J . Phys. Chem. 1985, 89, 1127-1 130
1127
Failure of the Pseudophase Model in the Acid-Catalyzed Hydrolysis of Acetals and p -Methoxybenzaldoxlme Esters in the Presence of an Anionic Micelle Marister Gonsalves, S6nia Probst, Marcos C. Rezende, Faruk Nome, Char Zucco, and Din0 Zanette* Departamento de Quimica, Universidade Federal de Santa Catarina, 88,000 Florian6polis, S.C., Brasil (Received: August 29, 1984)
The pseudophase model for micelle-catalyzed bimolecular reactions was tested for the acid-catalyzed hydrolysis of 2-(pnitropheny1)-1,3-dioxolane (l),acetyl p-methoxybenzaldoxime(2), and octanoyl p-methoxybenzaldoxime(3) in the presence of sodium dodecyl sulfate (SDS). At high acid concentrations the experimental data deviate appreciably from the behavior expected from the classical model. These discrepancies point to an additional catalytic pathway across the interfacial boundary operating at high [H']. This additional pathway in micelle-catalyzed reactions seems to be of general occurrence, and its existence cannot be overlooked in a complete theoretical description of catalytic micellar processes.
Introduction Micellar catalytic effects in bimolecular reactions are generally explained in terms of a favorable partition of the substrate between the aqueous and micellar phases. Rate enhancements are ascribed to an increase in the concentration of the reagents in the micellar phase, besides the changes in the reactivity of the substrate in the micelle. The kinetic treatment based on this phase separation model describes the observed rates as the sum of two contributions, an aqueous and a micellar term.'S2 Although the pseudophase model has been sucessfully applied to a large number of micelle-catalyzed reactions,l?*the validity of this treatment at high counterion concentrations has been recently q ~ e s t i o n e d . ~ Following -~ Bunton and co-workers' proposal of an additional reaction pathway across the micellar boundary,3a we investigated the dehydrochlorination of l , l , 1trichloro-2,2-bis@-chlorophenyl)ethane (DDT) and some of its derivatives in the presence of cationic micelles and high concentrations of b a ~ e . We ~ , ~have shown that the observed rates for these reactions deviate appreciably from the expected rates calculated from the pseudophase model and that these discrepancies could be accommodated by the introduction of a third reaction pathway across the micellar boundary. Thus, for the reaction of A + B in the presence of a micelle, the observed rate is given by where subscripts w and m refer to the aqueous and micellar phases, respectively, and the last term is a contribution across the micellar boundary to the observed rate. So far, our evidence for this additional catalytic term was restricted to the base-assisted dehydrochlorination of the very hydrophobic chlorinated pesticides of the DDT family. In order to test the generality of this hypothesis, we decided to investigate some acid-catalyzed reactions in the presence of an anionic micelle. The acid hydrolysis of p-nitrobenzaldehyde diethyl acetal in the presence of sodium dodecyl sulfate (SDS) has been studied by Bunton and Wolfe? and this type of reaction was an obvious choice for us. We chose in addition the acid hydrolysis of two acyl benzaldoximes as a second test for our model, the kinetics and mechanism of such reactions being well established in the literature.' (1) Fendler, J. H. "Membrane Mimetic Chemistry"; Wiley: New York, 1982. (2) Fendler, J. H.; Fendler, E. J. 'Catalysis in Micellar and Macromolecular Systems"; Academic Press: New York, 1975. (3) (a) Bunton, C. A.; Romsted, L. S.; Savelli, G. J. Am. Chem. Soc. 1979, 101, 1253. (b) Cipiani, A.; Linda, P.; Savelli, G.; Bunton, C. A. J . Phys. Chem. 1983, 87, 5262. (c) Cipiani, A,; Savelli, G.; Bunton, C. A. J . Phys. Chem. 1983.87, 5259. (d) Bunton, C. A,; Gan, L. H.; Savelli, G. J . Phys. Chem. 1983, 87, 5491. (e) Broxton, T.J. Aust. J . Chem. 1981, 34, 2313. (4) Nome, F.; Rubira, A. F.; Franco, C.; Ionescu, L. G. J . Phys. Chem. 1982. 86. 1881. ( 5 ) Stadler, E.; Zanette, D.; Rezende, M. C.; Nome, F. J . Phys. Chem. 1984,88, 1892. (6) Bunton, C. A.; Wolfe, B. J . Am. Chem. Sac. 1973, 95, 3742. (7) Brady, 0. L.; Miller, J. J . Chem. SOC.1950, 1234.
Our results confirmed the expectation that, at high acid concentrations, the additional term introduced in eq 1 plays an important role in the overall micelle-catalyzed process.
Experimental Section
Melting points were obtained on a Koffler hot-stage apparatus and were not corrected. N M R spectra were taken on a Varian T-60 machine with Me@ as internal reference. Sodium dodecyl sulfate was purchased from Merck and purified by recrystallization from ethanol. 2-(pNitrophenyl)- 1,3-dioxolane (1) was prepared following a published procedure (mp 92 OC, lit? mp 90 "C). Acetyl p-methoxybenzaldoxime (2) was obtained by reaction of the oxime with acetyl chloride (mp 42-44 OC, lit? mp 40 "C). Octanoyl p-methoxybenzaldoxime (3) was prepared in a similar way (mp 45-46 OC (from hexane)): 'H NMR (CDC13)G 0.90 (m, 3 H), 1.35 (m, 12 H), 2.30 (m, 3 H), 3.80 (s, 3 H), 6.80 ( ~ , ~ H , J = ~ H Z ) , ~ S ~ ( ~ , ~ H , J =Anal. ~HZ),~.~O Calcd for C16H2303N:C, 69.29; H, 8.36; N, 5.00. Found: C, 69.73; H, 8.34; N, 4.92. Solutions of HCl were prepared with redistilled water and the acid concentrations determined by titrations with standard solutions of sodium hydroxide (Merck). Rates of hydrolysis of compounds 1-3 were determined by following (Shimadzu UV-210 A spectrophotometer) the appearance of p-nitrobenzaldehyde at 259 nm for compound 1 or the disappearance of the acyl benzaldoximes 2 and 3 at 275 nm. The temperature for the kinetic runs was maintained at 25.0 f 0.1 OC by using a water-jacketed cell compartment. Individual pseudo-first-order rate constants were obtained from linear plots of In (A, -A,) (compound 1) or In (A, -A,) (compounds 2 and 3) vs. time. All plots were linear for at least 90% of the reaction, with correlation coefficients greater than 0.99. The binding constants for compounds 1 and 2 were determined by standard procedures.1°
Results and Discussion The acid hydrolyses of 2-@-nitrophenyl)- 1,3-dioxolane (1) and of acyl p-methoxybenzaldoximes (2) and (3) were studied in the present of sodium dodecyl sulfate (SDS). 02 N e C H < : ]
i
CH 3O-H=N-O-C-R
1
2. R CH3 9
3.R C7H15
The acid hydrolysis of acetals is first order with respect to the substrate and to the acid A bimolecular mechanism is also postulated for the acid hydrolysis of acyl benzaldoximes such as 2 and 3.' Following the ion-exchange
~
0022-3654/85/2089-1127$01 S O / O
(8) Fife, T.H.; Jao, L. K. J . Org. Chem. 1965, 30, 1492. (9) Crawford, R. J.; Woo, C. Can. J. Chem. 1965, 43, 1532. (10) Winters, L. J.; Grunwald, E. J . Am. Chem. SOC.1965, 87, 4608.
0 1985 American Chemical Society
1128 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
model proposed by Chaimovich and Quina,'' the pseudefmt-order rate constant k,, for our bimolecular reactions, involving an uncharged substrate and the univalent proton, in the absence of buffers, is given by k,,
=
[HIT [(k2m/ V(KsKH/Na)[Na]b/ [Na]f + k2wl (1 + KscD)[1 + KH/Na[Nalb/[Nalfl
(2)
where the subscripts T, b, and f refer to the total, bound, and free ionic species, respectively, and [HI and [Na] are proton and sodium ion concentrations. K, is the binding constant for the substrate, P i s the partial specific volume per mole of micellized detergent, k2, and kzw are second-order rate constants in the micelle and water, respectively, CDis the concentration of micellized surfactant, and KHpa is the selectivity coefficient for the H+/Na+ exchange. In order to express k, in terms of experimentally available variables, we must be able to estimate the ratio of concentrations of bound and free sodium ions, [NaIb/[Nalf, for any concentration of surfactant and/or acid. Starting from the definition of KH/Na as the ratio of concentration of bound and free ions,' KH/Na = [H]f[Na]b/[H]b[Na]f,and assuming this ratio to be unity, following Bunton's observations that there is little difference in the binding of hydrogen and sodium ions to the SDS micelle,I2 we can write [NaIb/[Nalf = [HIb/[Hlr. Equation 2 then becomes k+m =
[HlT[(kz,/V)K,[Hlb/[H]f + k2wl ( l + KsCD)(l + [Hlb/[Hlf)
(3)
For a micellar solution of SDS containing the reactive counterion H+ and no added salts, we can write the equation" CD + cmc + [Hlb [Hlb KH/Na
=
[HIT - [Hlb ( l - .)CD -
(4)
IH1b
Gonsalves et al. TABLE I: Pseudo-First-Order Rate Constants for the Hydrolysis of Compounds 1-3 at 25 OC and Constant Acid Concentration
102[SDS], M 0.10 0.12 0.13 0.20 0.30 0.50 0.70 1.oo 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
i04k,,, s - ~ cmpd 2b 3.45
cmpd 1'
cmpd 3b 6.77 11.61 14.21 16.05 16.74 15.59 15.54 14.86 13.08 12.44 12.16 11.20 10.06
5.92 7.57 8.60 10.04 11.03 10.83 11.56 10.80
4.40 9.20 12.50 11.80 10.20
9.98 9.95 9.50
7.80
"[HCI] = 0.03 M. b[HCI] = 0.1 M. TABLE II: Bindiiag Comrtants K , and Second-Order Rate Constants k h and kk for the Hydrolysis of Compounds 1-3 in the Presence of SDS at 25 OC
K,
substrate 1
52 71 30000
2
3
kZw,M-' s-I 8.64 X lo-' 3.11 X 3.11 X lo-'
k2,, M-ls-] 1.18 X 6.00 X 5.00 X 10"lb
k2,fkz, 0.14 0.19 0.16
'At an acid concentration of 0.03 M HC1. bAt an acid concentration of 0.1 M HC1.
where CY is the degree of ionization of the micelle. With the assumption made above, KH/Na = 1, we obtain lo
which allows us to express the ratio [HIb/[H]f as a function of the total concentration of acid, since [HIT = [HIb [HIr. Substituting this ratio into (3), one obtains the final expression (6),which gives k@ in terms of the total hydronium and surfactant concentrations, [HIT and cT, respectively.
+
where Y=
cD(1 - a)
+
and
CT = CD + cmc The pseudo-first-order rate constants k,, for each substrate at constant [H'] and various surfactant concentrations are given in Table I. Plots of k,, against SDS are shown in Figure 1. All curves exhibit maxima, in agreement with the pattern expected for micelle-catalyzed bimolecular reactions of this type.' Values of k2wand K, for all substrates were determined experimentally and are given in Table 11. Because of its low solubility in water, values of kzwand K, for compound 3 could not be obtained directly. The second-order rate constant in the aqueous phase kzwwas assumed to be the same for the related esters 2 and 3. As for the binding constant K,, an estimated value of 30 OOO was used throughout this work, obtained by adjustment of eq 6 to the experimental data. This estimation is readfly justified (11) Quina, F. H.; Chaimovich, H. J. Phys. Chem. 1979, 83, 1844. (12) Bunton, C. A.; Ohmenzetter, K.;Sepulveda, L.J. Phys. Chem. 1977, 82, 2000.
0
1
2
3
4 5 6 10'CSDSl.M
7
8
Figure 1. Variation of the pseudo-first-order rate constants with the concentration of surfactant for the hydrolysis of substrates 1 (e),2 (u), and 3 (0)at 25 OC and constant acid concentrations. Theoretical curves are drawn following eq 6.
by considering eq 6 . For highly hydrophobic substrata, K,CD >> 1, the pseudo-first-order rate constant k,, is rather insensitive to large variations of Ks. Equation 6 was used to estimate values of k2,/ P for all substrates. Using values of k, and Ks of Table 11, (Y = 0.18,l3 and M (for compound 1, [H+] = 0.03 M)6 and 7.0 cmc = 1.1 X X lo4 M (for compounds 2 and 3, [H+] = 0.10 M),I4 by proper ~
~~
(13) Romsted, L.S . Ph.D. Thesis, Indiana University, Bloomington, IN, 1975.
Hydrolysis of Acetals and p-Methoxybenzaldoxime Esters
The Journal of Physical Chemistry, Vol. 89, No. 7, 1985 1129
adjustment of eq 6 to the experimental data (see Figure l), one arrives at k,,/Pvalues of 4.70 X 2.40 X and 2.00 X 10” s-’ for compounds 1,2, and 3, respectively. The second-order rate constants in the micelle, kzmrobtained by multiplying these values by an effective volume per mole of detergenP V = 0.25 M-I, are therefore 1.18 X 6.00 X lo4, and 5.00 X lo4 W1 s-’ for compounds 1,2, and 3, respectively. For sake of comparison with the aqueous phase, these values are given in Table 11, together with the corresponding ratios k2,/kZw. In order to test the existence of a new catalytic pathway in our reactions, the hydrolyses of compounds 1-3 were followed at high acid concentrations, in the presence of a constant concentration of surfactant. An appreciable contribution of the aqueous-phase term to the overall rate constant was observed for substrates 1 and 2 at high acid concentrations. This contribution originated from the relatively low hydrophobicity of these substrates and tended to mask any contribution from a reaction pathway across the interfacial boundary which might be present. This problem was obviated by employing higher concentrations of SDS in solution, thereby depleting the aqueous phase of the substrate, and thus diminishing the contribution of the aqueous phase to the observed rate. The observed pseudo-first-order constants at constant surfactant and increasing acid concentrations are given in Table 111. By using values from Table 11, we can test eq 6 , which should describe k,, as a function of the total acid concentration [HIT. For the calculation of the theoretical rates k+,,,,a constant value of 3.0 X lo4 M was assumed for the cmc of the surfactant. This corresponded to an extrapolated value of the surfactant cmc at [H+] = 1 M, by utilizing data for SDS solutions at different acid concentrations6 and assuming a linear relationship between In cmc and In The use of an average value of the cmc for the whole range of acid concentrations employed may be open to question. However, the approximations involved do not alter the calculated k+,,,values significantly. For example, for all substrates, a hundredfold variation of cmc values in the range 10-3-10-5 M caused only a negligible (1%) average deviation in the values of the calculated rates k,,. Figure 2A-C compares some of the experimental data with calculated curves of kJ, vs. [HITclearly, in all of these examples, the differences between the experimental data and the calculated values of k,, based on the classical pseudophase model are far too large to be explained in terms of the variation of the cmc or a with increasing acid concentrations. The possible hydrolysis of SDS at high [H+] might be invoked as an alternative explanation for these deviations. It might be argued that partial hydrolysis of the surfactant and the consequent decrease of the micellar concentration would force the less hydrophobic substrates 1 and 2 to react in the aqueous phase, where a first-order dependence on [H+] would be expected. This argument however, which is based on a process not likely to occur under our experimental conditions, cannot explain the same discrepancies observed with the more hydrophobic substrate 3. These large deviations clearly demonstrate the failure of the pseudophase model at high acid concentrations. According to the classical model, the SDS micelle should become saturated and the observed pseudo-first-order rates reach constant values at high [H+]. The observed rates on the contrary increase linearly with the total acid concentration, indicating that a new catalytic pathway is operating in this acid range. The incorporation of a new catalytic term into eq 6 yields relationship 7, which describes the corrected pseudo-first-order rate constant k‘$, in terms of [HIT for the whole range of acid concentrations studied. k’J, = kJ, -tkZm/w[H]T (7)
A 0
CHCI1.M
/
.---
__----- ___----
0
lo/
a
.---
1
2 .o
CHCI1.M
0
(14) Mukerjee, P.; Mysels, K. J. “Critical Micelle Concentrations of Aqueous Surfactant Systems”;National Bureau of Standards: Washington, DC, 1971. (15) (a) Shinoda, K.; Soda, T. J . Phys. Chem. 1963, 67, 2072. (b) Mukerjee, P. J . Phys. Chem. 1%2,66, 1733. (16) Shinoda, K.; Nakagawa, T.; Tamamushi, B.; Isemura, T. “Colloidal Surfactants”; Academic Press: New York, 1963.
Y
75t
2.0
I.O
30
CHC13.M
Figure 2. Variation of the pseudo-first-order rate constants for the hydrolysis of compounds 1-3 at 25 OC and increasing acid concentrations: (A) substrate 1, [SDS] = 0.1 M; (B) substrate 2, [SDS] = 0.1 M; (C) substrate 3, [SDS]= 2.5 X lo-) M. Full and broken lines correspond to theoretical curves obtained from eq 7 and 6, respectively.
Gonsalves et al.
1130 The Journal of Physical Chemistry, Vol. 89, No. 7 , 1985 TABLE III: Pseudo-First-Order Rate Constants for the Hydrolysis of Substrates 1-3 at 25 ‘C and Various Acid Concentrations 1 0 3 ~s-I .
IHCI1, M 0.003 0.01 0.03 0.05 0.07 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.oo 1.30 1.53 2.00 2.50 3.07
compd 1 0.07 M 0.10 M SDS SDS 0.14 0.40 1.02
0.064 0.29 0.78
2.24 4.24
3.80
5.33
4.96
7.70
7.04
8.51
8.51
11.25 15.05
11.20 15.75
compd 2 0.10 M
0.0025 M
0.0050 M
SDS
SDS
SDS
0.14 0.63 0.77 1.24 1.74 2.13 2.12 2.45 3.1 1 3.02 2.81 2.55 2.41 4.82
0.48 0.60 1.85 2.11 2.09 1.96 2.25 2.03 2.71 2.96 2.75 2.90 2.92 3.06
0.90 1.47 2.03 2.30 2.19 1.78 1.89 2.1 1 2.54 2.91 2.37 2.92 2.86 2.94
0.83 1.24 1.72 1.95 1.58 1.64 3.02 2.86 2.50 2.80 2.7 1 2.91 2.95 2.87
0.32 0.82 0.85 1.86 1.12 1.39 1.54 1.73 2.13 1.75 2.17 2.24 2.55 2.99
0.26 0.60 1.18 1.46 1.76 2.23 2.20 2.25 3.1 1 3.04
6.22 7.58
4.30 4.99 6.18 7.26
4.35 4.28 5.75 7.00
3.74 5.26 6.17 6.58
3.47 4.62 5.78 7.25
6.16 7.89
The incorporated term k2,,,[HlT takes into account the contribution of an additional pathway across the micellar boundary, which becomes the dominant catalytic factor at high acid concentrations. The resulting theoretical curves obtained from (7) fit the experimental data fairly well (Figure 2A-C). Values of constants kzmlwfor substrates 1, 2, and 3 are 6.50 X 2.50 X and 1.80 X M-’ s-l, respectively. The above results reinforce our previous reports of a catalytic pathway across the interfacial boundary for the micelle-catalyzed basic dehydrochlorination of DDT and derivative^.^^^ A similar situation is encountered in the acid-catalyzed processes studied in this work. For relatively low concentrations of the reactive counterion, a phase separation model applies, which assumes a closed thermodynamic system and partition between the micellar and aqueous phases. This model however is no longer acceptable when the reactive counterion concentration is increased. An additional reaction pathway across the micellar boundary becomes important then, and eventually the dominant catalytic factor in the overall process. From a theoretical point of view it is possible to rationalize the new catalytic pathway by considering the effect caused by electrolytes on the micellar structure. The addition of sodium chloride to micellar solutions of SDS causes the micelle to grow and become rod shaped,”.18 a structural change which should also be provoked by the addition of hydrochloric acid. The electrolyte addition causes the ionic head groups to get closer in the rod-shaped micelle, decreasing the repulsion between them, Le., decreasing the surfactant head-group area aO.l9 This closer packing of the head
compd 3 0.020 M SDS
0.050 M
SDS
0.10 M SDS
2.61 2.32 4.62
groups causes an increase in the concentration of the accompanying counterions [H’]. One arrives at a similar conclusion if the effect of the increasing electrolyte concentration on the diffuse double-layer “thickness” is considered. Since the surface tension of the hydrocarbon-water interface should be constant, the decrease in a,, must be paralleled by a decrease in the Debye length.’9*20 Therefore, a considerable decrease of the “thickness” of the diffuse double layer is expected with increasing electrolyte concentrat i ~ n . ’The ~ ~net~ effect ~ is an overall increase in concentration of [H’] in the inner part of the diffuse double layer. As a consequence, the possibility of reaction between a micellar bound substrate and ions adsorbed at the inner part of the double layer, close to the shearing surface, increases. It appears at this stage that the failure of the classical pseudophase model at limit situations is a general phenomenon. A complete theoretical description of micelle-catalyzed reactions cannot overlook processes across the interfacial boundary. We are currently investigating other reactions with a view to widening our evidence of this proposed pathway and deepening our understanding of these processes.
Acknowledgment. We are grateful to the Conselho Nacional de Pesquisa (CNPq) and to the Financiadora de Ektudos e Projetos (FINEP) for financial assistance. Registry No. 1, 2403-53-4; 2, 3848-39-3; 3, 94929-79-0; SDS, 15121-3; p-MeOC6H4CH=NOH, 3235-04-9; CH3C(0)CI,75-36-5; CH3(CH,),C(O)CI, 11 1-64-8. (19) Mitchell, D. J.; Ninham, B. W. J . Chem. SOC.,Faraday Trans. 2
(17) Wennerstrom, H.; Lindman, B. Phys. Rep. 1979, 52, 1. (18) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J . Chem. SOC., Faraday Trans. 2 1976, 72, 1525.
1981, 77, 601.
(20) Barlow, C . A. In ‘The Electrical Double Layer”; Eyring, H., Handerson, D., Jost, W., Eds.; Academic Press: New York, 1970.
