noyce, H.
S. Avarbock, and W. L. Reed, J . Amer. Chem.
Soc., 84, 1647 (1962).
(13) I
