1313
THEBR$NSTEDa AND ISOTOPE EFFECTS FOR VINYLETHER HYDROLYSIS potentiostatic or galvanostatic conditions, respectively, a t U 5 4 V. It is suggested that the oxidation of dissolved CO involves the type 2 species as intermediates a t U < 0.4 V. This interpretation is substantiated by the shape of the potentiostatic I-U curve, which consists of an initial part with small oxidation rates between 0.1 and 0.4 V and of a second part with larger rates above 0.5 V. The decrease of Qz between 0.1 and 0.4 V implies that the rate of adsorption of type 2 species is insufficient to maintain full coverage with increasing oxidation rates. Adsorption of type 2 species is the rate-determining step at U 2 0.4 V. The type 1 species are considered the intermediates during the oxidation of dissolved carbon monoxide a t I > 10 mA. The oxidation rates of type 1 species in
The Brgnsted
a
Figure 9 are comparable to the rates of the oxidation of CO in Figure 5 and 6 . The coverage Q1/sQ1 decreases with potential at currents which are approaching the limit imposed by mass transport. Insufficient supply which leads to a smaller adsorption rate is the reason for the decrease of Q1/sQl. Partial mass-transport control was also concluded for the CO oxidation on smooth platinum free of Goad. I n contrast to the voltammetric measurements on smooth platinum,1~2e the decrease of Q 1 / ~ & 1 with U occurs here in a potential region in which the coverage with OHad and Oad is small.
(26) P. Stonehart, Electrochim. Acta, 12, 1186 (1967).
and Isotope Effects for Vinyl Ether Hydrolysis'.
by Maurice M. Kreevoy and Robert Eliasonlb Department of Chemistry, Unieersity of Minnesota, Minneapolis, Minnesota
66466 (Received October 8, 1967)
Like other reactions in which proton transfer is rate determining, ethyl vinyl ether hydrolysis is shown to obey the Br$nsted catalysis law with a variety of carboxylic acids, but acids of other structure give rates substantially varying from those predicted by the carboxylic acid correlation. The value of a for carboxylic acids is 0.66. The over-all solvent isotope effect, k ~ / k is~ 3.2 , f 0.1. The competitive isotope effect, H H / H D , is 7.0 f 0.1. If proton transfer is directly from M 8 0 +to the substrate, these lead to a primary isotope effect, ( k H / k D ) I , of 4.8 and a secondary solvent isotope effect, ( ~ E / ~ D ) I I o, f 0.66. From the latter an isotopic a, ai,of 0.56 is obtained. Competitive tritium isotope effects have been measured and the Swain-Schaad relations are obeyed. The reaction coordinate seems to be largely proton translation.
A number of papers have recently appeared establishing that vinyl ether hydrolysis is general-acid c a t a l y ~ e d ,shows ~ ~ ~ a solvent isotope effect, k ~ / k D , around 3 when catalyzed by mineral acid in water,2-6 and shows a substantially larger isotope effect when catalyzed by molecular formic acid.4 It has also been shown that the proton, once covalently affixed to the substrate, does not revert to the ~ o l v e n t . ~These observations convincingly show that vinyl ether hydrolysis proceeds through the mechanism given in eq 1 and 2. The mechanism is given for the specific case of ethyl vinyl ether hydrolysis, with which this paper is concerned. CH2=CHOCzH6 CH&H-OCzHs+
+ H + 3 CH&H-OCZHS+
+ HzO -+
(1)
series of ~
a
CH&H=O
~ -t s
~
+ CzH50H
p
~
(2)
The present paper provides a Brpinsted a for a series
of carboxylic acids and separates k ~ / into k ~ primary and secondary solvent isotope effects. From the latter, an isotopic a is obtained in general agreement A test of the Swain-Schaad with the Brpinsted a. relation is also described.' Some details of transitionstate structure can be surmised. (1) (a) Supported, in part, by the National Science Foundation through Grant No. GP-5088; (b) Hercules Corp. Summer Fellow, 1965; Du Pont Co. Summer Fellow, 1966; Ethyl Corp. Fellow, 19661967. (2) P. Salomaa, A. Kankaaperh, and M. Lajunen, Acta Chem. Scand., 20, 1790 (1966). (3) A. J. Kresge and Y . Chiang, J. Chem. SOC.,Sect. B , 53 (1967). (4) A. J. Kresge and Y . Chiang, ibid., Sect. B , 58 (1967). (5) (a) D. M. Jones and N. F. Wood, ibid., 5400 (1964); (b) A. Ledwith and H. J. Woods, ibid., Sect. B , 753 (1966). (6) M. S. Shostakovskii, A. 5. Atavin, B. V. Prokoljev, B. A. Trofimov, V. I. Lavrov, and N. M. Driglazov, Dokl. Akad. Nauk SSSR, 163, 1412 (1965). (7) C. G. Swain, E. C. Stivers, J. F. Reuwer, Jr., and L. J. Schaad, J . Amer. Chem. SOC.,80, 5885 (1958). Volume 78, Number 4
April 1988
1314
Experimental Section Ethyl vinyl ether was purchased from Aldrich Chemical Co. and purified by distillation. Aqueous inorganic acids and bases were made up and standarized in the usual ways,8 as were formic acid solutions. Chloroacetic and cyanoacetic acids 17ere purified by distillation under vacuum. Chloroacetic acid hydrolyzes slowly to glycolic acid and HC1 in its aqueous solution^.^ I n our buffer solutions this hydrolysis was found, by periodic titration over a 2-month period, to consume -2% of the acid per meek. All the work described in this paper was done with solutions no more than 2 days old, whose pH had been verified with a pH meter. Kinetic measurements were made by conventional spectroscopic techniques and pseudo-first-order rate constants, ICl, could be replicated with discrepancies of no more than 5%. p-Nitrophenylhydraxone Collection. A mixture of 5 ml of ethyl vinyl ether with a solution of 1 g of p-nitrophenylhydrazine in 100 ml of 0.1 N hydrochloric acid was stirred vigorously for about 1 min, after which a yellow precipitate appeared. The precipitate mas collected and the solvent recovered by distillation so that its H : D ratio could be determined. The precipitate was purified by three recrystallizations from a methanol-water mixture and sublimed ; mp 125.9126.2" (lit.'O 128.5"). This sufficed to bring it nearly to analytical purity. Anal. Calcd for C~HgS302: C, 53.62; H, 5.06; N, 23.45. Found: C, 52.93; H, 5.00; N, 23.71. It was probably contaminated with a few per cent of the hydrazine, which would not influence the mass ratios in the region of the parent peak. The R H : R D ratio was determined mass spectro~ c o p i c a l l y . ~ ~ -The ~ 3 principal cause of imprecision and inaccuracy in these determinations was chemical decomposition at the temperatures required to volatilize the hydrazone. This was shown by a systematic variation in the ratio with time of residence in the heated inlet system. Results were extrapolated to zero time, but it can be seen that they are less precise by an order of magnitude than those previously reported for gases, using the same mass spectroscopic techniques. 2 , 4-Phenylsemicarbaxone Collection. Technique A . A mixture of 4 ml of ethyl vinyl ether and 50 ml of 0.1 M tritiated aqueous acid was stirred at 25" until the water-insoluble ether was consumed. The acetaldehyde was then driven over, by heating, into an untritiated neutral solution containing 0.5 g of 4-phenylsemicarbazide. The derivative precipitated after about 15 min and was purified by two crystallizations from methanol-water or ethanol-water mixtures. The melting point of the 4-phenylsemicarbazone of acetaldehyde, so prepared, lay between 131.8 and 133.8" (lit.14 151The Journal of Physical Chemistry
MAURICE11.KREEVOY AND ROBERT ELIASON l52"), but it is analytically pure. It is apparently polymorphic. Anal. Calcd for CgHllH30: C, 61.00; H, 6.26; N, 23.72. Found: C, 61.26; H, 5.99; N, 23.66. Technique B. This was the same as technique A, except that the acetaldehyde was carried over into the derivatizing solution by a nitrogen stream at room temperature, instead of by heating. With either technique a small amount of isotope exchange occurs after the acetaldehyde is formed. Tritium contents of derivatives prepared by both techniques were corrected for the tritium content of derivatives prepared from identical reaction mixtures, except that untritiated acetaldehyde was used in the place of ethyl vinyl ether. For both techniques these corrections amounted to about 10-1501, of the radioactivity of the derivative, but the corrections were much more reproducible by technique B. I n all cases the tritium content of the derivatized acetaldehyde and that of the water were determined by liquid scintillation counting, using either NuclearChicago, Model 724, or Beckman liquid scintillation systems. The chemical composition of all the counting solutions was the same; they contained 10.00 ml of the dioxane-based scintillation solution described by Friedman and Leete,15 1.00 ml of water, and 1.00 ml of methanol containing 10.0 mg of the phenylsemicarbazone of acetaldehyde. I n using these heterogeneous reaction mixtures it was assumed that the reaction takes place in the homogeneous aqueous phase, and the dissolved vinyl ethyl ether is continuously replenished from the organic phase. This assumption is supported by the typical results obtained in two experiments, using technique B, where the ethyl vinyl ether was added in small increments over a period of several hours, so that its solubility was never exceeded. The values of H K / H T obtained were 15.5 and 16.7. This can be compared with a mean value of 15.8 and an average deviation from the mean of 0.7.
Results All of the following results were obtained at 25.0" in aqueous solution. The best value of I C H ~ defined as kl/(H+) and determined from 22 measurements of kl (8) I. M. Kolthoff,,and E. B. Sandell, "Textbook of Quantitative Inorganic Analysis, The Macmillan Co., New York, N. Y., 1952. (9) A . J. Kresge, personal communication. (IO) E. Hyde, Chem. Ber., 32, 1813 (1899). (11) M. M.Kreevoy and R. A. Kretchmer, J . Amer. Chem. Soc., 86, 2435 (1964). (12) M. M. Kreevoy, P. J. Steinwand, and W. V. Kayser, (bid., 86, 5013 (1964). (13) M.M. Kreevoy, P. J. Steinwand, and W. V. Kayser, ibid., 88, 124 (1966). (14) P. P. T. Sah and T. S. Ma, J . Chinese Chem. Soc., 2, 32 (1934). (15) A. R. Friedman and E. Leete, J . Amer. Chem. Soc., 85, 2141 (1963).
1315
THEBR$NSTED CY AND ISOTOPE EFFECTS FOR VINYLETHER HYDROLYSIS ~
Table I : Dissociation Constants and Catalytic Coefficients KHA,iM
Acid
HCOOH ClCH2COOH NCCHzCOOH
HyPOi H+
1.77 1.36 3.40 7.16 55.5
x
x
x x
K ’ H A , ~M
3 . 2 9 x 10-4 2 . 4 1 x 10-8 6.03 x 1.57 X 55.5
10-4 a 10-3”
10-aa 10-3
k ~ ~M-1 l . sec-1 (5.92 0.10) x (3.36 =k 0.05) x (3.89 f 0.14) x (4.03 f 0.08) x 1 . 7 1 f 0.01
10-3 10-2 10-2 10-1
a G. Kortum, W. Vogel, and K. Andrussow, “Dissociation Constants of Organic Acids in Aqueous Solution,” Butterworth and Co., Ltd., London, 1961. * P. Salomaa, L. L. Schaleger, and F. A. Long, J.Amer. Chem. SOC.,86, 1 (1964). Assuming 55.5 for the activity Obtained as indicated in ref 20. e The cited uncertainties are 50% confidence limits (probable of water in dilute, aqueous solutions. errors).
in HCl and HC104, was 1.71 f 0.01 1M-l sec-l. It was determined by the least-squares criterion, with IC1 forced t o be zero when (H+) is zero.16 The best value of k~ was 0.57 0.01 M-’ sec-l. After the kinetic measurements were made, each reaction mixture was analyzed for H in the DzO, using the near infrared.” Atom percentages of H were between l and 2, averaging 1.1%. The small correction to 100% D was then made using the equations of Kresgel*and an ai of 0.6 (defined and obtained below). The resulting value of IGD was 0.53 1M-l sec-l. This gives the value of k H / k D as 3.2, with a probable error of 0.1. At higher total electrolyte concentrations IGH showed a M tendency toward higher values. With 1.9 X HClOd and NaC1O4 between 0.05 and 1.60 M, k~ is accurately governed by eq 3,1g with C, 0.31, and ~ H O , 2.00 M-’ sec-l. Since kHo is significantly above the log k H = log kHo
+ Cp
(3)
the product, described as techniques A and B in the Experimental Section. Fifteen experiments by technique A gave a mean value of 17.1, with an average deviation from the mean of 1.6 and a probable error of the mean of 0.4. Six experiments by technique B gave a mean value of 15.8, with an average deviation from the mean of 0.7 and a probable error of the mean of 0.3. The best value of X H / H T was taken as 16.4, and it is probably uncertain by about 0.2. Five determinations of RM/RT were made by technique B in water containing 97-98 atom % D. (The symbol, hl, is used to indicate hydrogen without specifying the isotope.) Each of these was used to evaluate XD/XT by means of eq 4. Experiments described below were used to evaluate X H / X = . For this purpose only an approximation is required in any event, as (HH/xD)(H/D)is only about 0.15. The (4)
value of IGH obtained experimentally in dilute solution, average value of XD/XT was 2.42, with an average the ionic-strength dependence of k~ must be somewhat deviation from the mean of 0.08 and a probable error of steeper with p < 0.05 M . 0.03. General-acid catalytic coefficients, HA, were obIn eight experiments in H20-D20 mixtures the tained for a number of molecular acids. These were product was trapped as the p-nitrophenylhydrazone and obtained from the slopes of plots of [ICl - IGH(H+)] its deuterium content was determined mass spectroagainst (HA) in buffered solutions of constant ionic scopically.ll-la The hydrogen : deuterium ratio of the strength (0.2 M , maintained by appropriate additions solvent was determined by the near-infrared techof NaC104)and nearly constant (H+). The method has nique.17 The average value of H H / X D obtained from been described in more detail previously. 2O As before these was 6.8, with an average deviation from the mean a least-squares criterion of fit was used to evaluate the of 0.8 and probable error of the mean of 0.3. slopes, HA, but in each case the line was forced through Discussion the origin. Table I lists the catalytic coefficients, their The present value of IGH at 25”, 1.71 M-’ sec-l, is in 50% confidence limits, the apparent dissociation constants, K’HA(in 0.2 M electrolyte), and the thermodynamic dissociation constants, KHA. For comparison (16) C. A. Bennett and N. L. Franklin, “Statistical Analyses in Chemistry and the Chemical Industry,” John Wiley and Sons, Inc., k H is also listed. New York, N. Y.,1964,pp 231-233. I n a series of experiments in HzO containing small (17) T. 6. Straub, M.S. Thesis, University of Minnesota, Minneapolis, concentrations of T, the product, acetaldehyde, was Minn., 1966,pp 48-54. and its level of (18) A. J. Kresge, Pure A p p l . Chem., 8,243 (1964). trapped as its 4-phenylsernicarba~one,~~ (19) A. A. Frost and R. G. Pearson, “Kinetics and Mechanism,” labeling was compared with that of the solvent. The John Wiley and Sons, Inc., New York, N. Y.,1961,p 152. quantity X H / K T is defined by (RH/RT),,od(T/H),01,.11-13 (20) M. M. Kreevoy, T. 9. Straub, W. V. Kayser, and John L. Two experimeptal techniques were used to isolate Melquist, J . A m e r . Chem. SOL, 89, 1201 (1967). Volume 72, Number 4 A p r i l 1968
1316
MAURICE
entirely reasonable agreement with the value 1.89 M - l sec-l of Salomaa, et U Z . , ~ and the value 1.81 M-l sec-l which can be inferred from the results of Kresge and Chiang3 a t 26.7’. The present value of k ~ / k 3.2, ~ , is a little larger than either that of Kresge and Chiang for the same compound, 2.95, or that of Salomaa, et a1.,2for chloroethyl vinyl ether, 2.46. It is similar to the value, -3.4, found by Shostakovskii, et aLI6 for 2-hydroxyethyl vinyl ether. However, none of the previous groups report analysis for H in their “completely deuteriated” reaction mixtures. Moreover, even using the nominal H content, 0.501,,and the equations of Kresge,18we extrapolate the measured ratio of Kresge and Chiang to 3.02. They obtained the value, 2.95, by a linear extrapolation. The present values of k H . 4 for carboxylic acids and the value, 1.5 X lo-* M-l sec-l, which can be ascribed to acetic acid at 2503 obey the Brqinsted catalysis law,21 with deviations not much larger than their experimental uncertainties. They give an a of 0.66 f 0.04 by the method of least squares.22 As in previous work20kH is much too small (by a factor of -15) to fit the correlation line and is too large by a similar factor. The value of a is in rather poor quantitative agreement with the value 0.51 obtained by Salomaa, et ~ 1 . ,for ~ methyl isopropenyl ether. Partly, the difference arises from the choice of acids. Salomaa’s study included only four acids, of widely varying structure, including H+. A line drawn between k H and any other point on the Br@nstedplot would be of much lower slope than the line defined by carboxylic acids. Simple theory and its more obvious elaborations would lead one to expect that HH/HT should be equal to the product (HH/HD)(HD/HT). The success of this relationship, within the combined statistical uncertainties, lends confidence in the individual values. The “low temperature” Swain-Schaad relation is shown in eq 5 and 6.7 (They are not independent.) The value of H H / H D predicted by eq 7 and H H / H T is 6.96, with a probable error of 0.10. The predicted
HD/XT
=
(.%H/HT)0’a07
(6)
value of H D / H T is 2.36 f 0.03. These are in good accord with the experimental values, 6.8 -I: 0.3 and 2.42 f 0.08. The low-temperature Swain-Schaad relation has recently been shown to be very useful in analyzing aromatic hydrogen exchange results.23 The present experimental support for the relation, in a reaction of closely related type, strengthens this analysis (particularly in view of the recent production of evidence24 that the Swain-Schaad relation is not obeyed The Journal of Physical Chemistry
n%.KREEVOYAND ROBERTELIASON
in base-catalyzed abstraction of M from variously substituted acetophenones). Because of the relative unreliability of the direct experimental value of H H / H D the “best” value is probably that derived from H H / H T by means of eq 5, 6.96 -I: 0.10. Assuming that the proton is transferred directly from the M 3 0 +unit of the aquated proton to the substrate, the primary kinetic isotope effect, ( k ~ / k D ) 1 , can be obtained by multiplying XH/HD by Z.l2,l3 The latter is the constant required to convert (D/H)solvinto (D/H)hIaOand is assumed, in this paper, to have the value 0.69.25-28 The value of ( ~ H / ~ D ) so I obtained is 4.8. From the relation k ~ / k =~ (~H/JcD)I ( k H / k D ) I l , the secondary solvent isotope effect, ( k H / kD)ll, then takes the value 0.66. An isotopic a, ai, can readily be obtained from ( ~ H / ~ D ) Iand I 1 by means of eq 713,’*,20and has the value 0.56. The significance of ai is the same as that of a) except that the acid “strength” is varied by varying the isotopic composition of the untransferred part of the aquated proton, instead of varying R in RCOOH. The numerical similarity of the two a’s supports the model used for the tran(7) sition state. The value of ai is experimentally indistinguishable from the value 0.6 suggested by Kresge and Chiang to fit rates of ethyl vinyl ether hydration in H20-D20 mixtures or the value 0.52 suggested by Salomaa, et aZ.,2 to fit similar results on chloroethyl vinyl ether hydration but obtained by an entirely independent technique. I n conclusion then, the present paper supports and refines the mechanistic conclusions previously The results require that proton transfer is rate determining and suggest that it is a little more than half complete in the transition state. The magnitude of HH/HD requires that the largest component of the reaction coordinate is proton t r a n ~ l a t i o n . ~ ~ Most of the parameters of this reaction are very similar to those of allylmercuric iodide cleavage,13 but HH/HD is significantly smaller. This difference suggests the inclusion of slightly more heavy-atom motion in the present reaction coordinate than in that for allylmercuric iodide cleavage. (21) J. N. Br$nsted, Chem. Rev., 5 , 231 (1928). (22) Reference 16, pp 36-43, 231. (23) A. J. Kresge and Y . Chiang, J . Amer. Chem. Soc., 8 9 , 4411 (1967). (24) J. R. Jones and J. A. Rowlinson, personal communication. (25) A. J. Kresge and A. L. Allred, J . Amer. Chem. Soc., 8 5 , 1541 (1963). (26) V. Gold, Proc. Chem. SOC.,141 (1963). (27) H. H. Huang, R. R. Robinson, and F. A. Long, J . Amer. Chem. SOC.,8 8 , 1866 (1966). (28) Previously 12,1( a value of 0.7 has been assumed. (29) L. iMelander, “Isotope Effects on Reaction Rates,” The Ronald Press Co., New York, N. Y., 1960, p 20.
