J. Phys. Chem. 1981, 85, 1958-1960
1958
produced by dehydroxylation could probably act as the Lewis site, but the probability of existence of such an arrangement appears to be improbable (see further discussion). Further possible changes in the dehydroxylation products formed in the first step are reflected in the values of the Wiberg bond orders calculated for dehydroxylated clusters and depicted in Figure 1. For comparison, the Wiberg bond order of the Si-0 and A1-0 bonds of T606(OH)lz-type clusters, depending on the Si:A1 ratio, attain values of 0.80-1.00 and 0.55-0.65, respectively.l* It is apparent from the Wiberg bond orders for clusters containing tricoordinated Al that dehydroxylation leading to formation of tricoordinated A1 atoms results in strengthening of the Al-0 bond of this particular Al atom and insignificant changes in the values of the other bond orders. The calculations thus suggest that the cluster containing the tricoordinated A1 does not have a tendency
to split other bonds of the zeolite skeleton, although possible structural changes in bond lengths and valence angles are not excluded. Different conditions prevail in clusters containing tricoordinated Si. The unsaturated valences of the tricoordinated Si atom lead to strengthening of its Si-0 bonds, resulting in weakening of the remaining T-0 bonds of these three oxygen atoms. This effect is particularly noticeable for the corresponding Al-03 bond which becomes very weak, as is apparent from Figure 1. The calculations thus indicate the possibility of dissociation of this A1-0 bond, leading to formation of a >Si=O site and tricoordinated Al. The >Si-0 structure will not, however, probably be the final product of the dehydroxylation but will undergo further changes. For example, it could react with a water molecule to produce the >Si(OH), structure or interact with a skeletal Si atom leading to formation of five-membered ring and dicoordinated Al.
Proton Inventory of the Resin-Catalyzed Hydrolysis of Ethyl Acetate‘ Julio F. Mata-Segreda’ School of Chemistry. Universtty of Costa R i a , Cludad universitaria “Rodrig0 Facio”, Sen Josd 2060, Costa Rica (Received: January 20, 198 1; In Final Form: March 26, 198 1)
The kinetics of the hydrolysis of ethyl acetate catalyzed by the strongly acidic ion-exchangeresin Dowex 50W-X2 was studied at 25 “C in mixtures of light and heavy waters. The second-order rate constants vary with the deuterium atom fraction ( n )as 106k,/(s-’ mequiv-’) = (1.82 f 0.06)(1 - n + 0.83r1)~/(1- n + 0.69r1)~.This result suggests that the observed overall solvent isotope effect is generated by three protons changing their binding state in going from the reactant to the transition state.
Introduction Many features of active sites in enzymes are best described in terms of the hydrophobic nature of those catalytic entities. Therefore, it was thought to be useful to study reactions of biological interest taking place inside a polymer matrix, in order to explore the effect of a hydrophobic environment on the stability of model transition-state complexes. There have been many reports on the catalytic effect of ion-exchange resins on hydrolytic reactions3-10 and of (1) Resin Catalysis, part 3. For part 2, see ref 10. A preliminary version of this work was presented at the 5th IUPAC Conference on Physical Organic Chemistry, Santa Cruz, CA, Aug 1980. (2) Research Fellow of the Consejo Nacional de Investigaciones C i e n t h a s y TecnolBgicas (Costa Rica). (3) (a) Haskell, V. C.; Hammett, L. P. J. Am. Chem. SOC. 1949, 71, 1284. (b) Bernhard, S. A.; Hammett, L. P. Ibid. 1953,75,1798. (c) Ibid. 1953, 75,5834. (d) Bernhard, S.A.; Garfield, E.; Hammett, L. P. Ibid. 1954, 76, 991. (e) Riesz, P.;Hammett, L. P. Ibid. 1954,76,992. (f) Samelson, H.; Hammett, L. P. Ibid. 1956, 78, 524. (9) Chen. C. H.; Hammett, L. P. Ibid. 1958,80, 1329. (4)Nieto, A. J. Chem. Educ. 1974,50,846. (5)Barral, M. A.; de Cabrera, A. M. T.; Castro, A. A.; Parera, J. M. Rev. Fac. Ing. Quim. Univ. Nac. Litoral 1970,39,275. (6)Bhatia, S.:Raiamani, K.: Raikhowa, P.: Rao, M. G. Ion Exch. Membr. 1973,1, 127.(7)(a) Gold, V.;Liddiard, C. J. J. Chem. Soc., Faraday Trans. 1 1977, 73,1119. (b) Gold, V.;Liddiard, C. J.; Martin, J. L. Ibid. 1977,73,1128. Shenoy, S. C.; Rao, M. S.; Rao, M. G. J. Appl. Chem. (8)Rajamani, K.; Biotechnol. 1978,28,699. 0022-3654/81/2085-1958$01.25/0
theoretical contributions to the formulation of the kinetics in such systems.” The most relevant features of resincatalyzed reactions are the following: (1)The kinetics are first order in substrate concentration and first order in the resin bulk c~ncentration.~.~,’ (2) Diffusion processes are faster than chemical stepss6Vs (3) The reactions usually show lower values than the corresponding homogeneously catalyzed reactions, but more negative AS*.’OJ2 (4) The magnitude of solvent effects on the rate of these heterogeneous reactions is greater than the effect observed for the homogeneous analogue^.^ This work presents results on the effect of solvent isotopic composition on the rate of hydrolysis of ethyl acetate catalyzed by the strongly acidic ion-exchange resin Dowex 50W-X2, in order to obtain a quantitative description of the structure of the transition-state complex for the resin-catalyzed hydrolysis of simple esters. We shall also compare our results with the homogeneous counterpart, based on data found in the literature.13J4 ~~~~~
~
(9)Mata-Segreda, J. F. Rev. Latinoam. Quim. 1979,10, 57. (10)Mata-Segreda, J. F. Rev. Latinoam. Quim., in press. (11)(a) Helfferich, F. J. Am. Chem. SOC.1954,76,5567.(b) Helfferich, F. “Ion Exchange”; McGraw-Hill: New York, 1962. (12)(a) Regen, S. L.; Besse, J. J.; McLick, J. J. Am. Chem. SOC. 1979, 101,116. (b) Regen, S. L.; Besse, J. J. Ibid. 1979,101,4059.(c) Regen, S.L.; Heh, J. C. K.; McLick, J. J . Org. Chem. 1979,44, 1961. (13)Nelson, W.E.;Butler, J. A. V. J. Chem. SOC. 1938,957.
0 1981 American Chemical Society
Resin-Catalyzed Hydrolysis of Ethyl Acetate
The Journal of Physical Chemistry, Vol. 85, No. 13, 1981 1050
TABLE I : Second-Order Rate Constants for the Hydrolysis of Ethyl Acetate in Mixtures of Light,and Heavv Waters. Catalvzed by Dowex 50W-X2at 25 “C 106k,/ n
(s-’ mequiv-I)
0.000 0.233 0.500
1.82 f 0.06 1.99 f 0.04 2.21 * 0.01
n 0.671 0.997
106k, /
I
(s-l mequiv-l)
2.51 f 0.07 3.27 f 0.06
i
Proton Inventory Technique The so-called proton inventory technique has been used in the study of ~ h e m i c a l ’ ~asJ ~well as physical”J8 protolytic processes. The method consists of taking an inventory of those protons which change their binding state in going from the reactant state to the transition state. Rate constants are measured in mixtures of isotopic hydroxylic solvents (H20-D20, MeOH-MeOD, etc.) of deuterium atom fraction n. The behavior of the rate constant as a function of n is given by the general equation of Gross and Butler15916 hn/ho = n(1- n i
+ +i*n)/Il(l - n + +Rn) R
(1)
where &* is the isotopic fractionation factor for the ith exchangeable position in the transition complex. It gives the preference for deuterium over protium of the ith site comppared to the same preference in the bulk of the solvent, that is
@R
04 0
0 ; 25
o h
0.75
-4
a
Figure 1. Proton Inventory for the hydrolysis of ethyl acetate catalyzed by Dowex 50W-X2, at 25 O C . Line drawn by using 4 . = 0.83.
is the reactant-state counterpart of &*.
Experimental Section Materials. Ethyl acetate was from BDH, and D20 from Aldrich (Gold Label). The resin Dowex 50W-X2 was from Fluka in the Na+ form. The resin (500 g) was treated in a column with 1 L of 10% HCl and then washed with deionized water until no positive test was observed for either C1- (AgNO,) or H” (Congo Red). The capacity of the dry resin (4.04 mequiv g-l) was then determined by back-titration (NaOH/HCl) using Methyl Red as indicator. L 2 0 mixtures (L = H, D) were made gravimetrically. Kinetic Determinations. The kinetic determinations were carried out in a 500-mL double-walled glass reactor equipped for magnetic stirring and connected to a constant-temperature bath. All experiments were done at 25 OC. In a typical run, 100 mL of water and 20 mequiv of the resin were equilibrated in a reactor until the system reached the desired temperature. Then, 5 mL of ethyl acetate was added and 3-mL aliquots were taken at suitable times to titrate the acetic acid formed. Rate-Constant Calculations. The data gave good first-order plots, whose slopes were determined by using a linear regression program (with the proper modifications done) from the SAS DackaEte. The TSC(n) data (vide infra) were submitted to polynomial analysis by using SAS/REGR. l9 (14) Salomaa, P.; Schaleger, L. L.; Long, F. A. J.Am. Chem. SOC.1964, 86, 1.
(15) Albery, W.J. In “Proton Transfer Reactions”; Caldin, E., Gold, V., Eds.; Chapman and Hall: London, 1975. (16) Schowen, K. B. J. In “Transition States of Biochemical Procaasea”;Gandour, R., Schowen, R. L., Us.; Plenum Press: New York, 1978. (17) Mata-Segreda, J. F. Reu. Latinoam. Quin. 1979,10, 151. (18) Mata-Segreda, J. F.J.Phys. Chem. 1980,84, 446.
0,-
I
I
0,25
09%
1
n
0.7s
’ 9 0
Flgure 2. Proton inventory for the homogeneously (H30+) catalyzed hydrolysls of simple esters (ref 13 and 14). Line drawn by using 4
= 0.80.
Results The reaction rate follows simple second-order kinetics: d[LOAc] /dt = k,C,[AcOEt] (3) C, being the number of milliequivalents of resin used and kn the rate constant observed in a solvent mixture of deuterium atom fraction n. Table I and Figure 1 give the data. The solvent isotope effect is given in terms of the Gross-Butler equation15J6which in our case takes the form: k , / k o = TSC(n)/(l - n ln)3 (4)
+
where 1 = 0.69 is the isotopic fractionation factor of hy(19) Linear and polynomial regression done with the program SAS/ REGR, available from Centro de Informltica.
1960
The Journal of Physical Chemistry, Vol. 85,No. 13, 1981
dronium ion, and TSC(n) is a function of n which gives the transition-state contribution to the solvent isotope effect (see Figure 2). As described by Schowen,16 the TSC(n) data were fitted to polynomials of increasing order. Linear and quadratic terms were significant at the 99.9% confidence level, and the cubic term only at 80%. We conclude that at least two, and perhaps three, protons must contribute to the overall isotope effect. A cautionary note must be made at this point. The validity of this treatment is based on three premises: (a) AcOEt is equally stable in light and heavy waters, (b) 1 = 0.69 for a hydronium ion inside the resin matrix, and (c) the resin absorbs preferentially neither HzO nor DzO. Mata, Lindenbaum, and SchowenZ1found that for a well-formed H30+ in a poly(acrylic acid) matrix = 0.70 f 0.03 a value which makes our second assumption a certainty. On the other hand, KellomiikizZhas observed that sulfonic resins in the Li+, Na+, K+, and Ba2+forms do absorb preferentially HzO over DzO, but the expected isotope effect for the H+ form is relatively low (less than 2%). The overall isotopic composition will be considered to be equal to the situation inside the resin.
Discussion The polynomial fitting program does not pivot on physically significant &* values; thus the result is only a guide to the number of vibrationally active protons in the transition state. The kinetic studies of Yates and McClelland20 on the acidic hydrolysis of simple esters indicate that two water molecules are involved in the transition-state complex and that its possible structure can be depicted as follows: Hb%+./ Ha
I
o--H,l+ ,>' O-R'
Hd
Thus, it is safe to take the three-proton fitting as mechanistically meaningful. The best cubic fit for the resincatalyzed hydrolysis of ethyl acetate is (20) Yates, K.; McClelland, R. A. J.Am. Chem. SOC.1967,89, 2686. (21) Mata-Segreda, J. F.; Lindenbaum, S.; Schowen, R. L. J. Am. Chem. SOC.1977, 99,5916. (22) Kellomaki, A. Acta Chem. Scand., Ser. A 1978, 32, 747.
Mata-Segreda
TSC(n) = (1.001 f 0.006) - (0.58 f 0.07)n + (0.0005 f 0.18)n2- (0.05 f 0.11)n3 The k,-n data can be appropriately described by the relation TSC(n) = (1 - n I$*
+ r$*n)3
(5)
= 0.83 f 0.02
The kinetics of the acidic hydrolysis of ethyl formate and of methyl acetate was studied13J4 in LzO mixtures. The cubic relation for the homogeneous TSC(n) requires 4 * = 0.80 f 0.02. Although the data do not allow a clear-cut separation between the homogeneously catalyzed hydrolysis of ethyl formate and methyl acetate and our resin reaction, a small effect from the resin appears to exist on the structure of the transition-state complex (kD/kH = 1.80 f 0.07 for the resin system; kD/kH = 1.53 'asthe mean value for the systems cited above). The position of the transition state along the reaction coordinate can be calculated as x = log (0.83)/log (0.69) = 0.50. The bond order of C-OH,can be estimated as 3 / 2 and resembles the situation in gem-diols and hemiketals where 4 values range from 1.23 to 1.25.23 & also falls in the same category. Therefore, at this level of precision 4, and f$d are taken as 1.0. For the homogeneous reactions 4 * = 0.80 and x = 0.60. This is another case of an immature hydronium ion acting as a catalytic entity on the hydrolysis of esters." The characterization of the transition-state complex picture implies a large degree of electron polarizability of the hydrogen bonds and the nearby reacting orbitals, thus providing a basis for strong interaction through mutual p o l a r i z a t i ~ n a, ~process ~ which seems to operate better inside the aromatic (polarizable) polymeric matrix. Acknowledgment. I thank Vicerrectoga de Investigacih (UCR) for providing support through grant 02-09-46. (23) Mata-Segreda,J. F.; Wint, S.; Schowen, R. L. J. Am. Chem. SOC. 1974,96, 5608. (24) Hegazi, M.; Mata-Segreda, J. F.; Schowen, R. L. J. Org. Chem. 1980, 45, 307. (25) Hegazi, M.; Quinn, D. M.; Schowen, R. L. In "Transition States of Biochemical Processes"; Gandour, R., Schowen, R. L., Eds.; Plenum Press: New York, 1978; pp 369-70.
