Langmuir 1993,9, 61-65
61
Deacylation by Hydroxide Ion in Cationic Micelles. Reactivity at Micelle-Water Interfaces Raimondo Germani and Gianfranco Savelli’ Dipartimento di Chimica, Universith di Perugia, 06100 Perugia, Italy
Nicoletta Spreti Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universith di L’Aquila, 67100 L’Aquila, Italy
Giorgio Cerichelli and Giovanna Mancini Centro CNR sui Meccanismi di Reazione, Dipartimento di Chimica, Universith La Sapienza, 00185 Roma, Italy
Clifford A. Bunton Department of Chemistry, University of California, Santa Barbara, California 93106 Received March 16,1992. In Final Form: August 13,1992 Reaction of OH- with phenyl benzoate was examined in aqueous solutions of cetyltrialkylbromides (alkyl = Me, Et, n-Pr, n-Bu, CTABr, CTEABr, CTPABr, and CTBABr, respectively) and tetradecylquinuclidium bromide (TDQBr). With up to 0.1M OH- observedrate constantsincreasemodestlytomaxima, but in 0.6 M OH-, reaction in inhibited under all conditions. These results are fitted quantitatively by a pseudophase model that describes ion-micelle interactions in terms of equations of the form of the Langmuir isotherm. Second-order rate constants at the micellar surfaces are lower than in water by factors of 23-56, depending on the surfactant. Similar second-order rate constants were obtained for reaction in solutionsof CTAOH. At high [OH-] this low reactivity in the micellar pseudophase overcomes the rate-enhancing effect of concentration of OH- at the micellar surface. The classical pseudophase ion-exchange model fits the data qualitatively but not quantitatively, based on usually accepted values of the ion-exchange parameter and fractional micellar ionization, a. Nucleophilic deacylation of carboxylic esters has been examined extensively in aqueousmicellesof nonfunctional and functional surfactants.l Rate data have been treated quantitatively on the assumption that water and micelles can be regarded as distinct reaction regions.2 This pseudophase model fits rate data for reactions of nucleophilic anions, for example of OH-, with p-nitrophenyl alkanoates,and the distribution of OH- between aqueous and micellar pseudophases is often written in terms of equations similarto those used to describe interactions of ions with ion-exchange resins.2b-fJ Cationic micelles speed reaction of anions with hydrophobic substrates by bringing the two reagents together at a micellar surface. These rate enhancements are due largelyto concentration of reagents at the micellar surface and second-order rate constants at these surfaces are generallysimilarto those in ~ a t e r . ~Most B work in aqueous cationic micelles hae involved surfactants that have the MesN+head group. We are interested in the effects of the size of cationic head groups on micellar structure and rate (1) (a) Cordea, E.H., Gitler, C. h o g . Bioorg. Chem. 1973,2, 1. (b) Fender, J. H. Membrane Mimetic Chemistry; Wiley Interscience: New York, 1982. (2) (a) Martiiek, K.; Yataimhki, A. K.; Levashov, A. V.; Berezin, I. V. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.;Plenum Press: New York, 1977; Vol. 2, p 489. (b) Romsted, L. S. ibid., p 609. (c) Ra”, L. S., In Surfactants in Solution; Mittal, K. L., Lidman, B., Eda.; Plenum Press: New York, lQW,Vol. 2, p 1016. (d) Sudholter,E. J. R.; van der Langkruii, G. B.; Engberta, J. B. F. N. Red. Trov. Chim. Pays-Bas 1979,99,73. (e) Bunton, C. A.; Savelli, G.Adv. Phys. Org. Chem. 1986,22,213. (f) Bunton, C.A.; Nome, F.; Quina, F. H.; Rometed, L. S . Acc. Chem. Res. 1991 24,367. (3) Quina, F. H.;Chaimovich, H. J . Phys. Chem. 1979,83,1844.
0743-746319312409-0061$04.00/0
effects.4v6 An increase in head group size should reduce polarity and counterion concentration at micellar surfaces,46t7 which slowsreactions, but for someions, these effecta are offset by an increase in anionic nucleophilicity.4 Phenyl benzoate (PhOBz) reacts at a convenient rate with aqueous OH- and we examined this reaction over a wide range of concentrations of OH- and surfactant. The surfactant counterion was Br- or OH-, and with OH- we eliminate competiton between OH- and Br- at the micellar surface. The surfactants were6 ClsHssNRaBr (R = Me, CTAX; Et, CTEAX; n-Pr, CTPAX, n-Bu, CTBAX) and tetradecylquinuclidinium bromide, TDQBr.
Deacylation is generally written as involving initial formation of a tetrahedral intermediate which may return to reactants or be converted into products.* There is ~
~
~~~~~~~
(4) (a) Bacaloglu, R.;Bunton, C. A.; Ortega, F. J. Phys. Chem. 1989, F. J. 93,1497. (b) Bacaloglu, R.;Bunton, C. A.; Cerichelli, G.; -a, Phys. Chem. 1990,94,6068. (6)Bonan, C.; Germani, R.; Ponti, P. P.; Savelli, G.;Cerichelli, G.; Bacaloglu, R.; Bunton, C. A. J. Phys. Chem. 1990,94,5331. (6) Cerichelli,G.; Lucchetti,L.; Mancini,G.;Muzzioli,M. N.;Germani, R.; Ponti, P. P.; Spreti, N.; Savelli, G.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2, 1989,1081. (7) (a) Zana, R.J. Colloid Interface Sei. 1980,87,330. (b)Lianoe, P.; Zana, R. J . Colloid Interface Sci. 1982,88, 594. (8) Bamford, C. H.; Tipper, C. F. H. Ester Formotion and Hydrolyst; Elsevier: Amsterdam, 1972;Vol. 10 (ComprehensiveChemical Kinetica). Q 1993 American
Chemical Society
Germani et al.
62 Langmuir, Vol. 9, No. 1, 1993 0.15
0.10
vi
9 0
Y
Y
0.05
0.04
1 4
-
"
0.00
0.00o' ' ' ' 'o.obs
-
-
-
o.obo
0.0'1
o 0.0;5 [CTABr] , M
7
0.00 0,000
1
O.ob5
0.0'rO
[CTPA&],
I.~..~,.~rtrm
0.015
0.020
M
Figure 3. Reaction with OH- in CTPABr, symbols as in Figure
Figure 1. Reaction with OH- in CTABr, [OH-]: +, 0.01; 0,0.05; O, 0.1; 0 , 0 . 5 M. In 0.5 M OH- in the absence of surfactant kob = 0.23 s-l.
1. 0.12
0.10 0 P)
\r:
6 n
c
0
ui
Y
n
0
0.05
-
0.00 0.Ob0
0.0'05
0.0'10
[CTEABr],
I
~
0.015
~
0.020
~
~
M
Figure 2. Reaction with OH- in CTEABr, symbols as in Figure
~
0.04
-
Figure 4. Reaction with OH- in CTBABr, symbols as in Figure 1.
1.
0.15
Scheme I phco.oPh
+
OH- ---C PhCO-(0H)OPh
fst
PhCO;
+ PhO-
PhOBZ 0.10
evidence for concerted deacylations, based on Bronsted relations of rats to structure? but with such agood leaving group as phenoxide ion and OH- as nucleophile the ratelimiting step is almost certainly addition (Scheme I). Results Rate surfactant Profiles. Variations of overall fmtorder rate constante, koa,for reactions in the presence of
Br,withchangesinsurfactantstructureandconcentration and [OH-], are shown in Figures 1-5. In dilute OH-values of kobincrease with increasing [surfactant] and go through maxima, but the initial rate increase decreases as [OH-] is increased, and with [OH-] = 0.5 M added surfactant inhibite reaction under all conditions. The rate constants, kok, depend upon the head group bulk, and the rate-eurfactantprofdes suggest that second~~
(9) Williams, A. Acc. Chem. Reu. 1989,22, 387.
000 0000 1
,
,
#
,
/
I
,
/
I I I I I I I I
oobs
I I
I I I I I I I , I , ) I I I I I ( I I l I I I i
0010
[TDQBr],
0015
0.020
M
Figure 6. Reaction with OH- in TDQBr, symbols as in Figure 1.
order rate constante at the micellar surfaceare much lower than in water. T h e second-orderrata constantfor reaction in water, k,, is 0.39 M-l s-l for [OH-] S 0.1 M but for
Deacylution by Hydroxide Ion in Cationic Micelles 0'25
Langmuir, Vol. 9, No. 1, 1993 63 Table I. Fitting Parameterr for Reaction of PhOBe with Hydroxide Ion in Bromide Ion Micellem*
3
0.20
3r'
Figure 6. Reaction in CTAOH with the indicated added OH-.
reaction in 0.5 M OH- we take kw = 0.47 M-l s-l. This effect is due to an electrolyte effect at high [OH-]. Firsborder rate constmats for reaction in solutions of CTAOH are shown in Figure 6. With no added OH- rates increase modestly with [CTAOHI, but at high [OH-] reaction is inhibited by CTAOH. QuantitativeModels. Pseudophase Ion Exchange (PIE). This model assumes that OH- and Br- compete for the micellar surface as for an ion-exchange resin (eq 1)2b-f,3
= [OH,-] [ B r ~ ~ / ( [ o H ~ - ] [ B r w - l ) (1) Fractional charge neutralization of head groups, 8, is assumedto be constant (8 = 1- a,where CY is the fractional ionization). Values of [OHM]can be calculated in terms of KBroH and surfactant [D). "he concentration of monomericsurfactantis assumed to be the criticalmicelle Concentration,cmc, under the kinetic conditions, Le., [Dn] = [Dtl - cmc (Dt and Dn are total and micellized surfactant, respectively). The overall rate'constant is given by &OH
surfactant CTABr CTABr CTABr CTABr CTEABr CTEABr CTEABr CTEABr CTPABr CTPABr CTPABr CTPABr CTBABr CTBABr CTBABr CTBABr TDQBr TDQBr TDQBr
[OH-], M 0.01 0.05 0.1 0.5b 0.01 0.05 0.1 0.5b 0.01 0.05 0.1 0.5b
lWcmc, M 7 1 0.7
0 5 1 0.3
0 4
0.9 0.5
0
0.01
1
0.05
0.7 0.1
0.1 0.5b 0.05 0.1 0.5b
0 9 3
0
1@kM,8-l 8.0
8.0 8.5 9.0 6.0 6.0 6.5 7.0 5.0 5.0 5.0 5.0 5.0 6.5 6.0
6.0 7.0 7.5 9.0
AT 25.0 "Cwith K , = 2000 M-l, k, = 0.39 M-ls-l unlesespecified, KOH'= 55,45,25,12, and 50 M-1 for CTA, CTEA, CTPA, CTBA, and TDQ, respectively; KB; = 2000,1500,1100,750, and 1700 M-1, respectively, for the corresponding bromides k, = 0.47 M-18-1.
the value of the cmc, and it may be affected by a hydrophobic substrate. This failure of the PIE model with increasing [OH-] has been observed with other micellar-enhancedreactions, and data have been accommodated with an alternative ion-exchangemodel that describes ion-micelle interactions with equationsof the Langmuir form, equivalent to a massaction treatment.41sJ2J3 Mass-Action-Like Model. For reaction in solutions of CTAOH we write the distribution of OH-between water and micelles as
= [OHM-l/([OHw-l([Dnl- [OHMI)) (3) and with OH- and Br- ionic distributions are written as KOH'
KOH' = [OHM-l/([OHw-l([Dnl- [OHM-]- [B~M-]))(4)
= [BrM-l/([Brw-l([Dnl- [OHM-]- [BrMI)) (5) These equations predict that ionic concentrations at the kw[OHw-l + k&s[OH~-l micellar surface will increase with total concentrations, (2) koba 1+ Ks[Dnl whereas the PIE model, in its usual form, requires where kw, M-l s-l, is the second-order rate in water, k ~ , constancy of the ionic concentration.2b-f Equations 3-5 were initially developed for surfactant s-l, is that in the micellar pseudophase with concentration systems that contained only the reactive counterion,1% written as the mole ratio, [ O H M - I / [ D ~ Iand , ~ ~Ks, ~ M-l, and rate data in CTAOH were fitted with KOH'= 55 M-l, is the substrate binding constant to micellized surfactant. although slightly different values have been used sucEquations 1 and 2 can be coabined and the rate data cessfully by others.ls Values of Kx' are much higher for fitted by computar simulation, as dh"d elsewhere,alOJ1 ions such as Br-.4J2b The ion-exchange parameter, KBroH, is assumed to be Equations4 and 5 can be combined and used to fit kinetic unaffectad by changes in the head group, but 8 decreases data, and this treatment fits data in moderately concenwith increasing head group The rate surfactant trated OH- where the PIE model fails.12b profilea can be fitted by the PIE model only if k~ is Values of KOH'for the five surfactants were estimated increased by a factor of ca. 2 in going from 0.1 to 0.5 M from rate constants of reactions of OH- with methyl OH-, and even then fits are poor in dilute CTABr. For naphthalene-2-sulfonate in solutions of micellized surreaction in CTABr with 8 0.8 and KBroH = 14,we obtain factantswith OH-as counterions and, with selected values k~ = 0.044,0.066,0.070,and 0.09 s-l at 0.01,0.05,0.1, and of Kg;l4 are in Table I. Values of Kx' decrease with 0.5 M OH-,respectively. (Fits are aot shown.) We saw increasing head group bulk, except that they are similar similar increases in k~ with the other surfactants. Fits are insensitive to& with stronglybound substrates, except (12) (a)Bunton,C.A.;Gan,L.-H.;Moffatt,J.R.;Romsted,L.S.;Savelli, in very dilute aurfactant where the fitting is sensitive to G. J. Phys. Chem. 1981,85,4118. (b)Germani, R.;Ponti, P. P.; Savelli,
-
(10) Rodenan, E.;Vera, S. J. Phys. Chem. 1986,89,513. (11) (a) B r o a n , T.J; Chrhtie, J. R.; Chung, R. P.T.J. Org.Chem. 19SS,53,308L. (b) Broxton, T.J.; Christie, J. R.; Sango,X.J. Org. Chem. isa7,52,4814.
KB;
G.; Spreti, N.; Bunton, C. A,;Moffatt J. R. J. Chem. SOC.,Perkin TToM. 2, 1989, 401. (13) Vera, S.; Rodenae, E. J. Phys. Chem. 1986,90,3414. (14) Germani, R.; Savelli, G.; Romeo, T.; Spreti, N.; Cerichelli, G.; Bunton, C. A. Langmuir, preceding paper in this issue.
Germani et al.
64 Langmuir, Vol. 9, No.1, 1993 Table 11. Fitting Parameters for Reaction of PhOBz in CTAOH. [NaOHl, M l04cmc, M 102kM,8-l 8 1 0 0
0.1 0.29 0.50
12 11 13 14.5
2.00
At 25.0 "C with the data in Figure 6, and k, = 0.39 M-ls-l except where specified.
Table 111. Comparison of Second-OrderRate Constants in Water and Micelles ~~~~
surfactant CTABr CTAOH CTEABr CTPABr CTBABr TDQBr
~
~
102k2","M-' 8-l 1.17 1.71 0.88 0.71 0.83 1.04
~~
~
ui
n 0
Y
1.00
kz"/kw 0.030
0.044 0.023 0.018 0.021 0.027
Calculated from mean values of k M (Tables I and 11).
with the quinuclidinium and triethylammonium head group surfactants. The fitting is shown in Figures 1-5 based on the parameters in Table I. The mass-action-like model (eq 3) fitsvariations of kobwith varying [CTAOH] and [NaOHl within experimental errors of f5% (Figure 6 and Table
11). An alternative model of ion-binding considers both Coulombic and ion-specific interactions and ionic concentrationsat micellar surfaceare calculated from classical electrostaticsby solving the Poisson-Boltzmann equation (PBE).16J6This treatment has not been tested for large surfactant head groups. Discussion Comparison of Simulation Models and Reactivities. Values of k ~s-l, (eq 2), are defined unambiguously with concentration as a mole ratio, but to compare them with second order rate constants in water, kw,M-l s-l, we must write concentrations of OH- at the micellar surface as molarities, so that
k," = kMVM (6) where VMis the molar volume of the reaction region at the micellar surface. We take VM = 0.14 M-l, as discussed elsewhere.2e Values of kzm are in Table 111. Despite limitations the two ion-exchange models lead to similar values of k~ (or kzm) (Table I) for reaction with dilute OH-. Values of kzm (eq 6) vary linearly with VM, and problems in assigning values to this parameter have been noted.2c9eJ8JgWe take VM = 0.14 M-l, based on considerationsof the size of the trimethylammoniumhead group, but the value is higher, e.g., VM= 0.37 M-l, if it is based on the molar volume of a CU micelle. We can compare second-order rate constants, k ~ in, micelles of different surfactants with concentrations of OH- expressed as mole ratios (Table I), but the small micellar rate increases in dilute OH-and the inhibition in more concentrated OH- indicate that kw >> k p , for any (15) (a) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1986,89,4166. (b) Bunton,C. A.; Moffatt,J. R. J. Phys. Chem. 1986,90,538. (c) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R.J. Phya. Chem. 1989,93, 7851. (16) Rodenae, E.; Ortsga, F. J. Phys. Chem. 1987,91, 837. (17)Gunnarseon, G.; Jonseon, B.; Wenneretrom, H. J. Phys. Chem. 1980,84, 3114. (18) (a) Hicks, I. R.; Reinsborough,V. C. A u t . J. Chem. 1982,35,15. (b) Otero, C.; Rodenae, E. Can. J. Chem. 1986,63,2892. (19)Correia,V. R.;Cuccovia, I. M.; Chaimovich, H. J. Phys. Org.Chem. 1991,4, 13.
Figure 7. Predicted rate effects of CTAOH upon a hypothetical reaction with kzm = k,, see text.
reasonable value of VM. In water fiit-order rate constants increase approximately linearly with total [OH-], but the increase is lese than linear in micelles because concentration of OH- at micellar surfaces increases more slowly than total [OH-].2b*c The dependence of koa upon ECTAOHI is unusual, because reaction at high [OH-] is inhibited by CTAOH. Micellar incorporation of PhOBz brings it into a region of high concentration of OH-, but the low second-order rate constant in this region more thanoffsets the concentration effect. This kinetic behavior will be seen only when kw >> kzm (Table II), as shown by comparison of the plota in Figure 6 with the predicted plots in Figure 7 for a hypothetical substrate for which KSand kw are the same as for PhOBz, but with k p = kw. Our values of k p l k w for reaction of phenyl benzoate (Table 111) are lower than those quoted by Correia and ~o-workers~~ largelybecause they took VM= 0.37M-l which gives a factor of ca. 2.6 in the two seta of constants. Micellar Effects upon Second-Order Rate Constants. Second-orderrate constanta of reaction of anionic nucleophiles at micellar surfaces have usually been calculated by similar ion-exchangemodels, but with a variety of values of the parameters that describe ion-micelle interactions and of vM.2*3 It is difficult to compare results (i) For from different groups, but some trends are deacylations, values of k p l k w seem to be lower with a hydrophilic ion, e.g., OH-, than with a lesa hydrophilic ion, e.g., thiolate, carboxylate,or oximate.wa1 (ii)Electron donation in the substrate decreases k2mlk~.1"~Jgv22 (iii) Values of k p l k w are low if transition state formation requires localizationof charge on oxygen, asin deacylation, dephosphorylation,or reactions of some amides.213 (iv) If charge in the transition state is not strongly localized, value8of k p and kw will be similar,as in 5 ~displacements 2 and aromatic nucleophilic substitutions.4~~~12~14~1~ Interactions between cationic head groups and phenyl groups of PhOBz might hold it in a conformation that disfavors formation of a tetrahedral-like transition state (20) Reference 2c provideo an exteneive compilation of micellar rate data up to 1982 and data up to 1986 an, in ref L. (21) Cuccovia, I. M.; Schrotar,E. M.; Montiero,P. M.; Chaimovich,H. J. Org. Chem. 1978,43,2248. (22) (a) AI-Lohedan, H.; Bunton, C. A. d. Org. Chem. 1982,47,1160. (b) F u n d i , N.;Murata, A. Chem. P h .Bull. loSO,a8,805. (c) Vera, S.;Rodenae, E. Tetrahedron ISM, 42,143. (d) Broxton, T. J.; Fernando, D. R.; Rowe, J. E.J. Org. Chem. 1981,46,3622.
Deacylation by Hydroxide Zon in Cationic Micelles
(Scheme I). However, lH chemical shifta (300MHz) of the phenyl hydrogens of PhOBz bound to CTABr are within 0.01 ppm of values in D2O with 20% DMSO-& or 10% CD30D (v/v). Differences are slightly larger with CTBABr, but even here the largest is only by 0.05 ppm (for the ortho hydrogensof the phenoxy group). Therefore PhOBz is in a water-rich region at the micellar surface and micellar head groups do not seem to interact strongly with the phenyl groups. Micellar effects upon the spontaneous cyclization of 1 are informative,because althoughthisreaction has an SN2' like transition state, the rate constant in the micellar pseudophase is given directly without considerationof the diatribution of a second reagent.6 Rate constants at the micellar surface of CTAX are similar to those in water but increase with increasing head group bulk.
1
X = Br, I
Localizationof charge on an electronegative center, e.g, oxygen, in the transition state will occur most readily in protic solventsthat readily hydrogen bond to these centers, whereas a delocalizedcharge should interact more readily with cationicmicellar head groups. Electron-withdrawing groups in a substrate disfavor charge localization on oxygen,lQfor example, and it is reasonable to explain changes in kp/kw with changes in substrate structure and reaction mechanism in terms of these simple physical organic concepts. The generalization that kZm = kw is only a first appro~imation,2~~ even for reaction of nucleophilic anions, and especially when the bulky cationic head group is changed. In reactions of PhOBz and methylnaphthalene2-sulfonate (MeONs) with OH-, values of k p are insensitive to changes in head groups, but for reactions of 2,4-
Langmuir, Vol. 9, No. 1, 1993 66
dinitrochloronaphthalene with OH- and MeONs with C1or Br-, they increase modestlywith head group bulk (Table
IIIand ref 14). These comparisonsare based on a common value of VM. In the S~2-like cyclization of 1,Srate conatants are first order and independent of V M and concentration units, but relative rate constants in micellar and aqueous pseudophaaesincrease sharply with increasinghead group bulk.6 There are linear free energyrelatione between these rate effects and those upon the unimolecular decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ionu*%despite marked differences in mechanism. Relative micellar rate effects depend very much upon mechanism as well as on reaction molecularity and the nature of reactants and cannot be explained wholly in terms of variations in VM. Experimental Section Materials. The preparation and purification of the surfactanta have been described.6 Phenyl benzoate prepared by the Schotten-Bauman method and recrystallized (EtOH) had mp 69 OC. Kinetics. Reaction was followed spectrometrically by the increasing absorbance at 298 nm, with 10-4 M PhOBz. Substrate was introduced as a solution in MeCN (13'% (v/v)). All reactions were followed at 25.0 OC. Reaction in water is first order in OHup to 0.1 M and the second-order rate constant to 0.39 M-' s-l. This value increases to 0.47 M-l s-l with 0.5 M OH-.
Acknowledgment. Support of this work by CNR, Progetto Finalizzato Chimica Fine 11, Rome, MURSST, Rome, and the National Science Foundation (Organic Chemical DynamicsProgram) is gratefullyacknowledged. Registry No. CTABr, 57-09-0;CTEABr, 13316-70-6CTPABr, 25268-61-5;CTBABr, 6439-67-4;TDQBr, 36324-70-6; PhOBz, 93-99-2. (23) (a) Bunton, C. A. Pure AppE. Chem. 1977,49,969. (b)Cipiciaui, A.; Linda, P.; Savelli, G.;Bunton, C. A. J. Phye. Chem. 198s, 87,6262. (24) Cerichelli, G.; Mancini, G.; Luchetti, L.; Savelli, G.;Bunton, C. A. J. Phye. Org. Chem. 1991,4, 71. (26) The rata of thie reaction is a very sensitive probe of polarity and micromedium properties.utB (26) Kemp, D. S.; Paul, K. G. J. Am. Chem. SOC.1975,97,7305.
