83

CHAPTER III: NUCLEOPHILIC DEPHOSPHORYLATION

OF p-NITROPHENYL DIPHENYL PHOSPHATE IN

CATIONIC MICELLAR MEDIA#.

3.0 INTRODUCTION

The hydrolysis of phosphorous triesters has been proved as classical

reactions of fundamental importance in chemistry and biology1-12

. The widespread

use of toxic phosphates and phosphonates as insecticides, and their use as chemical

weapons, has led to the investigation of fast detoxification and decontamination

methods13

. Most of these compounds are hydrophobic phosphorous (V) esters or

phosphorylating agents. Such compounds are highly toxic to both target and non-

targeted organisms.

As chemical weapons cannot be used by raw hands due to safety and

licensing problems, a well known non-toxic and widely used simulant p-nitrophenyl

diphenyl phosphate (PNPDPP) (I) was employed in the present study.

O

O

P

O

O

NO2

(I)

Over the past years, many research groups14-32

have been used this

simulant for dephosphorylation reaction using different nucleophiles (Table 3.1).

However, there is still need to design and develop new reagents for detoxification. In

the preceding chapter, the nucleophilic substitution reactions of carboxylate ester

have been discussed. In this chapter, a study of the dependence of rate on a series of

-nucleophiles (hydroxamate ions) for the reaction of hydrolysis of PNPDPP have

been undertaken.

# Part of this chapter has been published in Langmuir, 2005, 21, 8664-8669.

84

3.1 REVIEW OF THE EARLIER WORK

Many research groups14-32

have carried out extensive studies on

phosphate ester hydrolysis over the past several years. A partial summary of the

hydrolysis / detoxification of PNPDPP using different nucleophiles are presented in

Table 3.1. A few distinguishing features of this survey are as under:

I. In view of the recognized ability of cationic surfactant micelles to accelerate the

cleavage of carboxylic acids, micellar catalyzed hydrolysis of p-nitrophenyl diphenyl

phosphate (PNPDPP) have been studied.

II. PNPDPP has become the unofficial “standard simulant” permitting comparisons

of the efficacy of many different cleavage reagents.

III. Among the many reagents, the iodosocarboxylates (iodosobenzoate and

iodosonaphthoate) and certain metallomicelles stand out as characterized by rapid

cleavage of PNPDPP and catalytic turnover55

.

IV. The oximate ions (2,3-butanedione monoximate etc.) are believed to act as -

effect super nucleophiles for the cleavage of PNPDPP in the presence of cationic

micelles56

.

V. The CTAB / ClO– system can also work efficiently as turnover catalyst to

decontaminate PNPDPP and other toxic chemical warfare agents21

.

VI. Bhattacharya et al.22-25

developed many effective nucleophiles like

monoperoxyphthalates, dialkylaminopyridine, 1-hydroxytriazoles and tetrazole derivatives

for dephosphorylation reactions.

85

Table – 3.1

Esterolytic Systems for the Cleavage of p-nitrophenyl diphenyl phosphate

PO

O

O O NO2

[CATALYST] (M) CONDITION kobs (s–1) REF.

I

C

O

O

O-

o-iodoxybenzoate

(1.0x 10–4)

1 mM CTACl

pH 8.0

6.45 x 10 –2

14

I

O

O

O-

o-iodosonaphthoate (1.0x 10–4)

5 mM CTACl

pH 8.0

0.26

15

C16H33N+Me2(CH2)2OH

(2.5 x 10-5)

0.01 M NaOH

pH 12.0

≈ 1.5

16

Cu (II) Complex of N, N, N’-trimethyl-

N’-tetradecylethylenediamine

(1.22 x 10–4)

10 mM

N-ethylmorpholine

2.02 x 10-2

17

N+

CHO

H3C

H3C

C12H25

( 8 x 10–3)

pH 9.0

2.00 x 10-2

18

Continued…

86

[CATALYST] (M)

CONDITION

kobs (s–1)

REF.

CH3CO

CH3

C NO

Butanedione monoximate

(7.5 x 10 –4 M)

pH 9-10

CTAX

0.12

19

C NO

C

H

O

CH3

anti- Pyruvaldehyde-1-oximate

[CTAOX] = 0.01 M

–

56

ClO –

Hypochlorite

pH 8.5

32°C

1.75mM CTABr

20.1 x 10–3

21

N

N

RH3C

(Dialkylamino) pyridine

O

║

R = (CH2)2CNH(CH2)3Me2N+C18H37

N,N-Dimethyl-N-octadecyl-N-[[3-[methyl(4-

pyridyl)amino]propanamido]propyl]ammonium

bromide.

pH 9.0

0.5 mM CTACl

≈ 8.5 x 10–3

22

O

HOO

COOH

Monoperoxyphthalates

(5.4x 10–4)

pH 8.5

1 mM CTACl

2.4 x 10–2

23

87

N

N

N

OH

1-Hydroxybenzotriazoles

(2.5 x 10 –5)

pH 8.2

0.01M CTAB

3.2 x 10 –3

24

N

N

N

OHOH

O

1-hydroxy-1-H-benzo[d] [1,2,3] triazole-

6-

carboxylic acid

pH 8.0

2.5 mM

Me2N

+—(CH2)m—N+–Me2

│ │

C16H33 C16H33

15.2 x 10 –3

25

NH

N

NN

H

Tetrazoles

(1.25 x 10-4 M)

pH 7.0

30.7 x 10–3

26

# The concentrations of catalysts are given in the parentheses.

88

VII. Besides these nucleophiles, many other inorganic oxygen containing

-nucleophiles (NH2OH, BrO–, HOO

–, F

– ions) covering the pKa range from –2 to

13.81 have been used for phosphate esters31,32

.

3.2 PRESENT INVESTIGATION

This chapter describes the systematic study of the hydrolysis of

PNPDPP by different hydroxamate ions (II) using cationic surfactants. The

hydroxamate ions are believed to act as -nucleophiles and are good deacylating and

dephosphorylating agents42-49

. The -nucleophiles possess lone pair of electrons on

an atom adjacent to the nucleophilic centre and show remarkable reactivity as

compared to normal nucleophile (k -nuc / k normal nuc.) with comparable pKa values.

N O

C O

R

R'

(II)

The extent of micellar catalysis is expected to be dependent on the

relative amount of substrate incorporated into the micelles. PNPDPP being highly

lipophilic gets more incorporated into the micelles. Due to biological and

environmental significance, the detoxification reaction is one of the essential aspects.

This chapter reports the esterolytic cleavage of p-nitrophenyl diphenyl phosphate

(PNPDPP) (Scheme I) by different hydroxamic acids (Table 3.2) using cationic

surfactants (Table 3.3).

Biomimetic models such as micelles23,29,33

, reverse micelles34

,

microemulsions35,36

, cyclodextrins13

, liposomes and vesicles37

have been used to

accelerate reactions of phosphate esters with nucleophiles. The quantitative

explanations of reactivity in micelles have been attempted using pseudophase model.

89

Table 3.2

Hydroxamic Acids Used in Present Investigation

R' C N

O OH

R

HYDROXAMIC

ACID

R’ R STRUCTURAL

FORMULA

MOL.

WT.

Acetohydroxamic Acid

(AHA)

CH3

H

C

N

O

OH

H3C

H

75.01

Benzohydroxamic Acid

(BHA)

C6H5

H

C

N

O

OHH

137

Salicylhydroxamic Acid

(SHA)

2-OHC6H4

H

C

OH

N H

O

HO

153.1

N-Phenylbenzo-

hydroxamic acid

(PBHA)

C6H5

C6H5

C

O

NOH

213

p-Fluoro-N-Phenyl-

benzohydroxamic acid.

(p-F-PBHA)

p-F- C6H4

C6H5

C

O

NOH

F

230

90

Table 3.3

Cationic Surfactants Used in Present Investigation.

RN+R’X

–

SURFACTANT R R’ X– STRUCTURE M.W.

Cetyltrimethylammonium

bromide (CTAB)

C16H33 (CH3)3 Br–

N+Br

364.5

Cetyltrimethylammonium

chloride (CTACl)

C16H33

(CH3)3

Cl–

N+Cl-

320

Tetradecyltrimethylamm-

onium bromide (TTAB)

C14H29

(CH3)3

Br–

N+Br-

336.4

Dodecyltrimethylammo-

nium bromide (DTAB)

C12H25

(CH3)3

Br–

N+

Br-

308.3

Cetylpyridinium bromide

(CPB)

C16H33

C5H5

Br–

N+Br-

384.4

Cetylpyridinium chloride

(CPC)

C16H33

C5H5

Cl–

N+Cl

358

Cetyldimethylethylammo

-nium bromide (CDEAB)

C16H33

(CH3)2

C2H5

Br–

N+Br-

378.5

91

P

O

O NO2 + Nu NO2OP

O

Nu +

Scheme- I

3.3 EXPERIMENTAL

Materials

N-Phenylbenzohydroxamic acid, p-fluoro-N-phenylbenzohydroxamic

acid and benzohydroxamic were prepared by literature method38

.

Salicylhydroxamic acid, acetohydroxamic acid, cetyltrimethylammonium

bromide, tetradecyltrimethylammonium bromide, dodecyltrimethylammonium

bromide, cetylpyridinium bromide and cetylpyridinium chloride were obtained

from Sigma /Aldrich. p-Nitrophenyl diphenyl phosphate was prepared at Defence

Research Development Establishment, Gwalior (Scheme II) by condensation of

diphenyl chlorophosphate with p-nitrophenol in the presence of triethylamine.

P

O

O

O

O Cl HO NO2+

Diphenyl chlorophosphate

p-nitrophenol

Triethylamine

P

O

O

O

O NO2

p-nitrophenyl diphenyl phosphate

Scheme II

A sample of PNPDPP was also obtained from Prof. C. A. Bunton, University of

California, USA.

Solution Preparation

The buffer (Borate 0.01M) solutions were prepared by using single

distilled water. Due to solubility problem, N-substituted hydroxamic acids (PBHA

and p-F-PBHA) were prepared in 20% (v/v) acetonitrile-water mixtures. The

92

solution of PNPDPP was prepared in 50% (v/v) acetonitrile. The pH of the

reaction medium was measured by using Systronics 335 pH meter.

Kinetics

All the reactions were followed at 27 0C ± 0.2

0C with Unicam

UV2-300 spectrophotometer equipped with Techne circulator (C-85A)

thermostated cell holder. The rate of nucleophilic reaction with PNPDPP was

determined by following the increase in absorption of p-nitrophenoxide anion

(400 nm). All the kinetic experiments were performed at an ionic strength of 0.1

M (with KCl). Buffer employed for whole reaction process was borate. For all the

kinetic runs the absorbance/time result fit very well to the first-order rate equation:

ln (A∞ -At) = ln (A∞ -A0) – kt (1)

The pseudo-first order rate constants can be determined by least

square fits. A progressive reaction profile is shown in Figures 3.1 and 3.2. The

spectrum exhibits an increase in absorbance at 400 nm with the formation of

p-nitrophenoxide ion during the course of reaction. The pKa values of hydroxamic

acids were determined pH meterically using Systronics (type-335) pH meter.

3.4 RESULTS AND DISCUSSION

Pseudo-first order rate constant for the reaction of p-nitrophenyl

diphenyl phosphate with hydroxamate ions (Scheme I) have been determined at 27° C

in 4 % (v/v) MeCN aqueous media with the nucleophiles in large excess over the

substrate. pH dependent rate constant increased with increasing pH in the range 6.6 -

11.0 pH. The rate of reaction shows drastic change at the pH where the deprotonation

of hydroxamic acid was found maximum, i.e. pKa of hydroxamic acid (Table 3.4).

The pKa of all the hydroxamic acids were determined in the presence and absence of

93

Figure 3.1 Repeat scans (1-15) every minute for the reaction of PNPDPP with PBHA in

CTAB (1.8mM) micellar media at 8.0 pH.

Figure 3.2 Repeat scans (1-20) every minute for the reaction of PNPDPP with BHA in

CTAB (2.74 mM) micellar media at 9.0 pH.

WAVELENGTH (nm)

AB

SOR

BA

NC

E

WAVELENGTH (nm)

AB

SOR

BA

NC

E

94

Table 3.4

pH-dependent pseudo-first order rate constants for the nucleophilic substitution reaction of

p-nitrophenyl diphenyl phosphate with N-phenylbenzohydroxamate (PBHA–) ion in micellar

solution at 270 C.

pH kobsd. 103/s–1

6.6 0.26

7.3 0.92

8.0 1.81

8.5 4.0

10.0 9.0

11.0 10.5

REACTION CONDITIONS

Temp. = 27 °C, = 0.1 M KCl, [PNPDPP] = 0.5 x 10– 4 M, [PBHA] = 0.5 x 10–3 M,

[CTAB] = 1.8 x 10–3 M, Medium = 4% (v/v) MeCN.

CTAB (Table 3.6). The effect of cationic surfactants on the pKa was not significant.

The pKa value, thus determined under micellar condition agreed with the value

determined pH-metrically in 10% (v/v) MeCN medium.

3.4.1 Effect of pH and Determination of pKa

The pH-rate constant profile for the reaction of PNPDPP with

N-phenylbenzohydroxamate ion in cationic micellar solution is typical of pH-

dependent nucleophilic reaction. Hydroxamic acids have been suggested to behave

either as NH or OH acids depending on solvents39-41

. Numerous studies indicate that

hydroxamic acids are OH, rather than NH acids in H2O39

. It is known that the anion

95

of hydroxamic acid (N–O–) acts as a reactive species in the hydrolysis of esters

41-43.

Consequently, the pKa for the conversion of the N–OH to N–O– form play important

role for the cleavage of phosphate esters.

A pH-rate constant profile for the nucleophilic cleavage of 0.05 mM

PNPDPP by 0.5 mM hydroxamate ion in CTAB micellar media (1.8 mM) gave the

apparent pKa values for each hydroxamic acids. Typically, the pseudo-first-order rate

constants for the reaction of PNPDPP were determined at different pH values in

between 6.7 to 11.0. The representative pH-rate constant profile for the cleavage of

0.05 mM PNPDPP by 0.5 mM N-phenylbenzohydroxamic acid in CTAB micellar

media (1.8 mM) at 270 C is shown in Figure 3.3. The plot of log kψ vs pH (Figure 3.3)

gave a break at pH 8.9 which was taken as a systematic pKa for the PBHA under

CTAB micellar conditions.

3.4.2 Effect of Nucleophile Concentrations

In order to investigate the nucleophilic catalysis of hydroxamate ions

for the decomposition of organophosphate, the reaction of PNPDPP in the

presence and absence of hydroxamate ions have been studied. By comparison, the

observed pseudo-first-order rate constant in the presence of hydroxamic acids

(kobs) and in buffer alone (k0), it is apparent that the addition of hydroxamic acids

under these conditions increases the rate of nucleophilic reaction of PNPDPP

significantly.

The nucleophile concentration dependent first order rate constant was

determined for the reaction of PNPDPP with hydroxamic acids in excess. Table

3.5 summarizes the data for the reaction of PNPDPP with different concentration

of N-phenylbenzohydroxamate ion at pH 9.1. Kinetic data provides additional

support for the hypothesis that hydroxamic acid is acting as a nucleophilic

96

Figure 3.3 Plots of first-order rate constants vs. pH for the reaction of

PNPDPP with N-phenylbenzohydroxamate ion.

catalyst for the reaction of PNPDPP. Equation (2) describes the reaction of

PNPDPP with nucleophiles, k0 defined in equation (3) correspond to the intercept

in the kobs versus [Nu] plot. The kH2O term may assume some significance for very

weak nucleophiles and at very low OH–

concentrations. At high pH the intercept,

got dominated by the kOH

– term.

kobs

= k0 + k

Nu [Nu] (2)

k0 = kH2O

+ kOH–[OH

– (3)

Plotting kobs vs. [Nu–] gave a straight line (Figure 3.4) with intercept k0. This

indicates that competition with other nucleophiles i.e. OH–

and H2O is not expected

and hydroxamate ions are very strong -nucleophiles41,43-49

for the nucleophilic attack

at the P center of PNPDPP and kobs is simply given by kobs=kNu

[Nu].

0

2

4

6

8

10

12

6 7 8 9 10 11 12

pH

ko

bs.

103/s

-1(A

)

-4

-3

-2

-1

0

(A)

(B)

log

kψ

(B

)

97

Table 3.5

Nucleophile concentration dependent first-order rate constants for the reaction

of PNPDPP with N-phenylbenzohydroxamate ion in micellar media.

[PBHA] x 10–3

M kHA

obsd.103/s–1

0.0 0.19

0.25 4.30

0.50 7.75

0.75 11.2

1.00 13.8

REACTION CONDITIONS

Temp.= 27oC, pH = 9.12, μ = 0.1 M KCl, [PNPDPP] = 0.5 x 10–4 M , [CTAB] = 1.80 x 10–3 M

0

2

4

6

8

10

12

14

16

0 0.25 0.5 0.75 1 1.25

[PBHA] 10-3

M

kH

Aobsd.1

03/s

-1

Figure 3.4 Kinetic plot of kHA

obs.103/s

–1 versus concentration of PBHA in the

micellar media.

98

3.4.3 Kinetic Studies in Micelles

The rate constant data for the nucleophilic substitution reaction of

PNPDPP with N-phenylbenzohydroxamate ion at 9.12 pH are given in Table 3.6. The

results follow typical biphasic pattern. Cationic micelle catalyzed the reaction and kψ

passed through maxima with increasing surfactant concentration. The rate maxima are

independent of the type of surfactants but the magnitude of rate constant depends on

type of surfactants. The rate-surfactant concentration profiles obtained with various

surfactants / catalysts are characteristic of micelle catalyzed reaction50

. The variation

increasing alkyl chain lengths of the surfactants, i.e. with increasing aggregation

number of micelle. The rate constants below the cmc is difficult to quantify due to

reactant induced micellization and interaction with non-micellized surfactants. The

reaction was slightly faster when the counterion was chloride than bromide ion

(Table 3.6). The fractional ionic dissociation, α, of micelle is often little affected by

the nature or the concentration of the counterion. In other words, the micellar surface

appears to be saturated with counterions, and the fractional coverage β = 1 – α, is

constant. If β is constant, the rate of reaction should increase as substrate is taken up

by the micelles, but once substrate is fully bound, the rate should be independent of

added surfactant or counterion.

Kinetic results indicate that CTAB is more reactive than TTAB. The

effect of DTAB is insignificant. The kψ values increase with increase in the order is

mainly due to the increase in electrical surface potential of the micelle and partially

due to an increase of hydrophobicity of palisade layer of micelle. The hydroxamate

ion concentration in the vicinity of the micellar surface is expected to increase with

increasing the aggregation number. The electrostatic attraction of the cationic head

groups of the surfactants at the micelle surface to the nucleophilic anion counterions

leads to augmentation of the local concentration of the nucleophile, while

99

Table 3.6

Kinetic rate data for the nucleophilic substitution reactions of p-nitrophenyl diphenyl phosphate with

N-phenylbenzohydroxamate ion in cationic micellar solutions.

[Surfactant]

x 103 / M

kobs. x 103 / s

–1

CPC CPB CTACl CTAB CDEAB TTAB DTAB

0 0.13 0.13 0.13 0.13 0.13 0.13 0.13

0.36 8.50 6.98 6.15 4.09 5.70 3.70 –

0.90 8.90 8.50 7.00 7.46 6.60 4.70 –

1.80 9.33 9.13 7.68 7.73 7.24 5.22 0.40

3.60 9.20 8.85 7.40 7.22 6.90 5.16 –

5.40 8.40 8.10 6.90 5.98 6.10 4.75 0.90

7.00 7.61 7.30 6.10 4.85 5.30 4.45 –

8.00 7.00 6.70 5.70 4.10 4.80 4.25 –

9.00 6.40 6.10 5.22 3.32 4.31 4.13 2.12

REACTION CONDITIONS:

pH = 9.12, [PNPDPP] = 0.5 x 10– 4 M, [PBHA] = 0.5 x 10–3 M, Medium = 4.0% MeCN, μ = 0.1 M KCl

100

incorporation of the substrate in the micelle leads to a higher local concentration of

the substrate50-53

.

This enhanced concentration of the reactants accounts for the higher rate

of reaction. Implicit in this explanation is the requirement that the reactive site of the

PNPDPP be situated in close proximity to the nucleophile, that is, at the micelle-water

interface, the Stern layer. The subsequent addition of the cationic surfactant after cmc

caused a decrease in the reaction rate possibly due to the decrease in the catalyst /

reagent concentration in the micellar pseudophase. The excess of unreactive

counterions (X–) compete with hydroxamate ions for available sites in the stern layer.

The rate of the nucleophilic reaction of anionic nucleophile depends on

the binding of the substrate molecule hydrophobically and electrostatically attraction

of anionic nucleophlies in to the micelle50,54

. Kinetic rate data for the reaction of

PNPDPP with various hydroxamate ions in micellar solution of cetylpyridinium

bromide shows that the rate of reaction increases with increasing hydrophobicity of

the nucleophiles. By comparing the reaction rate of N-substituted and unsubstituted

hydroxamate ions in aqueous media (Table 3.7), it can be concluded that the micellar

system shows differential reactivity than bulk aqueous media.

Table 3.7 shows the rate data for the reaction of PNPDPP with various

hydroxamate ions of different hydrophobicity. In aqueous media, reactivities of

p-F-PBHA and PBHA are comparable and slightly lower than SHA, BHA and AHA.

In aqueous micellar media p-F-PBHA and PBHA shows higher reactivity than SHA,

BHA and AHA. The solubility of PBHA and PNPDPP in pure buffered aqueous

solution was quite low. However, they are readily soluble in CTAB micelles. Since

these hydrophobic substrates also partition into the micellar pseudophase, increased

localization of catalysts and substrates lead to rate acceleration in the cleavage of

phosphate esters.

101

Table 3.7

Kinetic rate data for the reaction of p-nitrophenyl diphenyl phosphate with hydroxamate ions in cationic

micellar solutions of cetylpyridinium bromide at 270 C

a.

[CPB] / mM

kobs. x 103/s–1

p-F-PBHA PBHA SHA BHA AHA

(8.7)b (8.9)

b (7.2)

b (8.6)

b (9.2)

b

0 0.10 0.13 0.19 0.15 0.12

0.36 7.25 6.98 0.72 0.52 –

0.90 8.71 8.50 1.20 0.81 0.50

1.80 9.60 9.13 1.59 1.28 0.70

3.60 9.30 8.85 1.50 1.19 –

5.40 8.30 8.10 1.41 1.00 0.65

7.00 7.10 7.30 1.10 0.80 –

8.00 6.30 6.70 0.91 0.75 –

9.00 5.44 6.10 0.73 0.71 0.63

REACTION CONDITIONS:

pH = 9.12, [HA] = 0.5 x 10-3 M, [PNPDPP] = 0.5 x 10–4 M, a

μ = 0.1 M KCl .

b The pKa of nucleophiles in micelles are given in parentheses.

102

Similar observations have also been made for the reaction of Paraoxon

with hydroxamate ions42

. Paraoxon is less hydrophobic than PNPDPP therefore the

reactivity of hydroxamate ions in micellar solution is not significant (data not shown).

The hydrolysis reaction for PNPDPP proceeding via the steps outlined

in Scheme III. A point of interest was whether the catalysts were consumed as the

reaction proceeded or were regenerated and indeed a true catalyst. When the

concentration of PNPDPP was in large excess, the release of p-nitrophenoxide ion

also followed pseudo-first order kinetics and yielded a consistent kobs value. With

such a small proportion of the catalyst, the kinetics could only be first order if the

catalyst was not consumed during the course of reaction. There is no direct

experimental evidence for the complete regeneration of hydroxamic acids. Other

α-nucleophiles like o-iodosylcarboxylates55

, oximate56

, hydroperoxide57

and

hydroxybenzotriazoles24,25

rapidly cleaved phosphate esters with turnover in micelle.

P

O

PhO

PhO

O NO2 R' C N

O O

R+ P

O

PhO

PhO

ONO2Nu +

+ H2O

OHNO2P

PhO

PhOO

O

++Nu

Scheme- III

103

3.4.4 Quantitative Treatment of Rate Data in Micelles

The Pseudophase Model:

Rate effects on bimolecular reactions in association colloids are

rationalized by the pseudophase model51,58

, in which the aggregates and bulk solvent,

typically water, are regarded as distinct reaction regions. The overall reaction rate is

the sum of the rate in each pseudophase and depends upon the rate constants and

reactant concentrations in each pseudophase. A crucial requirement of this model is

that component distributes them much faster than the reaction.

The influence of cationic micelles on the reaction rate can be

quantitatively analyzed according to the model of the micellar pseudophase59,60

. We

assume that the presence of cationic surfactant does not change the pKa of the

hydroxamic acids in water. Under these experimental conditions, scheme IV can be

proposed for applying pseudophase model.

In this scheme IV subscript w and m indicates aqueous and micellar

pseudophases, respectively and Dn represents the micellized surfactant, that is

[Dn] = [DT] – cmc, where [DT] is the stoichiometric surfactant concentration and cmc

the critical micellar concentration, obtained under the experimental conditions as the

minimum surfactant concentration required to observe any kinetic effect.

Scheme IV considers the distribution of PNPDPP between the aqueous

and micellar pseudophases, KmPNPDPP

. This association constant of PNPDPP in

micellar systems with the value of KmPNPDPP

= 7000 M–1

, is agreed with literature

value56

. The distribution of the hydroxamate ion, HA, between both pseudophases is

considered through the distribution constant KmHA

. The different reactivities in the

104

aqueous and micellar pseudophases have been taken into account through the

corresponding second order rate constants: k2w and k2

m. The values of k2w have been

obtained by studying the reaction in the absence of the surfactants.

PNPDPP Dn+w PNPDPPm

+

HAmHAm + Dn

PRODUCTSk2mk2w

Km PNPDPP

Km HA

The hydroxamate concentration in the micellar pseudophase has been

defined as the local, molar concentration within the micellar pseudophase: [HA]T =

[HA]m / [DnV], where V is the molar volume in dm3 mol

–1 of the reaction region and

[Dn]V denotes the micellar fractional volume in which the reaction occurs. V was

assumed equal to the partial molar volume of the interfacial reaction region in the

micellar pseudophase, determined by Bunton59

as 0.14 dm3 mol

–1. Micellar binding

of both substrate, PNPDPP and hydroxamate ions HAs, is governed by hydrophobic

interactions and the equilibrium constants KmPNPDPP

and KmHA

are expressed by

referring these concentrations to the total volume of the observed rate constants, kobs,

based on scheme IV and on the above considerations, is given by the following

equation:

Scheme-IV

105

kobs. =

k2w

+k2

m

VKm

PNPDPPKm

HA[Dn]

Km

PNPDPP( 1 + [Dn] ) Km

HA( 1 + [Dn] )

[HA]T

Second order rate constants at the micellar interface and association

constants of the hydroxamate ions to the cationic micelles were obtained by fitting

equation 4 to the experimental data and listed in Tables 3.8 and 3.9. In Figures 3.5 and

3.6, the kψ calculated values with this treatment are shown by solid lines.

The results presented in Table 3.8 allow us to study the influence of the

nature of the micelle for the reaction of PNPDPP with N-phenylbenzohydroxamate

ion. From the fitting equation 4, k2w = 0.13 x 10

–3 M

–1 s

–1 was obtained in aqueous

medium. Likewise we obtained value of k2m

= 4.20 x 10–2

M–1

s–1

, for the highly

reactive CPB-PBHA combination. The CPB-PBHA system shows 323 fold micellar

catalysis (k2m

/ k2w) for the cleavage of PNPDPP.

A very important aspect to take into account when dealing with

nucleophilic nucleophilic reactions in micelles is the incorporation of different

nucleophiles into the micelle. Table 3.8 lists the substrate and nucleophile distribution

constants in cetylpyridinium bromide micelle. As stated earlier, N-substituted

hydroxamic acids are more hydrophobic, associate with CPB micelle through

hydrophobic interactions (Kmp-FPBHA

= 86.2 M–1

; KmPBHA

= 73.7 M–1

). The N–OH

groups are considerably ionized as N–O– at pH 9.1 and therefore also bind to

quarternary ammonium headgroup through electrostatic attractions. N-substituted

hydroxamate ions, p-F-PBHA and PBHA shows 351 and 323 fold micellar catalysis

(k2m

/k2w) towards reaction of PNPDPP, whereas SHA, BHA, and AHA shows around

36, 44 and 31 fold catalysis respectively.

(4)

106

Figure 3.5 Simulated rate-surfactant profiles for the reaction of p-nitrophenyl diphenyl

phosphate with N-phenylbenzohydroxamate ion (solid lines are predicted

values with model).

Fig.3.6 Simulated rate-surfactant profiles for the reaction of p-nitrophenyl

diphenyl phosphate with hydroxamate ions in cetylpyridinium bromide

micellar solutions (solid lines are predicted values with model).

○ CPC

● CPB

■ CTACl

■ CTAB

▲ CDEAB

▲ TTAB

○ p-F-PBHA

● PBHA

■ SHA

■ BHA

▲ AHA

k

ob

s. s

–1

ko

bs.

s–1

107

Table 3.8

Kinetic parameters obtained by applying pseudophase model for the Nucleophilic reaction of PNPDPP with

N-phenylbenzohydroxamate ions in the presence of cationic micelles.

wk2 /(M-1

·s-1

) PNDPP

mK /(M-1

) HA

mK /(M-1

) mk2 /(M-1

·s-1

)

PBHA (CPyCl) 0.13x10-3

7000 86.2 (3.930.15)x10-2

PBHA (CPyBr) 0.13x10-3

7000 73.7 (4.200.11)x10-2

PBHA (CTACl) 0.13x10-3

7000 73.4 (3.560.10)x10-2

PBHA (CTAB) 0.13x10-3

7000 118.6 (2.160.22)x10-2

PBHA(CDEAB) 0.13x10-3

7000 96.8 (2.590.09)x10-2

PBHA (TTAB) 0.13x10-3

7000 43.9 (3.790.07)x10-2

108

Table 3.9

Kinetic parameters obtained by applying pseudophase model for the Nucleophilic reaction of PNPDPP with

hydroxamate ions in the presence of CPB (Cetylpyridinium bromide) micelles.

wk2 /(M-1

·s-1

) PNDPP

mK /(M-1

) HA

mK /(M-1

) mk2 /(M-1

·s-1

)

p-F-PBHA 0.10x10-3

7000 94.1 (3.510.17)x10-2

PBHA 0.13x10-3

7000 73.7 (4.200.11)x10-2

SHA 0.19x10-3

7000 67.5 (6.780.92)x10-3

BHA 0.15x10-3

7000 50.5 (6.620.93)x10-3

AHA 0.12x10-3

7000 5.2 (3.670.47)x10-2

109

The study was undertaken with a view to develop a hydroxamate

function based ‘hydrolysing nucleophile’ in micellar medium to detoxify the toxic

phosphorus esters. For this purpose, PNPDPP was selected as simulant of nerve

agents as huge kinetic data are available in open literature on this substrate with other

nucleophiles, which could be compared with the data obtained with hydroxamic acids

(HAs). To achieve this target, the HAs and surfactants with varying structures were

selected for kinetic study and the best combination of ‘HA-Surfactants’ was

formulated as potential esterolytically detoxifying system against nerve agents.

110

REFERENCES

1. C. Cope, S. Karanth and J. Liu, Environ. Toxicol. Pharmacol., 2005, 19,

433.

2. M. S. Yargan and A. Tarik, Environ. Int., 2005, 32, 687.

3. J. P. Walker and S. A. Ascher, Anal. Chem., 2005, 77, 1596.

4. E. Longhinotti, J. B. Domingos, B. Szpoganicz, A. Neves and F. Nome,

Inorg. Chim. Acta, 2005, 358, 2089.

5. L. M. Schopfer, T. Voelkar, C. F. Bartels, C. M. Thompson and

O. Lockridge, Chem. Res. Toxicol., 2005, 18, 747.

6. E. D. Lonsihlio, A. Meneguez and E. Testai, Toxicol. Appl. Pharmacol.,

2005, 205, 237.

7. W. N. Sawaya, F. A. Al-Awadhi, T. Saeed, A. Al-Omar, A. Husain,

N. Ahmed, H. Al-Omirah, S. Al-Zenki, S. Khalafani, J. Al-Otaibi and

H. Al-Amiri, Food Chem., 2000, 69, 331.

8. X. Ji, J. Zheng, J. Xu, V. K. Rastogi, T-C Cheng, J. J. DeFrank and R. M.

Leblanc, J. Phys. Chem.,2005, 109, 3739.

9. J. B. Domingos, E. Longhinotti, V. G. Machadoc and F. Nome, Quin

Nova, 2003, 5, 745.

10. X. Cao, M. Mabrouki, S. V. Mello, R. M. Leblane, V. K. Rastogi,

T. C. Cheng and J. J. DeFrank, Colloids & Surfaces B: Biointerfaces,

2005, 40, 75.

111

11. J. F. Reichwein and B. L. Pagenkopf, J. Org. Chem., 2003, 68, 1459.

12. F. Tafesse, Inorg. Chim. Acta., 1998, 269, 287.

13. R. A. Moss and P. K. Gong, Langmuir, 2000, 16, 8551.

14. R. A. Moss, B. Wilk, K. Krogh-Jespersen, J. T. Blair and J. D. Westbrook,

J. Am. Chem. Soc., 1989, 111, 250.

15. R. A. Moss, A. T. Kotchevar, B. D. Park and P. Scrimin, Langmuir, 1996,

12, 2000.

16. C. A. Bunton and L. G. Ionescu, J. Am. Chem. Soc., 1973, 95, 2912.

17. F. M. Menger, L. H. Gan, E. Johnson and H. D. Durst, J. Am. Chem. Soc.,

1987, 109, 2800.

18. F. M. Menger and L. G. Whitesell, J. Am. Chem. Soc., 1985, 107, 107.

19. N. Ouarti, A. Marques, I. Blagoeva and M. F. Ruasse, Langmuir, 2000, 16,

2157.

20. N. Ouarti, I. Blagoeva, O. A. El Seoud and M. F. Ruasse, J. Phys. Org.

Chem., 2001, 14, 823.

21. D. K. Dubey, A. K. Gupta, M. Sharma, S. Prabha and

R. Vaidyanathaswamy, Langmuir, 2002, 18, 10489.

22. S. Bhattacharya and K. Snehalatha, Langmuir, 1995, 11, 4653.

23. S. Bhattacharya and K. Snehalatha, J. Org. Chem., 1997, 62, 2198.

24. V. P. Kumar, B. Ganguly and S. Bhattacharya, J. Org. Chem., 2004, 69,

8634.

112

25. S. Bhattacharya and V. P. Kumar, Langmuir, 2005, 21, 71.

26. S. Bhattacharya and P. K. Vemula, J. Org. Chem., 2005, 70, 9677.

27. C. A. Bunton, N. D. Gillitt and A. Kumar, J. Phys. Org. Chem., 1997, 10,

221.

28. C. A. Bunton, M. M. Mhala, J. R. Mofatt, D. Monarres and G. Savelli,

J. Org. Chem., 1984, 49, 426.

29. C. A. Bunton, Y. S. Hong, L. S. Romsted and C. Quan, J. Am. Chem. Soc.,

1981, 103, 5784.

30. H. Oumar-Mahamat, H. S. Tilk and R. S. Brown, Can. J. Chem., 1999, 77,

1577.

31. Yu. S. Simanenko, T. M. Prokop’eva, A. F. Popov, C. A. Bunton, E. A.

Karpichev, V. A. Savelova and K. K. Ghosh, Russ. J. Org. Chem., 2004,

40, 1337.

32. Yu. S. Simanenko, A. F. Popov, T. M. Prokop’eva, E. A. Karpichev,

V. A. Savelova, I. P. Suprun and C. A. Bunton, Russ. J. Org. Chem.,

2002, 38, 1286.

33. R. A. Moss, K. W. Alwis and J-S. Shin, J. Am. Chem. Soc., 1984, 106,

2651.

34. U. Costas-Costas, C. Bravo-Diaz, H. Chaimovich and I. M. Cuccovia,

Colloids and Surface A: Physicochem. Eng. Aspects, 2004, 250, 385.

35. A. B. Migorodskaya and L. A. Kudryavtseva, J. General Chem., 2002, 72,

113

1261.

36. A. Bieniecki and K. A. Wilk, J. Phys. Org. Chem., 2004, 8, 71.

37. Y. Okahata, H. Ihara and T. Kunitake, Bull. Chem. Soc. Jpn., 1981, 54,

2072.

38(a) U. Priyadarshini and S. G. Tandon, J. Chem. Eng. Data, 1967, 12, 143.

(b) S. K. Rajput, Ph. D. Thesis, Pt. Ravishankar Shukla University, Raipur,

1984.

39. O. Exner, M. Hradil and J. Mollin, J. Collect. Czech. Chem.Commun.

1990, 55, 3980.

40. A. Bagno, C. Commuzzi and G. Scorrano, J. Am. Chem. Soc., 1994, 116,

916.

41. I. H. Um, H. W. Yoon, J. S. Lee, H. J. Moon and D. S. Kwon, J. Org.

Chem., 1997, 62, 5939.

42. K. K. Ghosh, M. L. Satnami and D. Sinha, Tetrahedron. Lett., 2004, 45,

9103.

43. K. K. Ghosh, J. Vaidya and M. L. Satnami, Int. J. Chem. Kinet., 2006, 38,

26.

44. K. K. Ghosh, Y. S. Simanenko, M. L. Satnami and S. K. Sar, Indian J.

Chem., 2004, 43 B, 1990.

45. K. R. Fountain, C. J. Felkerson, J. D. Driskell and B. D. Lamp, J. Org.

Chem., 2003, 68, 1810.

114

46. K. R. Fountain, D. B. Tad-y, T. W. Paul and M. V. Golynskiy, J. Org.

Chem., 1999, 64, 6547.

47. K. R. Fountain, L. K. Hutchinson, D. c. Mulhearn and Y. B. Xu, J. Org.

Chem., 1993, 58, 7883.

48. K. R. Fountain and K. D. Patel, J. Org. Chem., 1997, 62, 4795.

49. C. A. Bunton, A. Blasko and N. D. Gillitt, Langmuir, 1997, 13, 6439.

50. L. S. Romsted, C. A. Bunton and J. Yao, Curr. Opin. Colloid Interface

Sci., 1997, 2, 622.

51. C. A. Bunton and G. Savelli, Adv. Phys. Org. Chem., 1986, 22, 213.

52. F. M. Menger, Acc. Chem. Res., 1978, 12, 111.

53. K. Martinek, A. K. Yatsimirski, A. V. Levashor and I. Berezin in

Micellization, Solubilization and Microemulsion, Vol. 2, K. L. Mittal Ed.,

Plenum Press, New York, 1977.

54. V. K. Balakrishnan, X. Han, G. W. VanLoon, J. M. Durst, J. Toullec and

E. Buncel, Langmuir, 2004, 20, 6586.

55. H. Morales-Rojas and R. A. Moss, Chem. Rev., 2002, 102, 2497.

56. S. Couderc and J. Toullec, Langmuir, 2001, 17, 3819.

57. C. A. Bunton and H. J. Foroudian, Langmuir, 1993, 9, 2832.

58. C. A. Bunton, J. Mol. Liq., 1997, 72, 231.

59. C. A. Bunton, N. Carrasco, S. K. Huang, C. H. Paik and L. S. Romsted,

J. Am. Chem. Soc., 1978, 100, 5420.

115

60. L. Garcia Rio, P. Herves, J. C. Mejuto, J. P. Juste, P. R. Dafonte, New J.

Chem., 2003, 27, 372.