Kinetics and Mechanism of Intramolecular General Base-Catalyzed

Langmuir 1996, 12, 261-268

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Kinetics and Mechanism of Intramolecular General Base-Catalyzed Methanolysis of Ionized Phenyl Salicylate in the Presence of Cationic Micelles Mohammad Niyaz Khan* and Zainudin Arifin Department of Chemistry, Universiti Malaya, 59100 Kuala Lumpur, Malaysia Received November 7, 1994. In Final Form: August 25, 1995X The methanolysis of ionized phenyl salicylate, PS-, has been studied in the absence and presence of 0.03 M cetyltrimethylammonium bromide, CTABr, at 0.01 M NaOH and within the CH3OH content range of 10-80 or 90% (v/v). The pseudo-first-order rate constants, kobs, remain unchanged with change in [CTABr]T from 0.0 to 0.03 M at g65% (v/v) CH3OH. The ratio, kobs(0.0 M CTABr)/kobs(0.03 M CTABr) decreases from 4.6 to 1.1 with increase in CH3OH content from 10 to 60% (v/v). At the constant temperature and [CH3OH]T, the rate constants, kobs, show a decrease with increase in [CTABr]T. These results are explained , for the in terms of a pseudophase model of micelle. At 30 °C, the pseudo-first-order rate constants, kMeOH M reaction of CH3OH with PS- in the micellar pseudophase and cmc of CTABr increase from 9.63 × 10-4 and 1.8 × 10-4 to 78.8 × 10-4 s-1 and 60.0 × 10-4 M, respectively, while the binding constants, K1, decrease from 8080 to 39 M-1 with increase in methanol content from 10 to 50% (v/v). Similar results are obtained at different temperatures ranging from 25 to 45 °C. The activation parameters, ∆H* and ∆S*, MeOH MeOH are larger than those for kNM (where kNM represents pseudo-first-order rate constant for the for kMeOH M reaction of CH3OH with PS- in the nonmicellar pseudophase) at 10, 20, and 30% (v/v) CH3OH. These results are attributed to the increased ground-state stability of a monomeric methanol in the micellar pseudophase compared to that in the nonmicellar pseudophase. The thermodynamic parameters, ∆H0 and ∆S0, for binding constant, K1, reveal the decrease from -8.6 and -10.5 to -14.1 kcal/mol and -31.9 cal/(K mol), respectively, with increase in CH3OH content from 10 to 30% (v/v). Significantly lower values MeOH compared to those of kNM at low contents of CH3OH are ascribed to (i) reduced [CH3OH] in the of kMeOH M micellar pseudophase, where PS- molecules exist, compared to [CH3OH] in the nonmicellar pseudophase and (ii) partial loss of the efficiency of intramolecular general base catalysis due to probable ion-pair formation between anionic site of micellized PS- molecule and the cationic micellar headgroups.

Introduction The occurrence of intramolecular general base (IGB) catalysis is believed to occur in many enzyme-catalyzed biochemical reactions.1 The reactions of H2O2 and CH3OH3 with ionized phenyl salicylate (PS-) have been shown to involve IGB catalysis. Normal micelles perhaps provide one of the best among the simplest models to study the effects of the dynamic microheterogeneity of the reaction medium on rates and mechanism of chemical reactions. Such studies may be considered as the partial model studies to many most complex biological reactions. The effects of micelles on the rates of intramolecular nucleophilic and general acid-base-catalyzed reactions have been extensively studied.4 We have studied the effects of anionic micelles on IGB catalysis in the hydrolysis,5 alkanolysis,6 and aminolysis7 of PS-. The systematic kinetic studies involving the effects of micelles on rates of organic reactions in mixed aqueousalkanol solvents have been carried out by a few investigators.8,9 The kinetic studies on the effects of micelles upon the solvolytic cleavages of organic compounds in mixed aqueous-alkanol solvents where alkanol molecules act X Abstract published in Advance ACS Abstracts, November 15, 1995.

(1) Dempcy, R. O.; Bruice, T. C. J. Am. Chem. Soc. 1994, 116, 4511. Dalby, K. N.; Kirby, A. J.; Hollfelder, F. Pure Appl. Chem. 1994, 66, 687. Page, M. I. J. Mol. Catal. 1988, 47, 241. Fersht, A. R. Enzyme Structure and Mechanism; W. H. Freeman: San Francisco, CA, 1977. (2) Copon, B.; Ghosh, B. C. J. Chem. Soc. B 1966, 472. Khan, M. N. J. Mol. Catal. 1987, 40, 195. (3) Khan, M. N. Int. J. Chem. Kinet. 1987, 19, 757. (4) Cerichelli, G.; Luchetti, L.; Mancini, G.; Savelli, G.; Bunton, C. A. J. Colloid Interface Sci. 1993, 160, 85. Bunton, C. A.; Savelli, C. Adv. Phys. Org. Chem. 1986, 22, 213. (5) Khan, M. N.; Na’aliya, J.; Dahiru, M. J. Chem. Res. (S), 1988, 116; (M), 1988, 1168. Khan, M. N. J. Chem. Soc., Perkin Trans 2 1990, 445. (6) Khan, M. N. Int. J. Chem. Kinet. 1991, 23, 837. (7) Khan, M. N.; Dahiru, M.; Na’aliya, J. J. Chem. Soc., Perkin Trans 2 1989, 623.

0743-7463/96/2412-0261$12.00/0

Scheme 1 PS-

k1 CH3OH

MS- + PhOH –CH3OH

k2 H2O

k3 H2O

o-HOC6H4CO2- + PhOH

as reactants appear to be nonexistent. Monohydric alcohols are known to hinder the micellization.9 We carried out the present study with the aims (i) to determine the effects of cetyltrimethylammonium bromide (CTABr) micelles on the rates of methanolysis of PS-, (ii) to determine the effects of mixed H2O-CH3OH solvents on kinetic cmc of CTABr, and (iii) to study the structural behavior of methanol in the presence of CTABr micelles. The observed results and the probable explanations are described in this paper. Experimental Section Materials. Reagent grade chemicals such as methanol, acetonitrile, phenyl salicylate, and cetyltrimethylammonium bromide (CTABr) were obtained from either BDH or Aldrich. All other chemicals used were also of reagent grade. The stock solutions of phenyl salicylate were frequently prepared in acetonitrile. Kinetic Measurements. The rates of methanolysis of ionized phenyl salicylate (PS-) in mixed solvents containing 0.01 M NaOH, 2% (v/v) CH3CN, and varying contents of methanol and water were studied spectrophotometrically by monitoring the appearance of product, phenolate ion, at 290 nm. The details of the procedure are described elsewhere.10 The complete reaction scheme for the cleavage of PS- under the presence of 0.01 M NaOH and mixed CH3OH-H2O solvents may be given by Scheme 1 where k1, k2, and k3 represent pseudo(8) Bertncini, C. R. A.; Nome, F. J. Phys. Chem. 1990, 94, 5875. Rubio, D. A. R.; Zanette, D.; Nome, F.; Bunton, C. A. Langmuir 1994, 10, 1151, 1155. (9) Bunton, C. A.; Gan, L.-H.; Hamed, F. H.; Moffatt, J. J. Phys. Chem. 1983, 87, 336 and references cited therein. (10) Khan, M. N. J. Chem. Soc., Perkin Trans 2 1989, 199.

© 1996 American Chemical Society

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Figure 1. Effect of methanol content on pseudo-first-order rate constants, kobs, for methanolysis of ionized phenyl salicylate, PS-, at 0.0 M CTABr for (.) and 0.03 M CTABr for (4). first-order rate constants for methanolysis and hydrolysis of PSand hydrolysis of ionized methyl salicylate (MS-), respectively, and PhOH is phenol. In mixed CH3OH-H2O solvents containing 0.01 M NaOH and 10% (v/v) CH3OH, the value of k1/k2 = 10 and 12 in the absence and presence of 0.03 M CTABr, respectively. Similarly, under such conditions, k1/k3 = 50 and 53 in the absence and presence of 0.03 M CTABr. Thus, under the experimental conditions of the study, k1/k2 g 10 and k1/k3 g 50. These results indicate that the rate of hydrolysis of transesterified product (MS-) is negligible compared to rate of methanolysis of PS-. The observed absorbance versus time data obeyed the first-order rate law under the entire experimental conditions of the study. The details of the data analysis are the same as described elsewhere.10

Figure 2. Plots showing the dependence of kobs upon [Dn] for methanolysis of PS- at 30 °C in mixed H2O-CH3OH solvents containing 2% (v/v) CH3CN and 10 (.), 15 (4), 20 (3), 25 (0), 30 (b), 35 (2), 40 (V), 45 ()), and 50% (v/v) CH3OH (triangle tilted right). The solid lines are drawn through the leastsquares calculated points.

Results PS-

(a) Cleavage of in CH3OH-H2O Solvents in the Absence and Presence of 0.03 M CTABr. A series of kinetic runs was carried out in the absence and presence of 0.03 M CTABr at 30 °C and at different contents of CH3OH (10-80%, v/v) in mixed CH3OH-H2O solvents containing 0.01 M NaOH and 2% (v/v) CH3CN. The pseudo-first-order rate constants (kobs) are shown graphically in Figure 1. (b) Effects of [CTABr] on the Rate of Methanolysis of PS- at a Constant Content of CH3OH in Mixed CH3OH-H2O Solvents. The reaction rates for methanolysis of PS- were studied at different [CTABr] keeping 0.01 M NaOH, 30 °C, 2% (v/v) CH3CN and a constant content of CH3OH in mixed CH3OH-H2O solvents. Similar observations were obtained at various contents of CH3OH (10-50%, v/v) and at different temperature (2545 °C). The pseudo-first-order rate constants (kobs) for these observations are shown in Figures 2-5. Discussion The values of molar absorption coefficients (290) are 700-900 M-1 cm-1 for PS- and MS-, 1700 M-1 cm-1 for PSH, 2700 M-1 cm-1 for MSH, 2350 M-1 cm-1 for PhO-, and 3250 M-1 cm-1 for o-OC6H4CO2- at 290 nm. The values of initial absorbance (A0obs at t ) 0) of the reaction mixtures containing 2 × 10-4 M phenyl salicylate, 0.01 M NaOH, and 2% (v/v) CH3CN were found to be unchanged with change in the contents of CH3OH from 10 to e90% (v/v) in CH3OH-H2O solvents. These absorbance values were also unaffected due to the presence of e0.18 M

Figure 3. Plots showing the dependence of kobs upon [Dn] for methanolysis of PS- in mixed aqueous solvents containing 2% (v/v) CH3CN and 10% (v/v) CH3OH at 25 °C (.), 35 °C (4), 40 °C (0), and 45 °C (3). The solid lines are drawn through the least-squares calculated points.

CTABr. These results show the presence of the 100% ionized form of the reactant, phenyl salicylate, into the reaction mixtures of all kinetic runs carried out in this study. The pseudo-first-order rate constants, kobs, for hydrolysis10 (in aqueous solvent containing 0.8% (v/v) CH3CN)

Intramolecular General Base Catalysis

Langmuir, Vol. 12, No. 2, 1996 263

larger than that of hydrolysis of PS- under similar experimental conditions.13 Thus, the effectiveness of IGB catalysis is apparently more pronounced in methanolysis (T2) than in hydrolysis (T1) of PS-. The detailed mechanism of IGB-catalyzed hydrolysis14 and methanolysis13 of PS- have been described in earlier report.13


(a) Analysis of Observed Data: kobs versus % (v/v), CH3OH in the Absence and Presence of 0.03 M CTABr. On the basis of the reaction mechanism of methanolysis of PS- as described elsewhere,13 the overall reaction involves PS- and CH3OH as the reactants. Thus, the apparent rate law for methanolysis of PS- may be given as

rate ) k[CH3OH][PS-]T


and methanolysis3 (in mixed aqueous solvent containing 80% (v/v) CH3OH and 0.8% (v/v) CH3CN) of phenyl salicylate were found to be independent of [OH-] within [KOH] range of 0.01-0.06 and 0.01-0.15 M, respectively, in the absence of CTABr. Similarly, kobs turned out to be independent of [-OH] within [NaOH] range of 0.01-0.04 M in the presence of 0.03 M CTABr.11 Under such conditions, the rate of hydrolysis of phenyl salicylate involves PS- and H2O as the reactants and the rate has been shown to be increased by nearly 106-fold due to the occurrence of intramolecular general base, IGB, catalysis (T1).12 The rate of methanolysis of PS- is nearly 100-fold (11) Unpublished observations.

(1)

where k represents second-order rate constant and [PS-]T is the total concentration of ionized phenyl salicylate. The observed rate law: rate ) kobs[PS-]T, and eq 1 predict that kobs should vary linearly with total concentration of methanol, [CH3OH]T, with essentially zero intercept provided [CH3OH] is directly proportional to [CH3OH]T. The plot of kobs versus [CH3OH]T is indeed linear but only at low contents of CH3OH in the absence of CTABr (Figure 1). The rate constants, kobs, revealed smooth and increasing negative deviations from linearity with increase in [CH3OH]T at high contents of CH3OH. As described elsewhere6 the most plausible reason for the nonlinear nature of the plot of Figure 1 is the self-association of CH3OH molecules even in the mixed CH3OH-H2O solvents with high contents of methanol. In the previous reports15 on alkanolysis of PS-, we considered the total concentration of methanol, [CH3OH]T, as the sum of the concentrations of monomeric, dimeric, trimeric, ..., and nmeric methanol. But one of the reviewers of this paper has pointed out that since the formation of dimeric, trimeric, ..., and nmeric methanol occurs by a stepwise association of CH3OH molecule, the [CH3OH]T should be given by eq 2.

[CH3OH]T ) [CH3OH] + 2[(CH3OH)2] + 3[(CH3OH)3] + ... + n[(CH3OH)n] (2) The assumption of equal association constant for one more addition of CH3OH molecule to an aggregate changes eq 2 to eq 3

[CH3OH]T ) [CH3OH]{1 + 2KA[CH3OH] + 3(KA[CH3OH])2 + ... + n(KA[CH3OH])n-1} (3) (12) Lajis, N. H.; Khan, M. N. Pertanika 1991, 14, 193. (13) Khan, M. N.; Audu, A. A. J. Phys. Org. Chem. 1992, 5, 129. (14) Khan, M. N.; Gambo, S. K. Int. J. Chem. Kinet. 1985, 117, 419. (15) Khan, M. N. J. Phys. Chem. 1988, 92, 6273. Int. J. Chem. Kinet. 1988, 20, 443. Khan, M. N.; Audu, A. A. Int. J. Chem. Kinet. 1990, 22, 37.

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Khan and Arifin

where KA represents the association constant for dimerization of CH3OH. Equation 3 can lead to eq 4 if KA[CH3OH] , 1.

[CH3OH] )

k[CH3OH]T 1 + 2KA[CH3OH]T

(4)

The observed rate law: rate ) kobs[PS-]T and eqs 1 and 4 can yield eq 5.

kobs )

k[CH3OH]T 1 + 2KA[CH3OH]T

(5)

The observed rate constants, kobs, obtained in the absence of CTABr, were treated with eq 5 and the nonlinear leastsquares calculated values of k and KA are (2.12 ( 0.06) × 10-3 M-1 s-1 and (15.6 ( 1.6) × 10-3 M-1, respectively. In terms of Scheme 1, kobs ) k1 + k2. But, under experimental conditions of the present study, k1/k2 g 10 and hence k2 may be neglected compared with k1. Thus, kobs = k1. The rate constants, kobs, obtained in the presence of 0.03 M CTABr, did not fit to eq 5 (Figure 1). The rate constants, kobs, increase almost linearly with increase in the contents of CH3OH until nearly 20% (v/v) CH3OH and then a further inrease in the contents of CH3OH causes a sharp nonlinear increase which in turn makes kobs deviate positively from linearity (Figure 1). The rate constants, kobs, were found to be unaffected due to the presence of 0.03 M CTABr at methanol contents of >65% (v/v) (Figure 1). It is therefore apparent that probably either there were no more micelles or there was no more micellar incorporation of PS- in the solvents containing 2 × 10-4 M PS-, 0.03 M CTABr, 0.01 M NaOH, 2% (v/v) CH3CN, and g65%, (v/v) CH3OH in mixed CH3OH-H2O solvents. The presence of CTABr micelles is reflected from the relative magnitudes of kobs obtained in the absence and presence of 0.03 M CTABr at CH3OH contents e65% (v/v) in mixed CH3OH-H2O solvents. The study on the effects of the concentrations of tetraalkylammonium ions, [R4N+], on KA revealed a decrease in KA with increase in [R4N+] and KA became almost zero at [Et4NBr] ) 0.24 M, [Pr4nNBr] ) 0.18 M, [Bu4nNBr] ) 0.12 M, and [Bu4nNI] ) 0.12 M.13 In view of these findings, the micellized methanol molecules, CH3OHM, may be assumed to exist in monomeric form, i.e., KA ) 0 for CH3OHM molecules. Under such conditions, eq 5 is reduced to eq 6

kobs ) k[CH3OH]T

(6)

which indicates that kobs, obtained under the conditions when 100% PS- molecules were micellized, must increase linearly with increase in the contents of CH3OH in mixed CH3OH-H2O solvents. But the observed data as shown in Figure 1 do not appear to follow eq 6 at CH3OH contents of >20% (v/v). The reason for these observations is probably the significant contribution of rate of methanolysis of nonmicellized PS- compared to that of micellized PS- due to decrease in the magnitude of binding constant, K1, of PS- with micelles with increase in the methanol content. The second-order rate constant, k, was calculated from eq 6 using kobs obtained within 10-20% (v/v) CH3OH and the value of k turned out to be (4.69 ( 0.36) × 10-4 M-1 s-1. (b) Analysis of Observed Data: kobs versus [CTABr] at a Constant % (v/v), CH3OH, and 30 °C. The observed rate constants, kobs for methanolysis of PS- decreased by 4.6-, 4.4-, 3.8-, 3.6-, 3.2-, 2.9-, 2.6-, 2.3-, and 1.9-fold with

increase in [CTABr] from 0.0 to 40% (v/v) CH3OH (Figure 11). (c) Effects of Temperature on kMeOH and K1 at 10, M 20, and 30% (v/v) CH3OH. A series of kinetic runs was carried out within the [CTABr]T range of 2.5 × 10-4 to 0.03 M at a constant content of CH3OH and temperature. The pseudo-first-order rate constants, kobs, at 10, 20, and 30% (v/v) CH3OH and at different temperature (25-45 °C) are shown in Figures 3-5. The rate constants, kobs, obtained at different [CTABr]T and at a constant temperature and CH3OH content, were found to fit to eq 7. The nonlinear least-squares calculated values of kMeOH M and K1 at the best kinetic cmc are summarized in Table 2. The best kinetic cmc was determined as described

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Khan and Arifin

Table 3. Effects of Methanol on Activation Parameters for Methanolysis of PS- in Micellar and Nonmicellar Pseudophase and Thermodynamic Parameters for Binding of PS- with CTABr Micelles in Mixed H2O-CH3OH Solventsa activation parametersb MeOH for kMeOH for kNM M CH3OH, % (v/v) ∆H*, kcal/mol -∆S*, cal/(K mol) ∆H*, kcal/mol -∆S*, cal/(K mol)

10 20 30

16.2 ( 1.8d 13.9 ( 0.1 13.8 ( 0.7

18.9 ( 5.6d 24.7 ( 0.3 24.0 ( 2.3

11.6 ( 0.6d 10.8 ( 0.4 11.1 ( 0.6

30.8 ( 11.9d 32.3 ( 1.2 30.6 ( 1.8

thermodynamic parametersc for K1 -∆H0, kcal/mol

-∆S0, cal/(K mol) -∆G0, kcal/mol

8.6 ( 1.4d (8.1 ( 2.0) 10.5 ( 4.7d (8.9)f 12.4 ( 0.7 (11.8 ( 0.5) 24.2 ( 2.3 (22.4) 14.1 ( 2.9 (14.3 ( 2.5) 31.9 ( 9.7 (32.0)

5.36e 4.91 4.43

a Experimental conditions are mentioned in Table 1. b Calculated from Eyring equation (eq 8) with rate constants, kMeOH and kMeOH, M NM mentioned in Table 2. c Calculated from thermodynamic equation (eq 9) and van’t Hoff equation (eq 10, parenthesized values) with binding d e constants, K1, mentioned in Table 2. Error limits are standard deviations. Calculated from the relationship: ∆G0 ) -RT ln K1 with T ) 308 K. f Calculated from the relationship: ∆G0 ) ∆H0 - T∆S0 with T ) 308 K.

earlier in the text. The fitting of the observed data to eq 7 is evident from the plots of Figures 3-5 where solid lines are drawn through the least-squares calculated points. and kMeOH obtained at The rate constants, kMeOH NM M different temperature (25-45 °C) were treated with the Eyring equation (eq 8) where

k)

KBT exp(∆S*/R) exp(-∆H*/RT) h

(8)

all the symbols have their usual meanings. The nonlinear least-squares calculated values of ∆H* and ∆S* at different contents of CH3OH are summarized in Table 3. The fitting of the rate constants, kMeOH and kMeOH , to eq 8 is evident NM M from the standard deviations associated with ∆H* and ∆S* as summarized in Table 3. The values of both ∆H* and ∆S* show a significant increase for the reaction occurs in the micellar pseudophase compared to that in nonmicellar pseudophase. The qualitative explanation for these results may be described as follows. The micellized water molecules exist in an environment of considerably high hydrophobicity. It is known that tetraalkylammonium ions are water-structure forming agents.20 In view of this fact, the micellized water molecules may be considered to be more structured compared to nonmicellized water molecules. The hydrogen bonding energy (DH-B) of water is 9.2-9.9 kcal mol-1 21,22 and that methanol is 5.6 kcal mole-1.21 The value of DH-B of a solvent is expected to be larger in the presence of a solvent-structure forming agent than that in absence of such an agent. This shows that mixed CH3OH-H2O solvent is more structured in the micellar pseudophase than in the nonmicellar pseudophase. The increase in the extent of solvent structure causes an increase in DH-B, i.e. an increase in the groundstate stability of the solvent molecules. The increase in the ground-state stability of the reactant (monomeric) is therefore expected to decrease both the enthalpy and entropy of the ground state of the reactant methanol. (20) Jencks, W. P. Catalysis in Chemistry and Enzymology; McGraw-Hill: New York, 1969. (21) Engberts, J. B. F. N. In Water, a Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1979; Vol. 6, Chapter 4. (22) Goel, A.; Murthy, A. S. N.; Rao, C. N. R. Indian J. Chem. 1971, 9, 56.

The binding constants, K1, obtained within the temperature range of 25-45 °C, were used to calculate the thermodynamic parameters, ∆H0 and ∆S0 (where ∆H0 and ∆S0 represent the enthalpy and apparent entropy of binding, respectively) from eq 9 using the nonlinear leastsquares technique.

K1 ) exp(∆S0/R) exp(-∆H0/RT)

(9)

The calculated values of ∆H0 and ∆S0 at different contents of CH3OH are summarized in Table 3. One of the referees has pointed out that the thermodynamics exactly lead to the van’t Hoff equation on the temperature dependence of ln K1 (eq 10).

d ln K1/d(1/T) ) -∆H0/R

(10)

The plots of ln K1 versus 1/T turned out to be linear at 10, 20, and 30% (v/v) CH3OH. The linearity of these plots indicates that the enthalpy change is constant in the range of temperatures and the entropy change is temperature dependent. The values of ∆H0 were also calculated from the integrated linear form of eq 10 using the linear leastsquares technique. The values of ∆G0 and ∆S0 were calculated by ∆G0 ) -RT ln K1 ) ∆H0 - T∆S0 at each temperature. These results are summarized in Table 3. It is interesting to note that within the temperature range of 25-45 °C, the maximum variations in ∆S0 are 0.8, 0.1, and 0.7 cal/(K mol) at 10, 20, and 30% (v/v) CH3OH, respectively. The ∆H0 values are essentially similar to the corresponding ∆H0 values calculated from eq 9. It is interesting to note that ∆H0 decreased from -8.6 to -14.1 kcal mol-1 while ∆S0 decreased from -10.5 to -31.9 cal deg-1 mol-1 with increase in the content of CH3OH from 10 to 30% (v/v). It must be noted that K1 ) K1°/n, where n represents an aggregation number of micelle. (We thank one of the reviewers for pointing this out.) The calculated values of ∆H0 are independent of n. But ∆S0 ) ∆S0° - R ln(n) (if n is independent of temperature) where ∆S0° represents the actual entropy of binding. The reasonably good fitting of K1 to eq 9 indicates that temperature dependence of n is insignificant within the temeperature range of the study. Acknowledgment. This investigation was supported in part by Universiti Malaya F Volte F161/94 granted to M.N.K. LA940881L

Kinetics and Mechanism of Intramolecular General Base-Catalyzed

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