J. Phys. Chem. 1986,90, 5858-5862
5858 002 I
004
006
008
010
I
I
I
I
for the various substrates at similar [OH-] and % (v/v) MeCN are consistent with the hydrophobicities of the substrates.
/'
Conclusion/Summary
I
I
I
2'3
1
IOz[%],
5
M
Figure 4. Bimolecular reaction of pNPDPP in le: 0,O.OlM NaOH and 10% (v/v) MeCN; m, 0.02 M NaOH and 20% (v/v) MeCN; 0 , O . O l M NaOH and 20% (v/v) MeCN; A, 0.01 M NaOH and 30% (v/v) MeCN. The lines are predicted by using the model.
[NaOH] a t fixed MeCN content. Added electrolyte should increase aggregation directly and because of deprotonation of le. The best-fit parameters for analysis of these bimolecular reactions using the model are compared in Table v, and Ks values
The self-association model discussed here gives a better fit than the simple mass-action model, eq 24, which fitted the data poorly in dilute 1, although it was satisfactory at high [l].5,6,9 It is important to note, however, that this new model contains assumptions which are reasonable but unproven, for example, that kM' and kMare independent of the size of aggregates; that K , is directly proportional to q; that K is independent of q; that monomer, dimer, and trimer do not bind substrate (or affect reaction rate); and that substrate does not affect the self-associationprocess. However, if these K and K, values are used as a guide, their relative values, as predicted by the model, are consistent with the effect of added electrolyte and organic solvent, the effect of deprotonation and charge on the self-association of tri-n-octylalkylammonium salts, and the binding of substrates to the aggregates.
Acknowledgment. Support of this work by the National Science Foundation (Chemical Dynamics Program) and the U S . Army Office of Research is gratefully acknowledged. Registry No. IC,79054-30-1; l e , 92642-02-9; 6-NBIC, 42540-91-0.
Deacylation In Microemulsions Vassilios Athanassakis, Clifford A. Bunton,* and Don C. McKenzie Department of Chemistry, University of California, Santa Barbara, California 931 06 (Received: March 18, 1986; In Final Form: June 3, 1986)
First-order rate constants for the reaction of pnitrophenyl benzoate @NPB) with OH- in microemulsionsof tert-amyl alcohol, octane, and cetyltrimethylammonium bromide (CTABr) or sodium dodecyl sulfate (SDS),or the alcohol-modified micelles, have been analyzed quantitatively in terms of reactant concentrations in the droplets or micelles. Second-order rate constants are very similar at the surfaces of cationic microemulsion droplets and aqueous CTABr micelles, but are smaller than in water. Overall reactions are very slow in anionic microemulsions because droplets bind substrate and exclude OH-.
The effects of self-assembling colloids on the rates of nucleophilic attack upon organic substrates in aqueous media are well documented. The colloids include micelles,* microemulsion d r ~ p l e t sand , ~ assemblies of hydrophobic tetralkylammonium ions," and the ability of these submicroscopic particles to bring reactants together, or keep them apart, is largely responsible for the rate effects. Oil-in-water (o/w) microemulsions are clear dispersions in water which generally contain an apolar solvent, a surfactant, and a cosurfactant which is typically a moderately hydrophobic alcohol or other polar s o l ~ e n t . ~ . The ~ * ~ surfaces of the droplets are (1) Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982. (2) (a) Sudholter, E. J. R.; van de Langkruis, G . B.; Engberts, J. B. F. N. Recl. Trav. Chim. Pays-Bas Belg. 1980,99,73. (b) Bunton, C. A. Catal. Rev. Sci. Eng. 1979, 20, 1. (c) Romsted. L. S . In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 2, p 1015. (3) (a) Hermansky, C.; Mackay, R. A. In. Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum: New York, 1979; p 723. (b) Mackay, R. A.; Hermansky, C. J. Phys. Chem. 1981, 85, 139. (4) (a) Okahata, Y.; Ando, R.; Kunitake, T. J. Am. Chem. SOC.1977,99, 3067. Kunitake, T.; Okahata, Y.; Ando, R.; Shinkai, S.; Hirakawa, S . Ibid. 1980, 102,7877. (b) Bunton, C. A,; Hong, Y.-S.; Romsted. L. S.; Quan, C. Ibid. 1981, 103, 5784. ( 5 ) (a) Prince, L. M., Ed. Microemulsions, Theory and Practice"; Academic: New York, 1974. (b) Danielsson, L.; Lindman, B. Colloids Surf. 1981, 3, 391.
probably similar to those of the so-called "swollen micelles" which are made up of a surfactant and a cosurfactent. Like normal micelles the surfaces of the droplets are ionic or polar and can take up apolar and polar (or ionic) solute^.^ Cationic microemulsions and swollen, or alcohol-modified, micelles sped reactions of anions with neutral molecules, although the rate enhancements tend to be smaller than with normal micelles. However, microemulsions are much better solubilizing agents than micelles and therefore have advantages as reaction media. Micellar effects upon thermal bimolecular reactions can be treated quantitatively by estimating the concentrations of both reactants in the micelles which are treated as a distinct reaction medium, Le., as a pseudophase.* Similar treatments have been applied to reactions in microemulsions and alcohol-modified micelle^,^^^.* although in these systems it is sometimes difficult (6) Borys, N. F.; Holt, S. L.; Barden, R.E. J . Colloid Interface Sci. 1979, 71, 526. Gonsalez, A.; Holt, S . L. J . Org. Chem. 1981, 46, 2594. Martin, C. A.; McCrann, P. M.; Ward, M. D.; Angelos, G. H.; Jaeger, D. A. Ibid. 1984, 49, 4392.
(7) Mackay, R. A. J . Phys. Chem. 1982,86,4756; Adu. Colloid Interface Sci. 1981, I S , 131.
(8) (a) Athanassakis, V.; Bunton, C. A.; de Buzzaccarini, F. J . Phys. Chem. 1982, 86, 5002. (b) Bunton, C. A,; de Buzzaccarini, G.; Hamed, F. H. J . Org. Chem. 1983, 48, 2457.
0022-3654/86/2090-5858$01.50/0 0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5859
Deacylation in Microemulsions TABLE I: Reaction of p N P B in Anionic Micelles and Microemulsions
SDS,'
run
%
1-AmOH,' %
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
7.92 7.90 7.89 7.92 7.91 8.02 7.92 7.90 15.8 25.1 15.8 26.4 10.0 10.0 10.0 9.90 9.95 14.9 9.94
1.11 4.00 8.04 8.05 7.99 8.04 16.1 16.2 25.8 16.2 27.4 20.1 20.1 20.1 19.9 20.0 16.8 29.6
octane,'
1OZINaOH],b
%
M
S-
1.oo 1.oo 1.oo 0.33 1.00
1.58 2.47 5.07 2.03 6.61 7.44 47.9 7.31 6.34 7.10 4.97 6.50 3.56 7.50 17.0 24.6 23.7 3.80 4.03
2.00
2.31 3.98 4.79 4.79 4.79 4.78 4.74 5.38 5.38
6.57 1.oo 1.00 1.oo 1.oo 1.oo 1.oo 2.00 4.93 9.85 9.85 1.oo 1.oo
102k,/[OH-], M-1 s-l
104~,,
1.58 2.47 5.07 6.97 6.61 3.72 7.29 7.31 6.34 7.10 4.97 6.50 3.56 3.75 3.45 2.50 2.41 3.80 4.03
'Percentages are by weight. bMolarity based on total solution volume. to estimate accurately the distribution of reactants between water and droplets. The present work covers the saponification of p-nitrophenyl benzoate @NPB) in microemulsions containing tert-amyl alcohol, octane, and cationic or anionic surfactant. The surfactants were cetyltrimethylammonium bromide (CTABr) or sodium dodecyl sulfate (SDS). Alcohol-modified micelles were also used. Reactions in aqueous acetonitrile or rerr-butyl or tert-amyl alcohol, in the absence of surfactant, were examined. The inhibition of reaction by anionic microemulsions was examined because it provides a simple method of following incorporation of substrate in the droplets.**' This work on the saponification of p N P B in microemulsions or alcohol-modified micelles is an extension of studies of deacylation of p N P B by the azide ion: and aromatic nucleophilic substitution and dephosphorylation by hydroxide ion.'"-12 We used tert-amyl alcohol as cosurfactant to avoid reaction with the alkoxide ion,'JO and solvent compositions were similar to those used earlier for reactions of 2,4-dinitrohalobenzenes with OH-.Io
Experimental Section Materials. The preparation or purification of substrate and surfactants has been d e ~ c r i b e d . ' ~ - 'tert-Butyl ~ and tert-amyl alcohol were redistilled. Kinetics. Reactions were followed spectrophotometrically at 400 nm in Gilford or Beckman spectrophotometers. The solvents were made up by weight with redistilled, C0,-free, water. Substrate was added in MeCN so that its final concentration was ca. 5 X M and the solution contained 0.05% MeCN. The first-order rate constants, k,, are in reciprocal seconds and all reactions were at 25.0 OC. Results and Discussion Reaction in the Absence of Surfactant. The effects of tert-butyl and tert-amyl alcohol and acetonitrile on reaction in the absence of surfactant are shown in Figure 1. The tertiary alcohols were used because primary and secondary alcohols can form nucleophilic alkoxide ions. Addition of organic solvents often increases the nucleophilicity or basicity of anions in water, probably by decreasing their hydration." However, the organic solvents slow the reaction over (9) Bunton, C. A.; Moffatt, J. R.; Spillane, S.J. J . Chem. Soc., Perkin T r a m 2. in Dress. (lO)AthLnassakis, V.; Bunton, C. A.; de Buzzacarini, F. J . Phys. Chem. 1982,86, 5002; J . Org. Chem. 1983,48, 2461. (11) Bunton, C . A.; de Buzzaccarini, F.; Hamed, F. H. J . Org. Chem. 1983.48.2457. (12) Biresaw, G.; Bunton, C. A,; Quan, C.; Yang, Z.-Y. J. Am. Chem. Soc. 1984, 106, 7178. (13) Parker, A. J. Chem. Reu. 1969, 69, 1.
3
R x H20
Figure 1. Effect of variation in the mole fraction of H,O upon the saponificationof pNPB with 0.01 M NaOH: 0 , 0, m, added t-BuOH, r-AmOH, MeCN, respectively.
most of the range of composition, and for low concentration of organic solvent tert-butyl alcohol and acetonitrile have qualitatively similar effects, based on their mole fractions, x ~ (Figure , ~ 1). The low solubility of tert-amyl alcohol in water forced us to use only a dilute solution, but this alcohol is a better inhibitor than tert-butyl alcohol. The increase of the rate constant at high concentration of tert-butyl alcohol is probably due to the decreased solvation of OH- becoming all-imp~rtant,'~ although we see only a slight rate increase at high concentrations of acetonitrile. However, comparison of second-order rate constants in mixed solvents can be misleading if they are calculated in terms of molarity. Molarity is a very convenient measure of concentration but it is of doubtful significance in mixed aqueous-rganic solvents. Hydrophilic ions will be solvated preferentially by the water molecules and the total solution volume is a poor measure of the effective amount of solvent. Our value of the second-order rate constant, kw, for reaction of OH- in water is 3.8 M-' s-l which is slightly higher than that of 3.2 M-' s-l from experiments in carbonate buffer or dilute OH-.12 Effects of Added Surfactants. Reactions in microemulsions or alcohol-modified micelles of SDS are very slow. In water the inhibition by SDS can be treated quantitatively on the assumption that OH- is excluded completely from the anionic micelle^,'^^'^ (14) (a) Menger, F. M.; Portnoy, C . E. J. Am. Chem. SOC.1%7,89,4698. (b) Bunton, C. A,; Robinson, L. J . Org.Chem. 1969, 34, 773. (15) Addition of large amounts of added salt allows some binding of OHto anionic micelles, and there is then a residual reaction with bound substrate.16
Athanassakis et al.
5860 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
TABLE II: Reaction of eNPB with OH- in Cationic Microemulsions and Micelles run
CTABr," %
t-AmOH," %
I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
3.34 3.31 3.32 3.33 3.33 3.33 3.33 6.67 13.3 5.71 5.71 11.4 10.0 10.0 10.0 10.0 13.3 8.0 13.7 17.5 23.3
0.81 3.33 6.66 6.66 6.66 6.66 13.3 26.7 11.4 11.4 22.8 20.0 20.0 20.0 20.0 13.4 24.0 27.4 35.0 23.3
Octane: %
2.86 2.86 5.72 10.0 10.0 10.0 10.0 13.3 8.0 13.7 17.5 23.3
102[NaOH],b M
1039, S-
k*lM-I [OH-], s-I
1 .oo 1 .oo 1 .oo 0.33 1.oo 1.97 3.94 1.oo 1.oo 1 .oo 2.00 1 .oo 1 .oo 2.00 4.93 9.70 1.oo 1.oo 1.oo 1 .oo 1.oo
42.6 37.7 31.0 6.19 18.5 35.9 66.9 6.8 4.8 6.5 12.1 3.0 3.0 5.8 14.5 23.7 4.4 2.9 3.1 4.3 5.1
4.26 3.77 3.10 1.86 1.85 1.82 1.70 0.68 0.48 0.65 0.61 0.30 0.30 0.29 0.29 0.24 0.44 0.29 0.31 0.43 0.51
'Percentages are by are by weight. bMolarity based on total solution volume.
and we can make that assumption here (Table I). If we assume that no OH- is taken up by the anionic micelles or droplets, and that the second-order rate constant in the bulk medium is that in water, the ratio of rate constants in the surfactant solutions to those in water gives the fraction of free substrate (Table I). This calculation is only an approximation because not all the tert-amyl alcohol will be bound to the micelles or droplets, and the residual amount in the bulk solvent will slow the reaction (Figure 1). In addition the presence of alcohol in the micelles or droplets will reduce their surface charge density, and therefore their ability to repel OH-,17and there may be some residual reaction in the micelles or droplets. These factors are probably responsible for the apparent increase in free substrate as the tert-amyl alcohol content is increased, but the effects are small and we conclude that the substrate is almost completely bound to the anionic micelles or droplets. We therefore assume that the substrate will be almost completely bound to cationic micelles or microemulsion droplets of CTABr because surfactant charge should not have a large effect on binding of a nonionic solute. Rate constants in cationic alcohol-modified micelles or microemulsions are given in Table 11. Overall second-order rate constants, k,/[OH-] are lower in these solutions than in water and the rate constants decrease with increasing amount of CTABr, but they are always very much larger than in solutions of SDS. Qualitatively addition of octane has little effect on reaction rates, as was found for reaction of azide ion with 2,4-dinitrochloronaphthalene? These results are understandable because reactions occur at the surfaces of micelles or droplets which should be little affected by an alkane which goes to the interior of a microemulsion droplet The observed second-order rate constants, k,/ [OH-], in micelles or microemulsions of fixed composition are approximately constant in dilute N a O H (Table 11). Quantitative Treatment of Kinetic Data. Aqueous micelles of CTABr speed reaction of pNPB with OH- by bringing the two reactants together. For a given [OH-] rate constants initially increase with increasing [CTABr] as substrate is transferred from water to the micelles, but they go through a maximum because with increasing [CTABr] competition between Br- and OH- in-
creases, and the increasing volume of the micelles dilutes OHin the micellar pseudophase.2~'2~18 Substrate is extensively bound in all our experiments in micelles and droplets and increasing concentrations of CTABr or tert-amyl alcohol should slow reaction, as is observed (Table 11). With constant [CTABr], added tert-amyl alcohol slows the reaction (runs 1-3 and 5 ) , probably by decreasing the charge density of the micelle and its ability to bind OH-.'7 This effect is seen with other reactions of Second-order rate constants in a micellar pseudophase can be calculated by estimating concentrations at the micellar s ~ r f a c e . ~ . ~ . ' ~ The various treatments depend upon the assumed reaction volume in the micellar pseudophase and, for ionic reactions, upon the assumptions made in estimating ionic concentrations at the micellar s u ~ f a c e s . * * ~ J ~ - ~ ~ In analyzing our rate data we follow the approach used earlier in treating reactions of OH- in alcohol-modified micelles, recognizing that it depends on untested assumptions.8 The fractional charge neutralization of the micelles, or droplets, by anions, (3, is given by
P = ([OH-,] + [Br-MI)/[CTABrMI
(1)
and the interionic competition by2,I8
where subscripts W and M denote the aqueous and micellar (or droplet) pseudophase. Equation 2 can be simplified if [Br-,] >> [OH-,], which is reasonable in our systems, so that
.335
(16) Srivastava, S. K.; Katiyar, S . S . Ber. Bunsenges. Phys. Chem. 1980, 84, 1214. Quina, F. H.; Politi, M. J.; Cuccovia, I. M.; Martins-Franchetti, S . M.; Chaimovich, H. In Solution Behavior of Surfacfanfs;Mittal, K. L., Fendler, E. J., Eds.; Plenum: New York, 1982; Vol. 2, p 1125. (17) (a) Larsen, J. W.; Tepley, L. B. J. Colloid Inferface Sci. 1974, 49, 113. (b) Zana, R.;Yiv, S.; Strazielle, C.; Lianos, P. Ibid. 1981, 80, 208. (c) Bunton, C. A,; de Buzzaccarini, F. J . Phys. Chem. 1982. 86,5010.
where [OH-,] is the stoichiometric concentration and all the CTABr is assumed to be micellized. Equations 1 and 3 give [OH-,] --
[CTABrl
-
[OH-,] Kg:(1/(3 - 1)
+1
(4)
(18) (a) Quina, F. H.; Chaimovich, H. J . Phys. Chem. 1979,83, 1844. (b) van de Langkruis, G. B.; Engberts, J. B. F. N. J. Org. Chem. 1984,49,4152. (19) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1985,89, 4166; 1986, 90, 538. (20) Almgren, M.; Rydholm, R. J. Chem. 1979, 83, 360. Lissi, E. A,; Abuin, E. B.; Sepulveda, L.; Quina, F. H. Ibid. 1984,88, 81. Funasaki, N.; Murata, A. Acta. Chem. Pharm. Bull. 1980, 28, 805.
The Journal of Physical Chemistry, Vola90, No. 22, 1986 5861
Deacylation in Microemulsions Equation 4 is consistent with reactions in solutions of CTABr being approximately first-order in [OH-T] (Table 11). The second-order rate constants, kM,in the micelles are written with concentration of OH- as a mole ratio of bound OH- to surfactant head groups bound alcohol (ROHM)
+
k, = ~ M R [ O H - /M[CTABr] ]
(5)
where
R = [CTABr]/([CTABr]
+ [ROHM])
The variations of rate constants with overall concentrations of OH-, CTABr, and tert-amyl alcohol can be used to calculate kM are specified. We take = 12 because provided that /3 and this value had been used earlier to fit data in aqueous and alcohol-modified micelles of CTABre8 However, various values have been used in other treatment^,^^^.^^,^^ and a treatment of ionic competition based on solution of the Poisson-Boltzman equation in spherical symmetry suggests that these ion-exchange parameters are constant only over limited ranges of surfactant and ionic con~entration.~~ Our treatment, in effect, assumes that we can treat the colloidal particles as if they are micelles whose properties are merely perturbed by addition of cosurfactant and hydrocarbon. For reactions in aqueous micelles, values of kM for a given reaction vary little with changes in concentration of surfactant and electrolytes, probably because micellar surfaces are insensitive to these changes.2lgB However, surfaces of microemulsion droplets should change with changes in relative concentrations of surfactant and alcohol; for example, the volume element of reaction will change with changes in R (eq 6 ) . Two assumptions have been made in estimating the molar volume element of reaction.* In one the volume element is taken as that of the alcohol plus half that of the surfactant, i.e.*l
gr
V M /= 2
MWCTABr
+ (1 - R ) MWROH -
1000
1000
(7)
where M W is molecular weight. In the second assumption the volume element is that of the surfactant plus that of the alcohol, i.e.
The second-order rate constants in the droplets are given by (9)
or k2” = k M VM’f
(10)
The second-order rate constants, kZ) and k2/1 have dimensions M-’ s-l and can be compared directly with the second-order rate constant in water, kw, M-’ s-l. Table I11 gives values of 6, R , and kM and of k2’ and k2” for reactions in alcohol-modified micelles and microemulsions. Over a wide range of solvent compositions valces of kZ) vary by a factor of ca. 3, and k2/1 by a factor of ca. 2, whereas values of k,/[OH-] vary by a factor of ca. 16 (Table 11). Values k2/ and k; are little affected by the presence of octane, which confirms the similarity of alcohol-modified micelles and microemulsion droplets. Our calculation of k i and k2/1 depends upon arbitrary assumptions; for example, we neglect the presence of water molecules at the droplet surface, and the values of R give only an approximation of the amount of tert-amyl alcohol in the micelle or droplet. However, the solubility of tert-amyl alcohol in water is low (ca. 5% w/w) so it is reasonable to take the total concentration of tert-amyl alcohol as [ROHM].22s23This assumption leads to values of VM’or VM”which may be too high, and the values of (21) Unit density is assumed for the solutes in calculating V’ and V”. (22) Bunton, C. A.; de Buzzaccarini, F. J . Phys. Chem. 1981, 85, 3142. (23) Hall, D.; Jobling, P. L.; Rassing, J. E.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 2 1977, 73, 1582.
TABLE III: Estimated Second-Order Rate Constants for Reactions in Microemulsions and Micelles
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
42.6 37.7 31.0 6.2 18.5 35.9 66.9 6.8 4.8 6.5 12.1 3.0 3.0 5.8 14.5 23.7 4.4 2.9 3.1
0.85 1.00 1.19 0.18 0.85 0.93 1.21 0.18 0.78 0.75 1.61 0.16 0.66 0.50 2.43 0.13 0.66 0.48 2.55 0.13 0.66 0.50 2.38 0.13 0.66 0.50 2.20 0.13 0.70 0.20 3.75 0.11 0.700.16 7.140.10 0.70 0.29 2.21 0.12 0.700.292.050.12 0.70 0.17 3.35 0.10 0.67 0.18 3.09 0.10 0.67 0.18 3.02 0.10 0.67 0.18 3.04 0.10 0.67 0.18 2.53 0.10 0.74 0.35 2.37 0.12 0.70 0.12 3.28 0.10 0.730.165.120.10
0.21 0.22 0.26 0.32 0.33 0.31 0.27 0.41 0.77 0.27 0.25 0.34 0.31 0.30 0.30 0.25 0.28 0.33 0.51
0.36 0.36 0.30 0.22 0.22 0.22 0.22 0.14 0.13 0.17 0.17 0.14 0.14 0.14 0.14 0.14 0.18 0.12 0.13
0.43 (1.44 0.48 0.53 0.56 0.52 0.48 0.53 0.93 0.38 0.35 0.46 0.43 0.42 0.43 0.35 0.43 0.39 0.67
‘From Table 11. k i and k2/1 in Table I11 will correspondingly be too high. The variation of k, with [CTABr] in water had earlier been analyzed by using the pseudophase ion-exchange model, giving the second-order rate of 0.3M-’ s-’ at the micellar surface.12 This value is similar to those in Table I11 although the calculations depend upon different assumptions regarding substrate binding and the volume element of reaction. Medium Effects of Micelles and Droplets. Values of kZ) and k2/1 are lower than that of kw (Table 111). Comparison of second-order rate constants in water and in micelles or microemulsion droplets depends upon the assumptions made in the calculations. For example, the second-order rate constants, kZ) or k?, increase if GF or VM’or VM”are increased. It is therefore difficult to compare second-order rate constants calculated by different groups, because of differences in the assumed ion-exchange parameters and volume elements of reaction. Despite these uncertainties several conclusions emerge; in particular second-order rate constants in the colloidal particles are generally similar to, or smaller than, those in ~ a t e r . ~ v ~ -In~ addition, - ~ ~ J ~ reactions -~~ of nonionic nucleophiles have lower second-order rate constants in the particles than in water, which is consistent with differences in the polarities of these media.25q26 Estimates of second-order rate constants, k i or k?; for dephosphorylation, deacylation, and aromatic nucleophilic substitution have been based on the same values of VM’or VM” and It is therefore reasonable to compare second-order rate constants of these reactions in colloidal particles, relative to those in water. Based on values of VMand used in the present work, second-order rate constants in alcohol-modified micelles or microemulsions are smaller than those in water for deacylation of pNPB for which kw = 3.8 M-’ s-l (Table 111). For dephosphorylation of p-nitrophenyl diphenyl phosphate kw = 0.48 M-I and kZ) = 0.05 M-’ and k2/1 = 0.1 M-’ s - ~ . But ~ for reaction of 2,4-dinitrofluorobenzenewith OH- kw = 0.12 M-’ s-l and kZ) is in the range 0.22-0.78 M-’ s-I and k2/1 is 0.4-1.1 M-’ s - ~ . ~ ~ There are similar differences for reactions in aqueous micelles. For example, second-order rate constants in water appear to be
e:.8
e?
(24) Martinek, K.; Yatsimirski, A. K.; Levashov, A. V.; Berezin, I. V. In Micellization, Solubilization and Microemulsiom; Mittal, K. L., Ed.; Plenum: New York, 1977; Vol. 2, p 489. Rodenas, E.; Vera, S.J . Chem. Educ. 1985, 62, 1120. (25) Menger, F. M. Pure Appl. Chem. 1979, 51, 999. Cordes, E. H.; Gitler, C. Prog. Bioorg. Chem. 1973, 2, 1. Bunton, C. A.; de Buzzaccarini, F. J. Phys. Chem. 1981, 85, 3139. Ramachandran, C.; Pyter, R. A.; Mukerjee, P. J . Phys. Chem. 1982, 86, 3198. (26) Some of these values of k’of k” may be low because of incomplete binding of the substrate, though the data on anionic droplets suggest that such effects are small.
J. Phys. Chem. 1986, 90, 5862-5865
5862 SCHEME I
L
t OH-
-
NO2
?I
NO2
droplets. For example, very hydrophobic substrates could be located deeply in the colloidal particles where they would be less accessible to hydrophilic anions. However, pNPB and 2,4-dinitrochloronaphthalene have similar micellar binding constants which does not support this h y p o t h e ~ i s . ' ~ J ~ Another explanation is that micelles, or other self-assembling colloids, have kinetic medium effects, related to mechanism and transition-state structure. For example, in anionic aromatic nucleophilic substitution the negative charge in the transition state is delocalized over a conjugated system and could interact readily with cationic head groups (Scheme I). In deacylation and dephosphorylation the charge in the anionic transition state is localized on oxygen atoms which will interact preferentially with ~ater.~',~~ This second explanation also seems to be applicable to reactions in normal aqueous micelles involving N< as well as OH-.30 This "solvent" effect of micelles or droplets is consistent with the rate decrease on addition of organic solvents to water (Figure 1). This inhibition is also observed for reaction of p-nitrophenyl diphenyl phosphate,8 whereas addition of organic solvents speeds aromatic nucleophilic substitution by OH-.* The transition states for deacylation and dephosphorylation should interact with hydrogen-bonding solvents such as water, and as water content is decreased destabilization of the transition state and stabilization of the apolar substrate opposes the rate-enhancing destabilization of hydroxide ion.
Acknowledgment. Support by the National Science Foundation lower than those in cationic micelles for reactions of OH- with 2,4-dinitrochlorobenzeneand 2,4-dinitrochl0ronaphthalene,'~*~~ (Chemical Dynamics Program) is gratefully acknowledged. whereas the opposite sequence is formed for deacylation of pNPB Registry No. pNPB, 959-22-8. and dephosphorylation of p-nitrophenyl diphenyl phosphate. One possible cause of these different mcdium effects is that (28) The structure of the transition state in aromatic nucleophilic substithe substrates have different average locations in the micelles or (27) (a) Bunton, C. A.; Cerichelli, G.;Ihara, Y.; Sepulveda, L. J . Am. Chem. Soc. 1979,101,2429. (b) Bunton, C. A. Pure Appl. Chem. 1977,49,
tution is assumed to be similar to that of a Meisenheimer complex.29 (29) Bunnett, J. F. Q. Rev. Chem. SOC.1958, 12, 1 . Miller, J. "Aromatic Nucleophilic Substitution"; Elsevier: New York, 1968. (30) Bunton, C . A.; Moffatt, J. R.; Rodenas, E. J . Am. Chem. SOC.1982,
969.
104, 2653.
Reactions of Primary Amines with 2,4-Dinitrochlorobenzene in Microemulsions Vassilios Athanassakis, Clifford A. Bunton,* and Francesco de Buzzaccarini Department of Chemistry, University of California. Santa Barbara, California 931 06 (Received: March 18, 1986; In Final Form: June 3, 1986)
Reactions of 2,4-dinitrochlorobenzenewith n-butyl-, n-amyl-, and n-hexylamine have been examined in microemulsions of n-octane and cetyltrimethylammonium bromide (CTABr) or sodium dodecyl sulfate (SDS) with amine as cosurfactant. The second-order rate constants in the microemulsion droplets are similar for reactions of n-butyl- and n-amylamine, but are lower for reaction with n-hexylamine, especially in the anionic microemulsions. With this exception second-order rate constants are similar to those in water.
Oil-in-water (o/w) microemulsions contain an oil, a surfactant, and a cosurfactant which is often a medium chain length alcohol, but can be a moderately hydrophobic molecule, and water is the bulk solvent.' Microemulsions are good solvents for ionic and nonionic solutes and their use as reaction media is well documented.'b.24 They are similar to aqueous micelles in their ability (1) (a) Prince, L. M., Ed. Microemulsions: Theory and Practice; Academic: New York, 1977. (b) Mackay, R. A. Adv. Colloid Interface Sci. 1981, 15, 131.
(2) (a) Hermansky, C.; Mackay, R. A. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.;Plenum: New York, 1979; p 732. (b) Mackay, R. A.; Hermansky, C. J . Phys. Chem. 1981, 85, 739. Mackay, R. A. Ibid. 1982,86, 4156. (3) Borys, N . F.; Holt, S. L.; Barden, R. E. J. Colloid Interface Sei. 1979, 71, 526. Gonsalez, A.; Holt, S. L. J . Org. Chem. 1981, 46, 2594. Martin,
C. A.; McCrann, P. M.; Ward, M. D.; Angelos, G. H.; Jaeger, D. A. Ibid. 1984, 49, 4392.
to influence reaction rates. Overall rate effects are usually smaller in microemulsions, but their higher solubilizing power may be useful. Bimolecular ionic reactions in microemulsions have been well studied, but effects on spontaneous reactions and reactions of nonionic nucleophiles have also been examined to a limited extent. lbsZd n-Hexylamine is a cosurfactant in microemulsions of cetyltrimethylammonium bromide (CTABr) and octane, and its reactivity toward 2,4-dinitrochlorobenzene (DNCB) in micro(4) (a) Athanassakis, V.; Bunton, C. A.; de Buzzaccarini, G. J . Phys. Chem. 1982, 86, 5002. (b) Bunton, C. A.; de Buzzaccarini, F. Ibid. 1982, 86, 5010. (5) Bunton, C. A.; de Buzzaccarini, F. J . Phys. Chem. 1981, 85, 3139. (6) Bunton, C. A.; de Buzzaccarini, F. J . Phys. Chem. 1981, 85, 3142.
0022-3654/86/2090-5862$01 .50/0 0 1986 American Chemical Society
