Langmuir 1997, 13, 3959-3964
3959
Effects of Cationic Micelles on Rate of Intramolecular General Base-Catalyzed Aminolysis of Ionized Phenyl Salicylate M. Niyaz Khan,* Zainudin Arifin, M. Noh Lasidek, M. Amin M. Hanifiah, and George Alex Department of Chemistry, Universiti Malaya, 50603 Kuala Lumpur, Malaysia Received September 23, 1996. In Final Form: April 28, 1997X Effects of cetyltrimethylammonium bromide (CTABr) micelles on second-order rate constants (knobs) for the nucleophilic reactions of amines (n-butylamine, piperidine, and pyrrolidine) with ionized phenyl salicylate (PS-) reveal a decrease of nearly 4-fold for n-butylamine, 13-fold for piperidine, and 17-fold for pyrrolidine with an increase in [CTABr]T from 0.0 to 0.01 M at 35 °C. These results show that the binding constants (KN) of amines with CTABr micelles vary in the order KN for n-butylamine > KN for piperidine > KN for pyrrolidine. The observed data are explained in terms of the pseudophase model of the micelle. The values of KN for n-butylamine, piperidine, and pyrrolidine turn out to be so small that they could not be determined kinetically. The significantly lower rate of aminolysis in the micellar pseudophase compared to that in the aqueous pseudophase is attributed to both micellar medium effect and different locations of amine and PS- in the micellar pseudophase.
Introduction Effects of micelles on rates of uncatalyzed, specific acid-, and specific base-catalyzed hydrolysis of esters have been studied extensively.1 But the systematic studies on the effects of micelles on rates of aminolysis of esters are almost nonexistent. The main reason for the limited attempts on such studies is the requirement of the use of amine buffers in these studies. The presence of micelles (especially ionic) changes the pKa of acids, and thus, it complicates the kinetic treatment of rate data obtained in the presence of buffers. If the rate of hydrolysis of a substrate is insensitive to hydroxide ion concentration within its range ca. 0.0050.050 M, then the rate of aminolysis of such a substrate can be easily studied without the use of amine buffers. Such reaction characteristics led a few workers to study the effects of ionic micelles on the rates of the nucleophilic substitution reaction of aniline with 2,4-dinitrofluorobenzene2 and the rates of nitrosation of secondary aliphatic amines by alkyl nitrites.3,4 The effects of sodium dodecyl sulfate (SDS) micelles on the rates of aminolysis of ionized phenyl salicylate (PS-) have been studied without using amine buffers because the rate of hydrolysis of PS- turned out to be independent of [-OH] within the [-OH] range 0.005-0.06 M.5,6 Since the binding constant of PS- with SDS micelles7 is many-fold smaller than that with cetyltrimethylammonium bromide (CTABr) micelles,8 we became interested in exploring the effects of CTABr micelles on aminolysis of PS-. The results and their probable explanations for the effects of CTABr micelles X
Abstract published in Advance ACS Abstracts, June 15, 1997.
(1) (a) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular System; Academic Press: New York, 1975. (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 Press: New York, 1982; Vol. 2, p 1015. (2) Bunton, C. A.; Cerichelli, G.; Ihara, Y.; Sepulveda, L. J. Am. Chem. Soc. 1979, 101, 2429. (3) Iglesias, E.; Leis, J. R.; Pefia, M. E. Langmuir 1994, 10, 662. (4) Fernandez, A.; Iglesias, E.; Garcia-Rio, L.; Leis, J. R. Langmuir 1995, 11, 1917. (5) Khan, M. N. J. Chem. Soc., Perkin Trans. 2 1990, 445. (6) Khan, M. N.; Dahiru, M.; Na’aliya, J. J. Chem. Soc., Perkin Trans. 2 1989, 623. (7) Khan, M. N. J. Mol. Catal. A 1995, 102, 93. (8) Khan, M. N.; Arifin, Z. J. Colloid Interface Sci. 1996, 180, 9.
S0743-7463(96)00924-9 CCC: $14.00
on the rates of reactions of PS- with n-butylamine, piperidine, and pyrrolidine are described in this paper. Experimental Section Materials. All the reagents used were supplied by Fluka or Aldrich and were of the highest commercially available purity. The stock solutions of phenyl salicylate were prepared in acetonitrile. Kinetic Measurements. The rates of aminolysis of phenyl salicylate were studied spectrophotometrically by monitoring the disappearance of phenyl salicylate as a function of time at 350 nm and 35 °C. The observed data (absorbance versus time) were found to obey a pseudo-first-order rate law until more than 8 half-lives of the reactions. The details of the kinetic procedure and data analysis are described elsewhere.5
Results and Discussion In order to discover the effects of [CTABr]T ()total concentration of CTABr) on the rates of aminolysis of ionized phenyl salicylate (PS-), nearly 5-7 kinetic runs were carried out within the total amine concentration ([Am]T) range 0.003-0.06 or 0.10 M at a constant [CTABr]T and 35 °C. Pseudo-first-order rate constants (kobs) were found to fit to eq 1
kobs ) k0 + knobs[Am]T
(1)
where k0 represents the first-order rate constant for hydrolysis of PS- and knobs is the nucleophilic second-order rate constant for the reaction of amine with PS-. The rate constants k0 and knobs were calculated from eq 1 using a linear least squares technique. These calculated values at different [CTABr]T for n-butylamine and pyrrolidine are summarized in Table 1, and those for piperidine are in the Supporting Information. The fitting of the observed data to eq 1 is evident from some representative plots of Figure 1 and from the standard deviations associated with the values of k0 and knobs (Table 1 and Supporting Information). The perfect linear variation of kobs against [Am]T at different [CTABr]T (ranging from 0 to 0.01 M) indicates that any change in the micellar shape or characteristics with increasing values of [CTABr]T in the presence of 2 × 10-4 M PS- and different amounts of n-butylamine or piperidine or pyrrolidine is kinetically insignificant. © 1997 American Chemical Society
3960 Langmuir, Vol. 13, No. 15, 1997
Khan et al.
Table 1. Rate Constants, k0 and knobs, Calculated from Eq 1a amine n-butylaminee
g g g pyrrolidineg
104[CTABr] 0.0 0.0 0.2 0.4 0.6 0.8 1.0 2.0 3.0 4.0 5.0 7.0 10.0 20.0 40.0 60.0 80.0 100.0 0.0 0.2
T,
M
104k0, s-1 8.45 ( 8.66 ( 0.63 8.66 ( 0.94 9.12 ( 1.03 8.31 ( 1.06 7.61 ( 0.43 6.85 ( 0.23 5.47 ( 0.31 3.94 ( 0.53 3.08 ( 0.28 2.63 ( 0.48 2.10 ( 0.21 1.87 ( 0.18 1.64 ( 0.26 1.43 ( 0.74 1.53 ( 0.08 1.17 ( 0.10 1.36 ( 0.14 13.7 ( 5.0 (8.14)h
2.00 ( 6.54 (8.45)
0.6
6.34 ( 1.91 (8.45)
1.0
6.14 ( 2.75 (6.85)
3.0 4.0 5.0 7.0 10.0 20.0 40.0 60.0 80.0 100.0
103knobs, M-1 s-1
103kncalcd,c M-1 s-1
1.1f
7.54 6.85 4.86 3.89 3.31 2.93 2.46 2.08 1.60 1.34 1.25 1.21 1.18
10.0 ( 4.6 (8.45)
0.4
2.0
104k0calcd,b s-1
0.55f
6.13 ( 2.66 (4.86) 1.53 ( 2.85 (3.89) 5.25 ( 1.47 (3.31) 5.57 ( 1.83 (2.93) 1.53 ( 0.99 (2.46) 1.68 ( 0.53 (2.08) 1.00 ( 0.24 (1.60) 1.48 ( 0.21 (1.34) 2.22 ( 0.54 (1.25) 0.20 ( 0.05 (1.21) 1.75 ( 0.49 (1.18)
70.5 ( 68.7 ( 1.2 67.6 ( 1.7 69.3 ( 1.9 67.6 ( 1.9 61.1 ( 0.8 60.7 ( 0.4 39.9 ( 0.6 31.7 ( 1.0 24.7 ( 0.5 23.0 ( 0.9 17.2 ( 0.4 15.8 ( 0.3 16.2 ( 0.5 15.5 ( 1.2 16.0 ( 0.1 15.7 ( 0.1 17.3 ( 0.2 790 ( 15 813 ( 13 857 ( 14 862 ( 20 878 ( 20 849 ( 22 867 ( 6 859 ( 6 696 ( 8 696 ( 12 433 ( 8 441 ( 13 281 ( 9 273 ( 8 178 ( 5 188 ( 10 135 ( 6 145 ( 8 98.9 ( 3.0 95.2 ( 2.6 72.6 ( 1.6 71.2 ( 1.1 58.2 ( 0.7 55.5 ( 1.5 49.4 ( 0.7 50.1 ( 0.6 44.4 ( 1.6 48.3 ( 2.3 47.9 ( 0.1 45.6 ( 0.5 43.9 ( 0.7 45.1 ( 0.7
60.6 54.6 38.5 31.4 27.4 24.8 21.7 19.3 16.3 14.7 14.1 13.9 13.7
[Am]T,d
no. of runs
0.003-0.100 0.003-0.100 0.003-0.100 0.003-0.100 0.003-0.100 0.003-0.100 0.003-0.100 0.003-0.100 0.003-0.100 0.003-0.100 0.003-0.100 0.003-0.100 0.003-0.100 0.003-0.100 0.015-0.100 0.015-0.100 0.015-0.100 0.003-0.100 0.01-0.05
7 7 7 7 7 7 7 7 7 7 7 7 7 7 5 5 5 7 5
0.01-0.05
5
0.01-0.05
5
0.01-0.05
5
0.01-0.05
5
0.01-0.05
5
0.01-0.05
5
0.01-0.04
4
0.01-0.05
5
0.01-0.05
5
0.01-0.05
5
0.01-0.05
5
0.01-0.05
5
0.01-0.05
5
0.02-0.10
5
0.02-0.10
5
703 386 270 209 172 129 96.8 58.1 38.5 32.0 28.7 26.7
a [phenyl salicylate] ) 2 × 10-4 M, 35 °C, λ ) 350 nm, aqueous reaction mixture for each kinetic run contained 2% v/v MeCN. b Calculated 0 from eq 2 as described in the text. c Calculated from eq 6 as described in the text. d Total concentration of amine. e [NaOH] ) 0.005 M. f Error limits are standard deviations. g [NaOH] ) 0.02 M. h Parenthesized k values were obtained from eq 2 using k values obtained 0 0 for n-butylaminolysis of PS-.
The calculated values of k0 for piperidine and pyrrolidine are associated with considerably high standard deviations (Table 1 and the Supporting Information). This is the consequence of an insignificant contribution of k0 compared with knobs[Am]T toward kobs for more reactive amines such as piperidine and pyrrolidine. The more reliable values of knobs for piperidine and pyrrolidine were obtained from eq 1 by considering k0 as a known parameter. The k0 values at different [CTABr]T were obtained from kinetic data for less reactive n-butylamine. These k0 values are almost similar to the corresponding values obtained under similar experimental conditions with [Am]T ) 0.8 The knobs values calculated from eq 1 with k0 considered as a known parameter are also summarized in Table 1 and the Supporting Information. The knobs values for pyrrolidine and piperidine obtained from eq 1 at different [CTABr]T with k0 as an unknown or a known parameter are not significantly different (Table 1 and the Supporting
Information). These observations show the insignificant contribution of k0 compared with knobs[Am]T for these amines. The values of of k0 and knobs at [CTABr]T ) 0.0, 0.006, 0.008, and 0.010 M are unchanged with a change in [NaOH] from 0.005 to 0.020 M (Tablee 1 and the Supporting Information). This shows that, under such conditions, n-butylamine exists in the fully nonprotonated form. The values of knobs for piperidine obtained within the [CTABr]T range 0.0-0.002 M are ∼8-20% lower at 0.005 M NaOH than at 0.02 M NaOH. A simple calculation based upon the pKa of piperidine (in water) and pKw indicates the presence of ∼10% and
