Langmuir 1994,10, 2491-2492
2491
Decarboxylation of 6-Nitrobenzisoxazole-3-carboxylate. Enthalpy -Entropy Compensation in Micellar and Vesicular Catalysis: A Novel Analysis of Contrasting Rate Variations Marttand S. Patel,?Koos Bijma, and Jan B. F. N. Engberts' Department of Organic and Molecular Inorganic Chemistry, University of Groningen, Nrjenborgh 4, 9747 AG Groningen, The Netherlands Received February 28, 1994. In Final Form: May 2, 1994
The highly solvent dependent unimolecular decarboxylation of the 6-nitrobenzisoxazole-3-carboxylate, 1 (6NBIC), provides a popular probe to explore micellar,' vesicular,2p ~ l y m e r macrocyclic ,~,~~ a n t i b ~ d yand ,~ model-enzyme mediated ~ a t a l y s i s . ~ ~ , ~
Figure 1. Isokinetic relationship at 298 K for micellar surfactants 4 to 14 (0)and CTAB (0). all reactions showed excellent linearity (r > 0.998). The Gibbs energy of activation of the decarboxylation is dominated by enthalpy contributions.s
L
(1)
(2)
(3) u
Multiple linear regression analysis of solvent effects by Grate et a1.' on this reaction, have highlighted the importance of hydrogen-bonding and ion pairing effects as the major contributing factors in the rate variations. However, a plausible explanation for the striking differences in catalytic efficiency encountered in micelles and vesicular bilayers is still lacking, although Kunitake et alezahave shown that the fluidity of vesicular bilayers may play a role.2a Micropolarities of the Stern regions of micelles and bilayers formed from 1-alkyl-4-alkylpyridinium halide surfactants have been show to be similar,lf indicating that factors other than micropolarity must predominate in influencing the rate of decarboxylation in these systems. We now wish to report on the remarkable differences in the isokinetic relationships (enthalpy-entropy compensation effects)for decarboxylation of 1 in micellar and bilayer systems. The kinetic measurements were performed as described previously.ldJf The Eyring plots for t European Research Fellow (1992-1993).
(1)(a)Bunton, C. A.; Minch, M. J.;Hidalgo, J.;Sepulveda, L. J.Am. Chem. SOC.1973,95,3262-3272. (b) Bunton, C. A.; Kamego, A. A.; Minch, J.;Write, J. L. J.Org. Chem. 1976,40,1321-1327.(c) Bunton, C. A.; De Buzzaccarini, F. J.Phys. Chem. 1981,85,3139-3141. (d) Rupert, L. A. M.; Engberts, J. B. F. N. J.Org. Chem. 1982,47,50155017. (e) Sunamoto, J.; Iwamoto, N. S.; Kondo, H. Bull. Chem. SOC. Jpn. 1983,56,2469-2472. (0 Nusselder, J. J. H.; Engberts, J. B. F. N. Langmuir, 1991,7,2089-2096. (2)(a) Kunitake, T.; Okahata, Y.; Ando, R.; Shinkai, S.; Hirakawa, S. J.Am. Chem. SOC.1980,102,7877-7881. (b) Germani, R.; Ponti, P. P.; Savelli, G. N.; Cipiciani, A,; Cerichelli, G.; Bunton, C. A. J.Am. Chem. SOC.,Perkin Trans.2 1989,1767-1771. (3)(a)Suh, J.;Scarpa, I. S.;Klotz, I. M. J.Am. Chem. SOC. 1976,98, 7060-7064. (b) Smid, J.; Varma, A.; Shah, S. C. J. Am. Chem. SOC. 1979,101,5764-5769.(c)Zheng,Y .L.; Knoesel, R.; Galin, J. C.Po2ymer 1987,28,2297-2303. (d) Yang, Y. J.; Engberts, J. B. F. N. J. Org. Chem. 1991,56,4300-4304. (4)(a) Shah, S. C.; Smid, J. J.Am. Chem. SOC.1978,100,1426-1432. (b) Koyama, N.; Ueno, Y.; Sekiyama, Y.; Ikeda, K; Sekine, Y. Polymer 1986,27,293-298.(c) Schmidtchen, F.P. J.Chem.Soc., Perkin Trans. 2 1986,135-141. ( 5 ) Lews, C.; Kramer, T.; Robinson, S.; Hilvert, D. Science 1991,253, 1019-1022. (6)Kunitake, T.; Shinkai, S.; Hirotsy, S. J. Org. Chem. 1977,42, 306-312. (7)Grate, J.W.; McGill, R. A.; Hilvert, D. J.Am. Chem. SOC.1993, 115,8577-8584.
b SUrfUtvlt
SVUCNICS:
(a) w h m RX': CHISO,' (4). CHCHz SO,' (I). CH,(CHi)3SOi (6).
(b) where R'X = CHISO]' and n IS : I (ll),2 (12). 3 (13). where R X ' = CHICHISO j and n i s 2 (141, where RX
= CH,(CHz)IS03' and n IS 3 (15).
(e) 2C,N*2CI
Interestingly, a plot of enthalpies us entropies of activation for the decarboxylation of 1, reported here and taken from the literaturela (CTAB), showed a excellent linear relationship (r = 0.999, Figure 11, giving an isokinetic temperature (Tk)of 336 K for all micellar aggregates, 4 to 149(including wormlike micelles,1° lo), independent on the nature of the headgroup and counterion. Since these relationships may sometimes arise from artifacts, the validity of the isokinetic temperature was checked via the Peterson method," where plots of -log klT us 1lT gave intersections close to the isokinetic temperature obtained from Figure 1. The enthalpy us entropy of activation for decarboxylation of 1 in the presence of surfactant 16 (vesicles9 deviatessubstantially from the isokinetic relationship, indicating a change in interaction mechanism13between the aggregate and initial state and transition state, respectively. (8)Kemp, D. S.; Kenneth, G. P. J.Am. Chem. SOC.1975,97,73057311. (9)Analogous procedures to those in ref If were used in the preparation of surfactants. Counterion exchange was performed using sodium salts of the appropriate counterion on a column of Dowex 1x 8 (200-400mesh). Thepurityofthenovelcompounds4-15 waschecked via NMR and elemental analyses. (10)The visually observed viscoelasticity has also been quantified, through measurements of first normal stress differences, using a Bradender Rheotron rheometer at 298K, with cone-and-plategeometry, equipped with a normal F-sensor. Significant line broadening was also observed in 'H-NMR. (11)Peterson, R. C.J. Org. Chem. 1964,29,3133-3135. (12)The existence ofvesicular bilayers was shown by freeze fracture replicas examined with a Philips EM300 electron microscope, operating at 80 kV. The vesicles were prepared using the ethanol-injection method and had average diameters of 40-200 nm. We assume that the bilayer resides in the liquid-crystalline phase at room temperature. (13)Connors, K. A. Chemical Kinetics; VCN Publishers, Inc.: New York, 1990;pp 368-371.
0 1994 American Chemical Society
Notes
2492 Langmuir, Vol. 10, No. 7, 1994
24
I
1 ---.-."I
,/,LA-M
2o 16
-
,/ ,/
. ,
1
i/r. '
".,I
12 -25
-1 5
-5
5
15
25
Figure 2. Isokinetic relationships for bilayer system, 15 (O), and for type 2CnN+2C1surfactants,below T,(+) and above T, (0).
Calculation of activation parameters 6" rq"i activationenergiesfor the decarbo of the type 2C,,N+2C1 (where II et al.,*revealed two Werent AtHe and A*Se above and below temperature (T,) ofthe bilayers, respe~tively.~~ Our data for 15 nearly falls on the intersection of the linear relationship below T,,with an isokinetic temperature of 274 K(r = 0.997, Figure2), and above T,,with anisokinetic temperature of 324 K (r = 0.999, Figure 2) for the decarboxylation. Most likely the bilayers of 15 are in their fluid state, and therefore the data for this compound are expected to fall on the linear relationship above T,. Thus, the isokinetictemperature increases in the order bilayers (274 K, rigid, below T,)< bilayers (324 K,flexible, above T,)Q micelles (336 K, highly fluid surface).16 Isokinetic temperatures in the range 300-400 K indicate medium effects having a larger influence on enthalpy and entropy of activation than on the Gibbs energy of activation.ls The excellent isokinetic relationships imply that a single important interaction mechanism governs the catalytic effects for each of the three s y s t e m ~ , ~ presumably ~J~J~ desolvation of the carboxylate moiety of the initial state. (14) mota of A * P us ASSg gave a linear relationship: compounds 4 to 14, y = 22.16 4- 0.83C (r = 0.999); bilayers, belaw T,,y = 21.70 + 0.274~(r = 0.997);above T o I y= 21.13 0.8% (r = 0.999). Activation parameters taken from Kemp'e work* for pure eolvente do not fall on any of the relationships. (16) It isinterestington o t e t a n i s o W e r e l a t i o n 6 h i p a l e o ~ for mersed micelleaof CTAB and CTAC in OHCb'e with 8 ~ hkinetic 1 temperature of 517 K Plotting these data gave an intenectios of 18 kcal mol-' in the A*H9A*Se relationship, suggesting that different interactions are operating in these sy@ems. (16)(a) Leffler, J. E. Rates and Equilibria of Organic Reactions; Wiley;*NewYork, 1965, pp 315-403. (b)For p. recsnCdis+on of the background and eignificanceof ieokip&c ~ d a t i ~ ~see h ib pe r~t ,~W.; Jameson, R. T. Chem. SOC.Rev. 1989,18,477. (17)Alternatively, two or more ms ma be involvdifthey happen to have the same isobinetictemperature.' This seemshighly implausible.
+
1
Furthermore, we contend that the structure of the binding sitesfor 1in the adgregatasremainsessentiallyunchanged within the expeplmental temqerafittrerange (30-55 "C). Small (temperabe-induced) changes in the aggregation number of tbe mieaUes or aiae of the vesicles are known to be of minor impaftancs for the catalytic efficiency.lf The present results suggest that the thermal fluctuations at the bimhg sites for the substrate (i.e. at the surface of the aggregate) markedly affect the solvation processes which determine the catalysis of the decarboxylation. Surfactant molecules "twist,turn,and bob in and out of the surfaces".18 The catalykic efficiency for decarboxylation of 1 increases with decreasing fluidlike character of the aggregate surface (micelles Ivesicles (above T,)< vesicles (below T,)).However, slight discrepancies between rate of decarboxylation in vesicles above and below T,do not reflect this completely.% The small difference between the experimental temperature and isokinetictemperature mayexplain these ambiguities, with larger differences leading to greater rate variations within an isokinetic series.16 Therefore in micellar systems (Tk= 336 K)larger variations in rate occur at 298 K192aJ9than in (rigid) bilayer vesicles (Tk= 324 K) where rate variations are only modest.2 Enthalpyentropy compensationeffects have also been suggested to play an essential role in enzyme-mediated reactionsz0and are a manifestation of hydration effects at the surface of the protein.21 In summary, we have found that different isokinetic relationships hold for 'the unimolecular decarboxylation of 6-NBIC catdyzed by micelles and by vesicles. In the case of vesiclesWerent isokinetictemperatures are found for the bilayers above and below the phase transition temperature. We submit that these differences reflect the different surface dynaplics for the respective aggregates, a factor not previously considered in micellar and vesicular catalysis.
Acknowledgment. The authors thank the European
Community for their financial upp port (Project No. 3774712). We are also grateful to Dr. Arjen Sein for his assistance in electron microscopic experiments.
Supplementary Matemid Available: Iaobaric activation pwametera for the decarboxylation of 1 in the presence of various surfactante (4 to 15) and dimethyldiallcylammodum bromides, above and below T,, respectively (2 pages). Ordering information is given on any current masthead page. (18) Israelachvili,J. Intsnnolceukrr and Surfaee Forces, 2nd ed.; Academic Praee: New York, 1992; p 375. (19) G e W , R.; Ponti, P. P.; Romeo, T.; Savelli, G.; Spreti, N.; Cerichelli,G.;Luchea L.; Mancini, G.; Bunton, C. A. J. Phys. Org. chm.isss, a, 6 s 3 - s ~ . (20)Lilrhtenshtein, G. L. B i o f i k 1866,11,23. (21)(a) Lumry, R.; Rejender, S. Bwpolymers 1@70,9, 1125-1227. (b) Lumry, R.; Eyring, J. J. Phys. Chem. 1@64,68, 110.
