Chemical Reactions in Surfactant Solutions(Ⅲ). Nucleophilic and Micellar Catalyses on Hydrolysis of an Organic Phosphate by Sodium 2-Alkylbenzimidazole-5-sulfonates in Aqueous and CTABr Solutions

DAEHAN HWAHAK HWOEJEE (Journal of the Korean Chemical Society) Vol. 34, No. 6, 1990

Printed in the Republic of Korea

계면활성제 용액속에서의 화학반응(제

3

^보)

유기인산 에스테르의 가수분해반응에 미치는

2-

^{알킬벤즈이미다졸}

-5

■■술포네이트의 친핵적 및 미셀 촉매효과 洪永錦十•朴福鉉•朴漢銅

계명대학교 자연과학대학 화학과 (1990. 5. 26 접수)

Chemical Reactions in Surfactant Solutions(III). Nucleophilic and Micellar Catalyses on Hydrolysis of an Organic Phosphate by Sodium

2-Alkylbenzimidazole-5-sulfonates in Aqueous and CTABr Solutions

Young-Seuk Hong', Hee-Hyun Park, and Han-Seok Park

Department of Chemistry, College of Natural Science, Keimyung University, Taegu 704-200, Korea (Received May 26, 1990)

요 약 CTABr 미셀 용액속에서의 sodium 2-alkylbenzimidazole-5-sulfonate(R-BIS()3Na)의 음이온에 의해 추진되는p-nitrophenyldiphenylphosphate(p-NPDPP)^{탈인산화 반응은}Sodium benzimidazole-5-sul- fonate(BIS03 Na)의 음이온에 의해 추진되는 반응의 반응속도보다 현저히 감소하^, 알킬기의 길이가 me- thyl기에서 heptyl기로 길어질수록 감소의 정도가 더욱 크다. 이것은 CTABr을 포함하고 있지 않는 수용액 속에서의 BrSO3Na 및 R・B「SO3Na에 의한 탈인산화 반응속도가별차이가 없음을 감안할 때 이들 친핵체의 알킬기가 미셀 의사층(micellar pseudophase)내에서 입네장애(steric hindrance)로 작용하기 때문인 것으로 판단된다. 이것은 수용액과 미셀 용액속에서의 반응의 측정된 활성화에너지(AH*, AG* 및 AS*)의 값과도 정성적으로 일치하고 있다. 이러한입체장애가 반응속도에 미치는 영향을 정량적으로 계산하고자 시도하였다.

Nonyl^기에서 pentadecyl^{기까지의 긴 알킬기를 갖고 있는} R-B^「S()3Na^{는 그것들의} benzimidazole ^부분(BI moiety)^{이 친핵체로 작용할} 뿐 아니라, 이 분자들은 CTABr^{을포함하지 않는 수용액속에서 미셀을 형성하여}

반응을 촉진함을 알았다. .

ABSTRACT. Dephosphorylation of p-nitrophenyldiphenylphosphate (p-NPDPP) mediated by anions of sodium 2-alkylbenzimidazole-5-sulfonate (R-BI~SO3Na) in CTABr micellar solutions are obviously slower than that by anion of sodium benzimidazole-5-sulfonate (BI-SO3Na), and the reaction rates were decreased with increase of lengths of alkyl groups. This presents a striking contrast to the reactions in aqueous solutions without added CTABr, of which the reaction rates are on approximately same levels.

It seems due to steric effect of alkyl groups of R-BI~S03Na in the Stern layer of micelle, and it is sup­

ported by measured activation parameters (AH*, AG* and AS*) of the reactions in aqueous and micellar gutions. In addition to nucleophilic ability of benzimidazole moiety of R-BI-SO3Na on the reactions, these compounds with long alkyl groups (nonyl to pentadecyl) are micellized for themselves, and increase the reaction rates due to their micellar catalyses in aqueous solutions, not in이uding CTABr.

629

630 洪永錫•朴禧鉉•朴漢錫

INTRODUCTION

Two studies were made by C.A. Bunton and the author1- 2 on dephosphorylation of p-nitro- phenyldiphenylphosphate (p-NPDPP) mediated by benzimidazolide and naphth-2t3-imidazolide ions (BI~ and NI~) in micellar solutions of cetyl trimethyl ammonium bromide (CTABr) and in so­

lutions of the phase transfer catalysts, ethyl tri- n-octyl ammonium bromide and mesylate (TEABr and TEAMs, respectively). In these reactions evi­

dence that the imidazolide ions act as nucleophiles, not as general bases, was given.

Recently the author also discussed dephos­

phorylation of p-NPDPP mediated by some 2-alkylbenzimidazolide ions (R-BI-)3 and hydro­

lyses of carboxylic esters mediated by sodium 2-alkylbenzimidazole-5-sulfonates (R-BISO3 Na)4 in aqueous and CTABr solutions, in which nucleophilicities and steric effects of R-BI~

and R-BISO3Na were varied by a change of their alkyl groups. In spite of a sharp increase of the binding constants^) of the nonionic nucleophiles, R-BI, with increase of the number of methylene groups, reaction rates mediated by *

Core

Scheme 1 —The conceptual "anchor effect" proposed transition state of dephosphorylation mediated by R-Bl- in Stem layer, where S is 0 or 0 , and

—-P-OAr ——&OR ,、

砂)&O O nucleophile is R-BI~ or R-BISO3Na.

R-BI were decreased as compared with those by BI~, even though both the substrate and nucleophile should be located in the Stern layer of the micelle (Scheme 1). In the latter4, anions of R-BISO3Na with long alkyl groups (nonyl to pen- tedecyl) which form micelles of themselves increase the reaction rates due to their micellar catalyses in aqueous solutions, not in이uding CTABr.

In the present work we will discuss micellar property of anions of sodium 2-alkylbenzimida- zole-5-sulfonates (R-BI~SO3Na) with long alkyl groups, which are newly prepared, on de­

phosphorylation of p-nitrophenyldiphenylphos- phate (p-NPDPP).

EXPERIMENTAL SECTION

Materials. The preparation and purification of CTABr5, p-NPDPP6 and R-BI3 has been described. 2-Alkylbenzimidazole-5-sulfonic acids (R-BISO3H) were prepared by the addition of chlorosulfonic acid (C1SO3H) to stirred alcoholic solutions of R-BI at 5°C for 4 hours7. The remaining chlorosulfonic acid was decomposed by adding H2O, and the solutions were heated at 105-110 °C to eliminate HC1, and the product was crystallized on codling. The crude crystals were neutralized by adding NaHC03, and after recrystallization from H2O ^나le products (R- BISO3Na) were identified by IR and NMR spec­

troscopy.

Properties of R-BISO3Na. The apparent acid-dissociation constants(lCa) for deprotonation of R-BISO3Na in water and micellar solutions were measured by spectral titrati^이卩，호 monitored at 283 nm (Table 1). We calculated the concentra­

tion of micellar bound R-BI~SO3Na indirectly from the Ka values (we failed in the direct spec­

tral measurement of the concentrations of the materials because of complex shifts of the spectra.

If the spectra are simple, they should be usually

Journal of the Korean Chemical Society

계면활성제 용액속에서의 화학반응（제3^보） 631

Table 1. Ka values of sodium 2-alkylbenzimidazole-5-sulfonate, concentrations of their anions in aqueous and micel­

lar phase and their cmc NaSO3@》_R

1 H

KN Ka 106[R-BI-SO3Na]

in cmc water

in CTABr

c d e

R = -H 12.3 39.6 16.5 5.79 10.7

-ch3 3.98 14.8 6.89 1.95 4.94

-c3h7 3.83 13.7 6.43 1.88 4.55

-C5Hn 3.77 13.0 6.12 1.85 4.27

-C7H15 3.61 12.8 6.00 1.77 4.23

«9土9 3.42 12.6 5.90 1.68 4.22 3.40X10-3 A/

-CnH23 3.29 12.3 5.77 1.62 4.15 1.95x10-4 财

«13田7 3.18 12.0 5.67 1.57 4.10 1.64xlO・W

_c15h31 3.07 11.8 5.58 1.51 4.07 6.80x10-5

monitored at 283 nm in Beckman DU 8B UV-Visible spectrophotometer, apparent Ka values at optimum concentration of CTABr(8x 10~4A/) in dephosphorlyation of/>-NPDPP at 30± 0.2°C, the molar concetration of R-BESOsNa in 8 x 10-4A/ CTABr solution(10-2M carbonate buffer, pH 10.7), the molear concentration [/"] of R-BI-SO3Na in water solution (IO-2Ay carbonate buffer, pH 10.7), the molar concentration [/~] of R-BI_SO3Na in mic^이lar pseudophase ([시니馈by surface tensiometry

explained and calculate concentrations of the micel­

lar bound imidazoles from these.).

Determination of cmc of R-BISO3Na with long alkyl groups was made by surface-tensiometry (Table 1).

Kinetics. Dephosphorylations of p-NPDPP mediated by R-BrSO3Na and/or 01^厂 are shown in Scheme 2.

The formation of p-nitrophenoxide ion was followed spectrometrically1- 흥 on a Beckman DU-8B model in aqueous solution at 30 ±0.2 °C.

The first order rate constants for overall reaction, k牛,are in reciprocal seconds (sec-1). The meas­

ured rate constant(A:供)include contributions from

阿p財+雨=f

* (R-efSOaNa)

"◎A•"笋=心哙…。-

(p-NPDPP) °*<：Opb

Ar B -O~N02 H,0 j^OH~

A « -H, -CHj--

"PW+緣

Scheme 2.

reactions with OH- of 0.001 M carbonate buffer (pH 10.7) and with deprotonated R-BISO3Na (R- BI-SO3Na) (equation 1), where k^_7, k^H- and

are the first order rate constants with R- BISO3Na, OH- and R-BU SO3Na, respectively, and we assume that they are independent1-3.

r-i4-/?oh- + ^r~i- (1) Activation Parameters. Activation enthalpy

(厶H*), free energy ^(厶(구*) and entropy (AS*) were measured. Experimental temperatures are in the range 25-45 °C.

RESULTS AND DISCUSSION

Ka values of R-BISO3Na. Measured Ka valu­

es of sodium 2-alkylbenzimidazole-5-sulfo- nates (R-BISO3Na) in water decreased slightly with increase of length of the alkyl chain, because of inductive or steric effects (Table 1). On the other hand overall deprotonations of R-BISO3Na in micellar solutions gradually increased with increase of CTABr concentrations up to 10~2M and was followed by a gradual decrease, as found for BI1

Vol. 34, No. 6, 1990

632 洪永錫-朴禧鉉-朴漢錫

and R-BI3.

Concentrations of R-BISO3Na in Micellar Pseudophase. The concentrations of R- BI~SO3Na in aqueous and micellar solutions (car­

bonate buffer, pH 10.7) are calculated from their Ka values, and are shown in Table 1. Concentra­

tions in micellar pseudophase, [I^r were calculat­

ed from equation 2, where ^[兀] and [4^지are the concentrations of R-BI-S(^Na in aqueous and to­

tal reaction solutions, respectively. Values of the calculated KJ are given in Table 1.

[/>«一] = \J~ tot] ~^[丿叫] (2) The concentration ratios of R-BI~SO3Na and BI-SO3Na, [R-BU SO3Na]/[BI - S(&N ^히 in Table 2b, in the micellar pseudophase are much smaller than unity; for example, in 10-4 M CH3-BISO3Na solution the ratio is 0.45 and in 10-4 M heptyl-BISO3Na is 0.39, respectively.

First Order Rate Constants in CTABr Sol­

utions. To minimize perturbation of micellar structures, we used 6.66xl0~6Af p-NPDPP and 10-4 M R-BISO3Na solutions. And we as­

sumed that the cmc of CTABr in the reaction so­

lutions was 3xl0-4Af, instead of 8xlO~4M which is the cmc in pure water, because it was ex­

pected that the solutes decreased the cmc1- 3.

The values of k^_T for dephosphorylation of p-NPDPP in equation 1 are very small and can be neglected 0 爭=koH- + kR_r). Micellar ef­

fect upon the dephosphorylation of p-NPDPP are shown in Fig. 1, and the first order rate con- stants(A:申)increase with increase of CTABr con­

centrations, and reach maxima at ca. 8xl0~4 JW surfactant concentration, followed by a gradual decrease. These rate maxima are characteristics of bimolecular reactions of hydrophobic substrates and follow equation 3, where ^卩시 is concentration of micellized surfactants,^卩&] and [2Vm] are con­

centrations of nucleophiles in water and in micel­

lar pseudophase, Ks is binding constant to

Fig. 1. Micellar effects upon dephosphorylation mediated by R-BISO3Na; 6.66xlO-6A/ p-NPDPP, 1 x 10~4Af R-BISO3Na, carbonate buffer (pH 10.7), 30 ±0.2 °C.

CTABr, kw and km are second order rate cons­

tants, and 戒 and《are first order rate constants in aqueous and micellar pseudophase, respectively8.

L 知知KIMJ

kw=--- -- --- --- 1+K^丄切]

_ 氏园 ^+为차Ks

一 1+K皿- (3)

The mechanism of dephosphorylation of p- NPDPP mediated by BI- and R-BI- was ^안K)wn in previous studies1- % and we predict that the reactions mediated by R-BI~SO3Na also follow by this mechanism shown in Scheme 2, because the sul­

fonate group of the nucleophiles should not change the mechanism. However there are increases of overall reactivity of the nucleophiles, because of the negatively charged sulfonate group in positive­

ly charged Stern layer of micelle (cf. ref. 3 and Ta­

ble 2).

Joum이 of the Korean Chemical Society

계면활성제 용액속에서의 화학반응（제3^보） 633

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Vol. 34, No. 6, 1990

634 洪永錫-朴禧鉉•朴漢錫

Effect of R-BI-SOaNa Concentration in Ste­

rn Layer on the Reaction Rates 皿너 ^Steric^

E^ect. Dephosphorylations mediated by R- BI~SO3Na with R varried from methyl to heptyl in CTABr solutions (carbonate buffer, pH 10.7) are slower than that mediated by BI'SO3Na by fac­

tors of 0.14 to 0.25 fold. The larger binding cons­

tants (瓦)of substrate to micelle, generally lead to larger rate constants. However, rate constants mediated by R-BI~SO3Na decreased with increas­

ing Ka of the imidazoles as in the previous study of dephosphorylation of p-NPDPP mediated by R-BI~3 and hydrolyses of carboxylic esters by R BISO3Na4. We think that these decrea^욚卽 are due to one or two of the followings.

First, concentration of R-BI-SO3Na in 하｝e micellar pseudophase maybe only 0.14 to 0.25 fold of that of BI~SO3Na. We tried to determine these values to know whether or not thi용 assumption is correct.

Second, because of the bulky alkyl groups of R- BI~SO3Na, there is a steric effect, and the ^원teric hinderance could slow the reaction. And in addi­

tion alkyl groups of R-BrS03Na may penetrate into the core of micelle due to their hydrophobici­

ty and be relatively fixed at given position9. If this assumption is correct, the ability of R-BrSO3Na to attack p-NPDPP, as compared with that of BI'SOgNa, may be decreased of mobility of molecules. We tend to believe the second expla­

nation.

The relative rate constants^{麻_时眄品如「}

SO3Na in Table 2f) in the micellar pseudophase are decreased, as compared with the concentra­

tion ratios ([R-BI-SCLN^BTSQNa] in the pseudophase. For example, for IO-4 Mmethyl- BISO3Na the concentration ratio of the anions and the relative rate constant are 0.45 and 0.25 respectively (Table 2b and 2f). It is considered that the larger decrease of rate than that of concentration ratio is due to steric effect of the

Fig. 2. Relationship between alkyl groups of R-Bl- SOsNa and the first order rate constantratios 애妃BI-S6N그/性「ssNa) on dephosphorylations in aqueous and 8 x 10~4A/ CTABr solutions: carbonate buf­

fer (pH 10.7), 6.66x10-6“ 力 一 NPDPP, 1(尸3财 R-BISO3 Na, 30 ±0.2°C.

methyl group in Stem layer of CTABr micelle.

And, as shown in Fig. 2, same situation is occurred in all the reactions mediated by R- BI SO3Na with R varried from methyl to heptyl. However the rate-decrease followed by an in­

crease in the reactions mediated by k哗 alkyl- BI-SO3Na.

We assume that the difference (0.20) of the two ratios mentioned above is due to second effect, that is, a steric effect of ca. 44%. We de­

fine this steric effect (%) of alkyl groups of nu­

cleophiles as the equation 5, where A is theore­

tical first order constant-ratios (k R_Bi so3Na^

Brso3Na) which is equal to concentration-ra­

tios ([R-BI~S()3N히/[BI-SO3N히) in micellar pseudophase, and B is experimental rate constant-

steric effect(%) = 100(A—B)/A (5) ratios3. All the calculated steric effects are shown in Table 2g and Fig. 3. These steric effects are larg­

er when the alkyl group-lengths of R- BISO3Na are increased (from methyl to heptyl). This situa­

tion is similar to dephosphorylation3 of p-NPDPP mediated by R-BIL We proposed a model in the previous study3 and refered it as an "anchor ef­

fect^ of alkyl groups of the nucleophiles into micel-

Joumal of the Korean Chemical Society

계면활성제 용액속에서의 화학반응（제3^보） 635

Fig. 3. Calculated percentage%) of **Steric effect" in the whole decreasing of 知B广som그〃糸「soma) that are equal to [-BI_SO3Na]/[BI_SO3Na], B are experimental (AR-Bl-S6Na^件]-s6Na)•

lar core. This effect is characteristic of micellar reactions mediated by methyl to heptyl- BKSO3Na, (Table 2i), which is not observed in aqueous solution, and it means not due to differ­

ence of inherent reactivities of BI~SO3Na and R-BrSO3Na.

On the other hand, when we used nonyl to pentadecyl-BISO3Na as nucleophiles, the predict­

ed steric effect was sharply decreased (Fig. 3). This interesting appearance can be explained by micel- lization of R-BISO3Na with long alkyl group [se first order rate constants of the reactions in aque­

ous solution, not including CTABr, (Table 2h and Fig. 2) and cmc values in Table 1].

Nucleophilic and Micellar Effect of R-BI- SO3Na. R-BISO3Na with long alkyl groups (nonyl to pentadecyl) are good micellar forming sub­

stances (Table 1). The steric effects of anions of these compounds on dephosphorylation are shar­

ply decreased as compared with expected one (Ta­

ble 2g and Fig. 3). We conclude that this appearance is due to micellization of the nucleophiles (R-BI~SO3Na), and the reaction rates are increased. As shown in Table 2h, the first order rate constants of dephosphorylation medi­

ated by H- to heptyl-BI- SO3Na in aqueous solu­

tion without added CTABr micelle are very low and

ca, 10-5 sec-1.

However, the reactions mediated by nonyl- to pentadecyl-BI' SO3Na are increased by factors of 14 to 43 folds. It means that the expected large steric effect of long alkyl groups of R-BI_SO3Na is compensated by micellar effect of these nucleophiles. In other words, hydrophobic p- NPDPP comes into the Stern layer of micelles of R-BI" SO3Na and reacts with BI" moity of these functional micelles followed by an increase of the reaction rate. That is, R-BI~SO3Na with long alkyl groups have dual effects to accelerate the reaction rates by nucleophilicity of their BI- moi- ties and by micellar catalytic effect of these com­

pounds (R-BI~SO3Na with long alkyl groups act as imidazole-functional micelles in aqueous so­

lution).

Activation Parameter of the Reactions. We measured activation parameters of reaction in aqueous and micellar solution to compare inherent reactivities of anions of R-BI (R-BI ) and R- BISO3Na (R-BI'SO3Na) on dephosphorylation of p-NPDPP.

As shown in Table 3, activation enthalpies (△H*), free energies (AG*) of the reactions of p- NPDPP mediated by R-BI- and R-BrSO3Na in aqueous solutions, not including surfactant, are quite high. However, the values of the reactions in CTABr micellar solutions are r^이atively lower than values in aqueous solutions. It is consistent with increases of the reaction rates in micellar so­

lutions as compared with the reactions in aqueous solutions.

When we compard the parameters ^(厶矿and 厶G*) of the reactions mediated by R-BI- and R-BI-SO3Na in aqueous solutions, there are not much differences between the values of the reac­

tions by BI "and methyl-BI", and BI~SO3Na and methyl-BI- SO3Na. However, in micellar solutions there are large difference between the values. It means that the inherent reactives of BI" and R­

Vol. 34, No. 6, 1990

636 洪永錫•朴禧鉉•朴漢錫

BI- in water are almost equal, and the decrease of the reaction rates mediated by R-BI" (or R- BI~SO3Na) as compared with those by BI~ (or BI~SO3Na) are due to an interaction of R group with micelle, a steric effect.

On the other hand, when we compared the

△H* values of the reactions mediated by R- BI~ and R-BI~SO3Na in micellar solutions, ones having longer group (methyl to butyl) gradually leveled the parameters up. It is consistent with gradual decreases of the reaction rates by chang­

ing of R groups of the nucleophiles from methyl to heptyl (Fig. 1 and Table 1 and 2). And AS*

values of the reactions by methyl-BI~ in the micellar solutions were sharply increased as com­

pared with those of the reaction by BI~, while

△ G* values were not much increased. The in­

creased 4S* values in the micellar reactions indi­

cate that the reactants require more ordering in going to the transition states than ^나ley would do in the uncatalyzed reactions, in same way of en­

zymatic reactions10. Thus, the mechanism of the enzymatic10 and micellar11 reactions would impose upon the reactants its own orientation and steric requirements, a different means of achiving spat- ical proximity of the reactants, and its own solvent and concentration effects.

However, in the reactions mediated R-BI SO3Na with long alkyl groups, the 4H* val­

ues gradually decreased by changing the alkyl groups from nonyl to pentadecyl, and it is consis­

tent with gradual increases of the reaction rates (Table 1 and 2) due to abilities of micellization of R-BrSO3Na.

ACKNOLOWLEDGEMENT

This work was supported by the Bisa-

Research-Fmd of Keimyung University, Yong- Seuk Hong is greatly appreciate it.

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Journal of the Korean Chemical Society