3142
J. Phys. Chem. 1981, 85, 3142-3145
Reaction of n-Hexylamine with 2,4-Dinitrochlorobenzene in Microemulsions Clifford A. Bunton* and Francesco de Buzzaccarini hpatfment of Chemistry, UnlversW of California, Santa Barbara, California 93106 (Received: May 29, 1981)
Reaction of n-hexylamine with 2,4-dinitrochlorobenzene(DNC) is faster in microemulsions of n-octane, nhexylamine, and cetyltrimethylammoniumbromide (CTABr) than in water. The first-order rate constants, k,, for reaction in the absence of surfactant increase sharply with increasing concentration of n-butyl- or n-hexylamine in water but are similar in water and in water-tert-butyl alcohol. The rate enhancement in the microemulsion appears to be due largely to an increased concentration of reactants in the droplets.
Oil-in-water (o/w) microemulsions can form when oil in water is mixed with a surfactant and a cosurfactant.’ The cosurfactant is usually an alcohol, and the submicromicroscopic microemulsion droplets are believed to contain a hydrocarbon core surrounded by surfactant and cosurfactant-l A t high pH the alcohol in the droplet may be partially deprotonated and the alkoxide ion can react as a nucleophile, for example, in dephosphorylati~n.~*~ There are similarities between reactions in microemulsions and in micelles, because in both cases the aggregate behaves as a pseudophase which can influence the rate of a bimolecular reaction by exerting a medium effect, or by bringing the reactants together in a small volume, and so speeding reaction, or by keeping them apart, and so inhibiting its6 For example, deacylations and nucleophilic aromatic nucleophilic substitutions by nonionic nucleophiles are catalyzed by both cationic and anionic micelles,’-’0 but this rate enhancement is due to the concentration of reactants in the micellar pseudophase, and the second-order rate constant in this pseudophase is smaller than that in water. This unfavorable medium effect of the micelles is understandable, because the polarity of micelles, as measured by the apparent dielectric constant” or by a solvent parameter such as 2,12is lower than that of water. Shifts in fluorescence spectra are very similar in micelles and in micro emulsion^,^^ suggesting that microemulsion droplets also have a lower polarity than water. The aim of our work was to use an aliphatic amine as cosurfactant in an o/w microemulsion and to examine its reactivity in aromatic nucleophilic substitution,l* so that (1) (a) Prince, L. M., Ed. “Microemulsions: Theory and Practice”; Academic Press: New York, 1977. (b) Wmsor, P. A. Trans. Faraday SOC. 1948,44, 376. (2) Hermansky, C.; Mackay, R. A. In “Solution Chemistry of Surfactants”:Mittal. K. L.. Ed.: Plenum Press: New York, 1979; Vol. 2 p 723. (3) Aromatic nucleophilic substitution in these systems also occurs predominantly by attack of alkoxide ion.’ (4) de Buzzaccarini, F., unpublished results. (5) For discussions of micellar catalysis, see ref 6-9. (6) Fendler, J. H.; Fendler, E. J. “Catalysis in Micellar and Macromolecular Systems”; Academic Press: New York, 1975. (7) Martinek, K.;Yatsimirski, A. D.; Levashov, A. V.; Berezin, I. V. In “Micellization, Solubilization and Microemulsions”; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 2, p 489. (8) Bunton, C. A. Catal. Reo. Sci. Eng. 1979,20, 1. (9) Cordes, E.H.Pure Appl. Chem. 1978,60,617. (10)Bunton, C. A.; Cerichelli, G.; Ihara, Y.; Sepulveda, L. J. Am. Chem. SOC. 1979,101,2429. (11) Mukerjee, P. In ref 2, Vol. 1, p 153 and references cited. (12) Cordes, E. H.; Gitler, C. B o g . Bioorg. Chem. 1973, 2,1. (13).Bunton, C. A.; de Buzzaccarini, F. J. Phys. Chem., preceding paper in this issue. (14) For reviews of aromatic nucleophilic substitution, see ref 15. 0022-3654/81/2085-3142$01.25/0
TABLE I: Reactions in Amine-Water Mixtures”
1.22 2.41 6.51 19.4 49.9 80.0 95.0 100 45.0 50.1
0.303 0.606 1.68 5.60 19.7 50.5 82.3 100 14.7 15.2
100
100
0.166 0.332 0.890
0.142 0.275 1.18 12.7 31.9 183 351 362 30.ObvC 23.gb 280b
0.85 0.83 1.32
At 25.0 “ C with n-BuNH, ur-. !ss otherwise s p e c L d . With n-C,H,,NH,. 10 wt % Et,NBr.
we could attempt to apply quantitative models for micellar catalysis of bimolecular reactions to similar reactions in microemulsions.* We used a microemulsion of n-octane, cetyltrimethylammonium bromide (CTABr), and nhexylamine and examined the reaction of 2,4-dinitrochlorobenzene (DNC).
C‘
NO2
We compared reactions in the microemulsionswith those in homogeneous solution. One comparison was made with aqueous n-butyl- or n-hexylamine. The other was made by using dilute solutions of amine in aqueous tert-butyl alcohol because alcohols are effective cosurfactants in microemulsion formation.ls2 Addition of n-hexylamine to n-octane-CTABr gave stable, homogeneous, transparent mixtures. ‘ b ~ 6 The use of a nonionic nucleophile removes the problem of allowing for the effect of the microemulsion on deprotonation of a weak acid generating a good anionic nucleophile. Micellar effects upon deprotonation have been treated quantitatively by using acids whose deprotonation can be followed ~pectrophotometricallylly,1~ but it is difficult to treat deprotonation of an acid such as n-butyl alcohol2 (15) Miller, J. “Aromatic Nucleophilic Substitution”;Elsevier: New York, 1968. (16) We could not form microemulsions using n-butylamine as cosurfactant, although they form readily with n-butyl alcohol.2 (17) Funasaki, N. J.Phys. Chem. 1979,83,237,1998; Bunton, C. A,; Romsted, L. S.; Sepulveda, L. Ibid. 1980,84, 2611.
0 1981 American Chemical Society
The Journal of Physical Chemistry. Vol. 85,No. 21, 1981 3143
Reaction of n-Hexylamine with DNC TABLE 11: Effect o f tert-Butyl Alcohol o n Reaction o f Amines with DNCa
RNH, W t % 10’XR” 1.54 3.04 3.22 5.89 2.80
1.56 3.08 2.38 5.97 1.10
M
1 0S 3- 1k $ ,
10312 I I [M-1 R N s-I gI,
0.166 0.327 0.252 0.633 0.332
0.304 0.629 0.490b 1.31 0.472c
1.83 1.92 1.94 2.07 1.43
a At 25.0 “ C with n-BuNH, in t-BuOH unless otherwise specified. With n-C,H,,NH,. In 60:40( w / w ) t-BuOH/H,O.
in either micelles or microemulsions, and this complicates the quantitatively treatment of reactions at high pH in microemulsions based on n-butyl alcohol, for example. Experimental Section Materials. The purification of CTABr has been described; lothe other solvents or reagents were distilled or recrystallized. Kinetics. Reactions at 25 “C were followed spectrophotometrically at 345 nm. The substrate was added in MeCN to 3 mL of the reaction solution so that its final M, and the solution conconcentration was ca. 3 X tained 0.05% MeCN. The first-order rate constants, k,, are in units of s-l. The microemulsions contained n-hexylamine as cosurfactant, but we used n-butylamine for some of the experiments in the absence of oil and surfactant because of the low solubility of n-hexylamine in water. Results and Discussion Reactions in the Absence of Surfactant. The reactions are first order with respect to amine provided that its concentration is relatively low (Tables I and 11). Water and n-C6H13NH2are immiscible at high water content, but we obtained homogeneous solutions when the amine content was relatively high. The reactivities of n-butyl- and n-hexylamine are similar in tert-butyl alcohol (Table II), and for reaction with dilute n-butylamine the first-order rate constants are approximately doubled as the solvent is changed from water to tert-butyl alcohol. Qualitative kinetic solvent theory suggests that nucleophilic aromatic substitution by an amine should be speeded by an increase in the polarity of the solvents,ls and this behavior is found with aprotic solvents, but rates change little when the solvent is changed from HzO to MeOH.19apb Although changing the solvent from HzOto t-BuOH has little effect on rate, the first-order rate constants increase very sharply as the amine content of water-amine mixture is increased. There is no simple relation between rate constant and either molarity or mole fraction ( ~ ~ ” 1 ) of amine, except with dilute amine. There is a small positive salt effect by Et4NBr (Table I). Microemulsion Reactions. The first-order rate constants for reactions in microemulsions are not very sensitive either to the amount of n-hexylamine in solution or to the mole ratios of amine to surfactant (Table 111). Added NaBr slightly speeds reaction. Micellar catalysis of bimolecular reactions is generally treated in terms of the concentrations of reactants in the micellar pseudophase,’-10 and we follow this approach in discussing reactions in microemulsions. However, we note that Mackay and co-workers introduced (18) Ingold, C. K.“Structure and Mechanism in Organic Chemistry”, 2nd ed.; Cornell University Press: Ithaca, NY, 1969; Chapter 7. (19) (a) Reference 15, Chapter 8. (b) Ritchie, C. D.; Sawada, M. J.Am. Chem. SOC.1977,99, 3754.
TABLE 111: Reaction o f n-Hexylamine with DNCa
H,O, run
%
1 2 3 4 5 6 7 8
60.0 74.9 87.5 84.1 92.0 89.3 83.7 83.7
CTABr, n-C,H,,- XRNH,/ lo3!$, % NH,,% X C T A B ~ S-
13.3 8.32 4.17 3.96 2.00 2.33 4.92 4.43
13.3 8.36 4.19 7.94 4.00 6.10 4.92 4.42
3.58 3.62 3.64 7.18 7.21 9.37 3.61 3.60
8.61 7.60 5.96 7.90 6.63 8.48 7.95b 9.46c
At 25.0 “ C with equal weights of CTABr and octane. 1.54 w t % NaBr. 2.98 w t % NaBr. TABLE IV: Estimated Second-Order Rate Constants for Reaction o f n-Hexvlamine
1 2 3 4 6 6 7b 8b
60.0 14.9 87.5 84.1 92.0 89.3 83.7 83.7
0.20 0.12 0.063 0.10 0.050 0.073 0.074 0.066
From Table 111.
1.3 1.1 0.9 1.0 0.9 1.0 1.2 1.4
0.27 0.17 0.084 0.12 0.060 0.084 0.098 0.088
1.8 1.6 1.2 1.2 1.0 1.2 1.6 1.9
With added NaBr.
the concept of phase volume, which is the volume of the organic solutes (surfactant, cosurfactant, and oil) relative to the total solution volume, and corrected their observed rate constants by assuming that the substrate would be in the organic solutes and the ionic nucleophile in the water. A correction of this type would be inappropriate when the nucleophile is nonionic and relatively hydrophobic. We assume that both reactants are fully incorporated with the microemulsion droplets.21 On one assumption the alkyl groups of CTABr penetrate the hydrocarbon core and the amine is disposed between the alkyl groups of the surfactant with the amino groups close to the surface of the droplet. We assume that reaction occurs in a region which contains all of the amine, but no hydrocarbon, and that half the length of the alkyl groups of the surfactant will be in this region. The volume of this element, V’, in 1L of solution can then be calculated from the composition of the microemulsion (we assume unit density for all of the components, but our calculation is insensitive to small changes in density). The corrected second-order rate constants, ki,are given by k i k, V’/ [RNH21 (1) +,2i20
where [RNH2] is amine molarity based on total solution volume. A second alternative assumption is that alkyl groups of the surfactant do not penetrate the hydrocarbon core and that reaction occurs in a volume element made up of surfactant and amine. The corrected second-order rate constants based on this assumption are given by = k,V”/[RNH,] (2)
kc
where V” is the volume of n-hexylamine + surfactant in 1 L of solution. (20) Mackay, R. A,; Letts, K.; Jones, C. In “Micellization, Solubilizntion and Microemulsiona”;Mittal, K. L., Ed.;Plenum Presa: New York, 1977; Vol. 2, p 801. (21)This assumption is a reasonable one in view of the low solubility of the reactants in water, and the ready micellar incorporation of DNC and hydrophobic solutes even in dilute surfacant?-”
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The Journal of Physical Chemistry, Vol. 85,No. 21, 1981
Bunton and de Buzzaccarini
The two sets of corrected second-order rate constants centration units, which for solution reactions are conven(Table IV) do not differ greatly. The values of k i are less tionally given in molarity. However, this choice is an ardependent than those of k{ upon the composition of the bitrary one, and molarity is an especially unsatisfactory microemulsions. They both rely on the assumptions in the measure of concentration for concentrated solutions.24 calculation, for example, upon the extents to which reacThe structure of o/w microemulsion droplets is generally tants are incorporated in the droplet, or surfactant is imassumed to be an oillike core surrounded by a layer of mersed in the hydrocarbon core, but changes in these surfactant and cosurfactant,le but this layer may also quantities will affect the rate constants to similar extents. contain water molecules, i.e., be similar to an aqueous The second-order rate constant for reaction of n-butylamine.I3 Extensive evidence for water penetration of amine with DNC in water is ca. M-l based on the micelles has been provided by Menger and his co-workexperiments with the lowest amine concentrations (Table e r ~ For . ~example, ~ ~ the ~ ~bimolecular, water-catalyzed I) and is similar to k,’ or k f l for reaction of n-hexylamine hydrolysis of bis(6nitrophenyl) carbonate in a micellar with DNC in the microemulsions (Table IV). pseudophase is only slightly slower than in water. This This agreement suggests that the second-order rate substrate binds strongly to the micelle, presumably in its constants for reaction of an n-alkylamine with DNC in the interior, so that water is apparently present in this region. outer layer of a microemulsion droplet are not very difWe see the same behavior for this reaction in a microferent from those in water. emulsion of octane, CTABr, and tert-amyl al~ohol,l~1~* so Despite this apparent agreement between second-order that water also appears to penetrate the microemulsion rate constants in microemulsion droplets and those in droplet. If this is correct, the outer layer of a microwater, there are problems with the treatment. Reactions emulsion droplet contains water, as well as amine and in water are first order with respect to amine only in dilute surfactant, so that reaction with DNC would not be occurring in a nonaqueous region, and comparisons of rate solution, and when amine is more than a minor component of the solvent, the first-order rate constants increase much constants in microemulsions and in a homogeneous solvent more rapidly than amine concentration, based on either should be based on a water-amine mixture, but the rate molarity or mole fraction (Table I). But a microemulsion constants in the microemulsions are lower than expected droplet should correspond to a concentrated solution of for reaction in a medium rich in amine (Tables I and IV). amine; for example, in our microemulsionsthe mole ratios One possibility is that the substrate is bound preferentially of amine to CTABr are >3.5, and therefore one might in the hydrocarbon core of the droplet, but this explanation is inconsistent with all of the evidence which suggests that expect the second-order rate constant in that medium to polar organic solutes do not go deeply into the apolar be larger than that in a dilute aqueous solution. The fact that they are similar suggests either that some medium interior of micelles but reside near the By effect is inhibiting reaction in the microemulsion droplet analogy we expect the pattern to be similar for o/w microemulsions.2J3 or that we must reconsider the basis of the comparison of Our tentative explanation of the apparent low reactivity rate constants in the two media. In any event, we must consider the nature of the microemulsion as a reaction of n-hexylamine in a microemulsion droplet is that the medium. Two factors could affect the properties of the arrangement of amine molecules in the droplet is such that microemulsion droplets as reaction media: (i) an electrothey do not participate as readily in reaction as does amine lyte effect and (ii) the composition of the outer layer of in a homogeneous solution. The transition state for rethe droplet. With regard to the first factor, the microaction of an amine with DNC should have a structure emulsion droplets are cations with associated counteransimilar to that of the tetrahedral intermediate 1 and be ions; whereas electrolytes were absent in most of the exCI NHR periments in amine-water mixtures (Tables I and 11), I added salt slightly speeds reaction, suggesting that the ionic surfactant in the microemulsion should not hinder reaction. With regard to the second factor, micelles are leas polar than water, as judged by their apparent dielectric I I I NO2 NO2 NO2 constants” or their 2 values as measured by fluorescent 1 spectral and second-order rate constants of amines with esters or halobenzenes are smaller in the micellar stabilized by a base which can accept a hydrogen bond pseudophase than in Micelles and alcohol-based from the forming ammonium ion center. With dilute microemulsions have very similar 2 suggesting amine the base will be a solvent molecule, water, or an that amine-based microemulsions will also be less polar alcohol, but, when amine is a major comparent of the than watera and that the medium effects on bimolecular solvent, it should be able to hydrogen bond readily with reactions of nonionic reactants should be similar to those 1, or the related transition state, in homogeneous solution. of micelles. However, this conclusion may be unwarranted However, the orientation of amine molecules in a microbecause, although generalizations about kinetic solvent emulsion droplet may make it difficult for them to solvate effects seem to be satisfactory for aprotic s o l ~ e n t s , ~ ~ J ~the * transition state, so that reaction is not as rapid as hydrogen bonding interactions are probably dominant in hydroxylic solvents and reaction rates will not relate in any (24) This uncertainty regarding the meaning of “concentration” in a simple way to parameters such as dielectric constant or microemulsion droplet is also present in similar micellar systems,’~loJTP 2 (cf. Table 11 and ref 19). although here the surfactant is usually in excess over the reagent so that An additional point is that comparison of second-order the problem is less acute. (25) Cuccovia, I. M.; Schroter, E. M.; Monteiro, P. M.; Chaimovich, rate constants in different media depends upon the con(22) Reactions of nonionic substrates and a nonionic nucleophile are at typically speeded by an increase in the polarity of the least for aprotic solvents. (23) The values of 2 in microemulsions me not materially affected by changes in the concentrations of the components.’*
H. J. Org. Chem. 1978,43,2248. (26) Menger, F. M. Acc. Chem. Res. 1979, 12, 111. (27) Menger, F. M.; Yoshinaga, J.; Venkatasubban, K. S.; Das,A. R. J. Og. Chem. 1981, 46, 415. Menger, F. M.; Bonicamp, J. M. J. Am.
Chem. SOC.1981,103, 2140. (28) We used a tertiary alcohol in these experiments to eliminate nucleophilic attack by the alcohol,’s and this experiment would not be feasible with a nucleophilic cosurfactant.
J. Phys. Chem. 1081, 85,3145-3150
expected from the concentration of amine in the droplet. We do not know how an amine arranges itself in a microemulsion droplet, but alcohol cosurfactants move readily from water to microemulsion dropletsBor micellesm (29) Bellocq, A. M.; Biais, J.; Clin, B.; Lalanne, P.; Lemanceau, B. J . Colloid Interface Sci. 1979, 70, 524. (30) Gettins, J.; Hall, D.; Jobling, P. L.; Rassing, J. E.; Wyn-Jones, E. J. Chem. SOC.,Faraday Trans. 1 1978, 74, 1957; Yiv, S.; Zana, R.; U1bricht, W.; Hoffmann, H. J. Colloid Interface Sci. 1981, 80, 224.
3145
and do not enter a rigid structure in the microemulsion. Therefore, by analogy we would expect amines to have considerablemobility in a microemulsion droplet, although they might not have as much conformational freedom as in an aqueous solvent.
Acknowledgment. Support of this work by the National Science Foundation Program) is gratefully acknowledged.
Ultrasonic Studies of the Complexation Kinetics of Cadmium Nitrate in Nonaqueous Solvents Shlnklchl Yamada’ and Ronald E. Verrall’ Depaflment of Chemistry and Chemical Engineering, University of Saskatchewan, Saskatoon, Canada S7N OW0 (Received: January 20, 198 1; In Fins1 Form: June 22, 198 1)
Ultrasonic absorption data obtained over the frequency range 5-95 MHz by the pulse method are reported for cadmium nitrate in methanol, dimethylformamide,and dimethyl sulfoxide in the temperature range 15-45 “C and the concentration range 0.02-0.25 mol dm-3. Ultrasonic relaxation spectra show a single relaxation. The kinetic data have been interpreted in terms of the Eigen mechanism. The rate constants for solvent exchange in the first coordination sphere of Cd2+were estimated at 25 “Cto be 1.1 X lo8 s-l in methanol, 6.5 X lo7 s-l in dimethylformamide,and 5.3 X lo’ s-l in dimethyl sulfoxide. Large negative values of the activation entropy suggest that an associative interchange mechanism I, is operative in this process.
Introduction Simple complexation reactions involving divalent cations of the first-row transition metals with unidentate ligands in aqueous solution have been generally represented by an Id mechanism: that is, an interchange process characterized by a dissociative activation mode. According to Eigen, Wilkins, and others,+? the rate and activation parameters for this process are only slightly dependent on the nature of the ligand and are very similar to those of solvent exchange reactions. Recently, kinetic studies of complexation reactions have been extended to nonaqueous systems to examine the applicability of this mechanism in other solvents besides water. At this time, available data show that the reactions involving many unidentate ligands show behavior similar to that of the corresponding aqueous systems. However, the same is not true of multidentate ligands, and discrepancies in rate constants and activation parameters between ligand substitution and solvent exchange have been discusseds in terms of steric requirements of the ligand in the ring-closure step, extra stabi-
lization of the outersphere complex, and solvent structure. The extensive complexation studies in nonaqueous solventa have been done mainly on the first-row transition-metal ions such as Mn2+,Fe2+,Co2+,and Ni2+,9and only limited data are available for the other labile metal ions. Despite the unique property of d’O configuration, there has been little work done on the complexation reaction of cadmium ion as compared to the partially filled d-orbital metal ions. The present study was carried out to obtain mechanistic information on the complexation reaction of cadmium ion in nonaqueous solvents. In this paper we report the kinetic results for the reaction between cadmium and nitrate ions in methanol, dimethylformamide, and dimethyl sulfoxide using ultrasonic techniques and discuss the details of the suggested mechanism.
Experimental Section Reagents. Cadmium Nitrate. Reagent-grade cadmium nitrate tetrahydrate was dried over P205under vacuum. The water content in the crystal was determined by Karl Fischer titration to be less than 0.015 mol/mol of Cd(N0312.
(1) On leave from the Department of Chemistry, Faculty of Science, Nagoya University, Chikusa, Nagoya 464, Japan. (2) C. H. Langford and H. B. Gray, “Ligand Substitution Processes”, W. A. Benjamin, New York, 1965. (3) M. Eigen, 2. Elektrochem., 64, 115 (1960); Pure Appl. Chem., 6 , 97 (1963). (4) R. G. Wilkins and M. Eigen, Adu. Chem. Ser., No.49, 55 (1965). (6) R. G. Wilkins, Acc. Chem. Res., 3, 408 (1970). (6) F. Basolo and R. G. Pearson, “Mechanisms of Inorganic Reactions”, Wiley, New York, 1967. (7) C. H. Langford, “Ionic Interactions”, Vol. 2, S. Petrucci, Ed., Academic Press, New York, 1971, Chapter 6. (8) J. F. Coetzee, “SoluteSolvent Interactions”, Vol. 2, J. F. Coetzee and C. D. Ritchie, Eds., Marcel Dekker, New York, 1976, Chapter 14. 0022-3654/81/2085-3145$01.25/0
Solvents. Reagent-grade methanol (MeOH), dimethylformamide (DMF), and dimethyl sulfoxide (MefiO) were dried over calcium hydride and twice distilled at atmospheric (MeOH) and under reduced (DMF and Me2SO) pressure. The water contents in these solvents were determined by Karl Fischer titration to be less than 5X (MeOH), 3 X lo9 (DMF), and 4 X lo4 (Me2SO) mol dm-3. None of these solvents showed any ultrasonic (9) Rate constants and activation parameters for the complexation reaction in nonaqueous solvents are compiled by J. F. Coetzee (ref 8).
0 1981 American Chemical Society
