kinetic study of oxidation of galactose by n-bromo phthalimide in the presence of cationic micelle...

Kinetic study of oxidation of galactose by N-bromophthalimide in the presence of cationic micelle in acidicmedium

Yokraj Katre • Savita Nayak • D. N. Sharma •

Ajaya K. Singh

Received: 11 March 2011 / Accepted: 29 May 2011 / Published online: 21 June 2011

� Springer Science+Business Media B.V. 2011

Abstract The kinetics of micellar catalyzed oxidation of galactose by N-brom-

ophthalimide was studied in the presence of acidic medium at 308 K. The oxidation

reaction exhibits first-order kinetics with respect to oxidant (N-bromophthalimide),

fractional order with respect to substrate (galactose) and positive fractional order

with respect to HClO4 on the rate of reaction. The rate of the reaction increased with

decreasing the dielectric constant of the medium. With a progressive increase in the

concentration of CTAB, the rate of reaction increased and after reaching peak kobs,

decreased at higher concentrations of CTAB. There catalytic roles are best

explained by Berezin’s model. The influence of salts on the reaction rate was also

studied. The various activation parameters have been calculated. The rate constant

and binding constant with the surfactant have also been evaluated. A suitable

mechanism consistent with the experimental findings has been proposed.

Keywords Micellar catalysis � Oxidation � Kinetics � Galactose � Berezin’s model

Introduction

Carbohydrates have been reported to be a biologically important substance whose

microbiological and physiological activities depend largely on their redox behavior.

Y. Katre (&) � S. Nayak � D. N. Sharma

Department of Chemistry, Kalyan Post Graduate College, Bhilai Nagar, Durg 490006, India

e-mail: [email protected]

S. Nayak


A. K. Singh

Department of Chemistry, Vishwanath Yadav Tamaskar Post Graduate Autonomous College,

Durg 490023, India


123

Res Chem Intermed (2012) 38:179–193

DOI 10.1007/s11164-011-0335-6

Carbohydrates serve as the chief fuel of biological systems supplying living cells

with usable energy. They are the body’s primary source of energy. Energy is stored

in the complex molecular structure of the carbohydrates [1–4]. The kinetics of

oxidation of sugars has been the subject of extensive research in recent years [5–10].

The versatile nature of N-halogeno compounds is due to their ability to act as

sources of halonium cations, hypohalite species, and nitrogen anions, which act as

both bases and nucleophiles. N-bromophthalimide (NBP), like other similar N-halo

imides, may exist in various forms in acidic medium, i.e., free NBP, protonated

NBP, Br?, HOBr, (H2OBr)?. NBP has found widespread application in organic

transformations. It is widely applicable in industrial process for the synthesis of

drugs, pharmaceuticals, and agrochemicals. It is extremely stable in solid state when

kept out of light and moisture. Its standard solution has excellent storage qualities.

The oxidation of reducing sugars in micellar system is also reported [11–14]. The

catalyzed and non-catalyzed oxidation of organic compounds has been studied in

detail using organic oxidants such as N-halo compounds [15–20]. There are several

reports available in the literature on the oxidation of reducing sugar by oxidants

such as N-bromoacetamide, N-bromosuccinimide, permanganate ion, and inorganic

oxidant, such as Cu, Cr, transition metal ion [21–26]. However, the details of

micellar effect on oxidation of galactose (Gal) by NBP are yet unknown. This

prompted us to study the micellar effect on the kinetics of the oxidation of Gal by

NBP in the acidic medium.

Experimental procedures

Materials

N-bromophthalimide (NBP) was used as obtained (Sigma-Aldrich, Germany, 99%

pure). The melting point of the sample was found to be 481 K. Solutions of NBP

were prepared in 80% distilled acetic acid and stored in a black-coated flask to

prevent photochemical deterioration. The prepared solution was then standardized

iodometrically [27] against the standard solution of sodium thiosulphate using

starch as an indicator. A standard aqueous solution of the cationic surfactant, cetyl

trimethyl ammonium bromide (CTAB), 99% pure, was obtained from Sigma-

Aldrich. It was recrystallized and its solution was prepared just before the

experiment. The solution of CTAB was acidified with 50% acetic acid. The standard

solution of mercuric acetate (S.d.fine) was acidified with 20% acetic acid. Perchloric

acid (A.R.) diluted with double distilled water and was standardized by acid–base

titration. All other standard solutions of KCl, KBr, and phthalimide were prepared

using triple distilled water. Triple distilled water was distilled over KMnO4 in an

all-glass (Pyrex) distillation setup.

Kinetic measurements

[HClO4] was used for the acidic medium. NBP is dissolved in acetic acid and

[Hg(OAc)2] was used only as a Br- scavenger. The reaction was carried out in

180 Y. Katre et al.

123

glass-stoppered Pyrex vessel whose outer surface was coated black to eliminate

photochemical effects. The reaction mixture containing the requisite amounts of

[Gal], [Hg(OAc)2], [CTAB], [HClO4], [CH3COOH] and water were taken in the

vessel. The reaction vessel was kept in the thermostat water bath at 308 K for

thermal equilibrium and the solution was left to stand for 15 min to attain

equilibrium. A measured amount of the oxidant solution, also thermo stated at the

same temperature, was rapidly added to the mixture. Measuring unconsumed NBP

iodometrically using starch as the indicator monitored the progress of the reaction.

The course of the reaction was studied for two half-lives. The pseudo-first-order rate

constants were calculated from log [remaining NBP] versus time.

Stoichiometry and product analysis

Various sets of experiments were performed with different [NBP]:[Gal] ratios,

under the condition of [NBP] � [Gal]. The stoichiometry of the reaction was

established by equilibrating the mixture consisting of NBP, Gal, CTAB, HClO4,

Hg(OAc)2 and acetic acid for 72 h. After completion of the reaction, the

unconsumed NBP was estimated. It was found that one mole of Gal is oxidized

by one mole of NBP. Thus, the ratio of consumption of reductant to oxidant is 1:1.

C6H12O6 þ NBPþ H2O! d-Galactono-1,5-lactone þ NHPþ HBr

After the kinetic experiment was completed, a part of the oxidized reaction

mixture was treated with alkaline hydroxylamine solution, and the presence of

lactone in the reaction mixture was tested by FeCl3–HCl blue test [28, 29]. To the

other part of the reaction mixture, barium carbonate was added to make the solution

neutral [30]. FeCl3 solution that had been colored violet with phenol when added to

this reaction mixture gave a bright yellow coloration [31], indicating the presence of

aldonic acid. It is concluded that lactone formed in the rate-determining step.

Results and discussion

Effect of varying [NBP]

To find out the order with respect to [NBP], the kobs values were determined at

different NBP concentrations at constant concentrations of other reactants at 308 K.

The plots of log [NBP] versus time were found to be straight lines, indicating that

the order with respect to oxidant was one (Table 1).

Effects of varying [Gal]

The rate of reaction increased from 2.44 9 10-4 to 8.95 9 10-4 s-1 with

increasing concentration of Gal from 2.5 9 10-2 to 15 9 10-2 mol l-1 at constant

concentration of other reactants at 308 K. The results are summarized in Table 1.

The plot of log k versus log [Gal] was linear with a fractional slope (0.71),

indicating fractional order with respect to [Gal]. The result is presented in Fig. 1.

Kinetic study of oxidation of galactose 181

123

Effect of varying [HClO4]

The rate constant increased from 3.04 9 10-4 to 7.73 9 10-4 s-1 with an

increasing [H?] from 0.15 9 10-2 to 0.4 9 10-2 mol l-1. The plots of log k versus

log [H?] produced a straight line with a slope is positive and less than unity

indicating that order with respect to [H?] ion is positive fractional order. The results

are presented in Table 1 and Fig. 2.

Effect of varying [CH3COOH]

In order to determine the effect of dielectric constant (polarity) of the medium on

the rate of reaction, the micellar catalyzed oxidation of Gal by NBP was studied in

Table 1 Effect of [NBP], [Gal], [H?], [Hg??] and acetic acid percentage on the rate of oxidation

reactions

104[NBP]

(mol l-1)

102[Gal]

(mol l-1)

103[H?]

(mol l-1)

104[Hg??]

(mol l-1)

[CH3COOH]

%

104kobss-1

0.5 5.0 2.5 2.0 50 4.55

1.0 5.0 2.5 2.0 50 4.54

2.0 5.0 2.5 2.0 50 4.53

3.0 5.0 2.5 2.0 50 4.54

4.0 5.0 2.5 2.0 50 4.55

1.0 2.5 2.5 2.0 50 2.44

1.0 3.5 2.5 2.0 50 3.33

1.0 5.0 2.5 2.0 50 4.54

1.0 7.5 2.5 2.0 50 5.85

1.0 10.0 2.5 2.0 50 6.96

1.0 15.0 2.5 2.0 50 8.95

1.0 5.0 1.5 2.0 50 3.04

1.0 5.0 2.0 2.0 50 3.56

1.0 5.0 2.5 2.0 50 4.54

1.0 5.0 3.0 2.0 50 5.53

1.0 5.0 3.5 2.0 50 6.53

1.0 5.0 4.5 2.0 50 7.74

1.0 5.0 2.5 4.0 50 4.55

1.0 5.0 2.5 6.0 50 4.56

1.0 5.0 2.5 8.0 50 4.55

1.0 5.0 2.5 2.0 30 7.49

1.0 5.0 2.5 2.0 40 6.13

1.0 5.0 2.5 2.0 50 4.54

1.0 5.0 2.5 2.0 60 3.68

Reaction condition [CTAB] = 9 9 10-4 mol l-1, Temperature = 308 K

182 Y. Katre et al.

123

various compositions of acetic acid. The reaction rate constant decreased with

increasing acetic acid percentage from 30 to 60% (Table 1). The data clearly reveal

that the rate decreases with an increase in the percentage of acetic acid, i.e., with

decreasing dielectric constant or polarity of the medium.

The effect of dielectric constant of the medium on the rate constant of a

reaction between two ions has been described by the well-known equation [32]

given below

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5

2 + log[Gal]

4 +

lo

g k

Fig. 1 Plot of log k versus log [Gal] at 308 K. Reaction condition [NBP] = 1 9 10-4 mol l-1,[Hg??] = 2 9 10-4 mol l-1, [CTAB] = 9 9 10-4 mol l-1, [H?] = 2.5 9 10-3 mol l-1, CH3COOH =50%

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8

3 + log [H+]

4 +

lo

g k

Fig. 2 Plot of log k versus log [H?]. Reaction condition [NBP] = 1 9 10-4 mol l-1, [Hg??] =2 9 10-4 mol l-1, [CTAB] = 9 9 10-4 mol l-1, [Gal] = 5 9 10-2 mol l-1, CH3COOH = 50%,Temperature = 308 K


123

logkobs = logk0o�ZAZBe2N

2:303 4pe0ð ÞdABRT� 1

Dð1Þ

where k0o’ is the rate constant in a medium of infinite dielectric constant, ZA and ZB

are the charges of reacting ions, dAB refers to the size of activated complex and T is

absolute temperature and D is the dielectric constant of the medium.

Equation (1) shows that if a plot is made between 4 ? log k and 1/D, a straight

line having a slope equal to {-ZA ZBe2N}/2.303(4pe0) dAB RT will be obtained.

When log k values observed for the oxidation of Gal were plotted against 1/D, the

straight line shown in Fig. 3 was obtained.

Effects of varying [Hg(OAc)2]

Change in [Hg(OAc)2] to the reaction mixture showed an insignificant effect on the

rate of oxidation. It has been reported earlier that Hg(II) can act as a homogeneous

catalyst, cocatalyst, and oxidant [33]. In order to ascertain the role of Hg(OAc)2, in

addition to its action as a Br- scavenger, several experiments were performed with

different initial concentrations of Hg(OAc)2 in the presence of NBP under similar

experimental conditions (Table 1). The kinetic observations in the presence of NBP

showed that the reaction rate was almost constant with the increase in concentration

of Hg(OAc)2 negating its role as catalyst and cocatalyst in the reaction. The reaction

did not proceed under the similar concentrations with Hg(OAc)2 without using

NBP, indicating noninvolvement of Hg(OAc)2 as an oxidant. Thus, in view of such

kinetic observations, Hg(OAc)2 acts only as a Br- scavenger [34] due to formation

of the complex [HgBr4]2-. Therefore, all the experiments were carried out in the

presence of Hg (OAc)2.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.01 0.02 0.03 0.04

102/D

4 +

lo

gk

Fig. 3 Plot between log k and 1/D. Reaction condition [NBP] = 1 9 10-4 mol l-1, [Gal] =5 9 10-2 mol l-1, [Hg??] = 2 9 10-4 mol l-1, [CTAB] = 9 9 10-4 mol l-1, [H?] = 2.5 9 10-3

mol l-1, Temperature = 308 K

184 Y. Katre et al.

123

Effects of varying phthalimide

Addition of phthalimide (a reduced product of the oxidant) rate of reaction remained

constant, indicates that it does not effect on the rate of reaction.

Effect of salts

With ionic surfactants, the addition of electrolytes such as KCl or KBr to the

surrounding solution at first acts primarily to modify interactions between micelles.

However, at sufficiently elevated salt concentrations, micelles grow above their size

in pure water. The salt effect on the Gal-NBP reaction was studied in the presence of

CTAB micelles at 308 K, keeping other variables constant. The salt effect on

micellar catalysis should be considered in the light of its competition with the

substrate molecule, which interacts with the micelles electrostatically and

hydrophobically, and structural changes, which occur on salt addition [35]. The

effect of added salts on the rate of reaction was also explored because salts as

additives, in micellar systems, acquire a special ability to induce structural changes

which may, in turn, modify the substrate–surfactant interaction. Positive effects of

[Cl-] and [Br-] on the oxidation velocity were found. The results are presented in

Table 2.

Test for free radicals

The generation of free radicals during the course of the oxidation could be

confirmed by using acrylonitrile monomer. A known amount of acrylonitrile was

added in a reaction mixture containing NBP, Gal, HClO4 and acetic acid. No

precipitate was found in the reaction mixture. This indicates that no free radicals are

formed in the reaction mechanism.

Activation parameters

The reactions were studied at different temperatures to obtain various values of

activation parameters. From the Arrhenius plots of log k versus 1/T, activation

energy and other thermodynamic parameters for Gal with surfactant have been

calculated. The values of negative DS# (entropy of activation) and positive DH#

(enthalpy of activation) suggest the formation of more ordered activated complexes

and the transition state is highly solvated. The value of energy of activation shows

that the reaction is slow, and enthalpy is controlled. The values of activation

parameters show that the activated complex so formed is more bulky in the presence

of surfactant. The results are presented in Table 3 and Fig. 4.

Mechanism

Aldoses exist predominantly as cyclic hemiacetals that are in equilibrium with a

cyclic form and an open chain. The monosaccharides are considered as a polyol and

the reactivities of –OH groups can be influenced by the presence of the carbonyl


123

group. Aldohexoses exist mainly as pyranoid and furanoid forms, the former being

more stable. The pyranoid form mainly exists in a chair conformation. On the

contrary, various species of NBP in aqueous media are given in reactions (2–5).

Table 2 Effect of variation of

[CTAB] and inorganic

electrolytes on the oxidation

reaction

Reaction condition

[NBP] = 1 9 10-4 mol l-1,

[Hg??] = 2 9 10-4 mol l-1,

[Gal] = 5 9 10-2 mol l-1,

[H?] = 2.5 9 10-3 mol l-1,

CH3COOH = 50%,

Temperature = 308 K

104[CTAB]

mol l-1108[KCl]

mol l-1108[KBr]

mol l-1104k s-1

0 – – 2.20

3.0 – – 2.33

4.0 – – 2.54

5.0 – – 2.68

6.0 – – 3.00

7.5 – – 3.98

8.5 – – 5.01

9.0 – – 4.54

10.0 – – 4.32

11.0 – – 4.09

12.0 – – 3.80

13.0 – – 3.63

14.0 – – 3.36

– 2.0 – 5.44

– 0.4 – 6.59

– 0.6 – 7.04

– 0.8 – 7.90

– 1.0 – 8.93

– – 0.2 5.23

– – 0.4 6.31

– – 0.6 7.33

– – 0.8 8.02

– – 1.0 8.85

Table 3 Temperature effect

and activation parameters for

CTAB catalyzed reactions of

oxidation of Gal by NBP

Reaction condition

[NBP] = 1 9 10-4 mol l-1,

[Hg??] = 2 9 10-4 mol l-1,

[Gal] = 5 9 10-2 mol l-1,

[H?] = 2.5 9 10-3 mol l-1,

CH3COOH = 50%,

[CTAB] = 9 9 10-4 mol l-1

# = Activation parameter

Parameters 104k s-1

298 K 2.33

303 K 3.45

308 K 4.54

313 K 5.59

DEa (kJ mol-1) 31.717

DH# (kJ mol-1) 29.157

DS# (Jk-1 mol-1) -50.154

DG# (kJ mol-1) 44.603

logPz 2.04

186 Y. Katre et al.

123

NBPþ Hþ � NHPþ Brþ

ð2Þ

NBPþ Hþ � NBPHð Þþð3Þ

NBPþ H2O� HOBrþ NHPð4Þ

HOBrþ Hþ � H2OBrð Þð5Þ

When HOBr was assumed as the reactive species, the derived rate law failed to

explain negligible effect of phthalimide. When (H2OBr)? was taken as reactive

species, the rate law obtained showed first-order kinetics with respect to hydrogen

ion concentrations, contrary to our observed positive fractional order in [H?].

Therefore, the possibilities of involvement of (H2OBr)? and cationic bromine (Br?)

as reactive species were ruled out. Hence, neither of these species could be

considered as the reactive species. When (NBPH)? was taken as reactive species of

NBP, the derived rate law fully explained the positive fractional effects of [H?] ions

and zero effect of NHP. Thus, (NBPH)? is considered as the reactive species. It led

to a rate law explaining all the kinetics observations and other effects. Hence, in the

light of kinetic observations, (NBPH)? was assumed to be the main reactive species

for the present reaction. On the basis of above experimental findings the following

mechanism can be proposed (Scheme 1).

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

3.15 3.2 3.25 3.3 3.35 3.4

104/T K

5 +

lo

gk

Fig. 4 Plot of log k versus 1/T at 308 K. Reaction condition [NBP] = 1 9 10-4 mol l-1, [Gal] =5 9 10-2 mol l-1, [Hg??] = 2 9 10-4 mol l-1, [CTAB] = 9 9 10-4 mol l-1, [H?] = 2.5 9 10-3 mol l-1,CH3COOH = 50%


123

In reactions (6–8), X- is intermediate species. On the basis of the above reactions

(6–8), the rate in terms of intermediate between substrate and oxidant can be

expressed as

rate ¼ k X�½ � ð9ÞWith the help of reaction (6), we can write Eqs. (10) and (11) as follows:

K1 ¼NBPHþ½ �

NBP½ � Hþ½ � ð10Þ

NBPHþ½ � ¼ K1 NBP½ � Hþ½ � ð11ÞWith the help of reaction (7), we can write Eq. (12) as:

K2 ¼X�½ �

Gal½ � NBPHþ½ � ð12Þ

With the help of reaction (8), we can write Eq. (13) as:

O

(R)

(R)

OH

H

H

HO

H

H

OHHOH(S)

(S)

CH2OH

b-D-Galactopyranose

C

C

O

N

O

Br + H+ K1

C

C

O

N

O

(6)

N-bromophthalimide (NBP)

Br

H

+

C

O

N

C

O

Br

HK2

(NBPH+)

(NBPH+)

O

(R)

(R)

OH

H

H

HO

H

H

OHHOH(S)

(S)

CH2O-

O

(R)

(R)

OH

H

H

HO

H

H

OHHOH(S)

(S)

CH2O-

+ NHP +H+

Slow

Fast

H+

O(R)

OH

H

H

HO

H

OHH(R)

(R)

CH2O-

+

O

Br-

[X-]

[X-]

(7)

(8)

Scheme 1 Reaction mechanism

188 Y. Katre et al.

123

X�½ � ¼ K2 Gal½ � NBPHþ½ � ð13ÞSubstituting the value of [NBPH?] we get Eq. (14)

X�½ � ¼ K1K2 Gal½ � NBP½ � Hþ½ � ð14ÞOn substituting the value of [X-] from Eq. (14) to Eq. (9), we get Eq. (15)

rate¼ kK1K2 Gal½ � NBP½ � Hþ½ � ð15ÞAt any moment in the reaction, the total concentration of [NBP], i.e., [NBP]T can

be shown as follows

NBP½ �T¼ NBP½ � þ NBPHþ½ �þ X�½ �On substituting the value of [X-] from Eqs. (11) and (14) to the above equation,

we get Eq. (16)

NBP½ �T¼ NBP½ � þK1 NBP½ � Hþ½ � þK1K2 Gal½ � NBP½ � Hþ½ � ð16ÞEquation (16) can also be written as

NBP½ � ¼ NBP½ �T1þ K1 Hþ½ � þ K1K2 Gal½ � Hþ½ � ð17Þ

From Eqs. (15) and (17), we have

rate ¼ kK1K2 Hþ½ � Gal½ � NBP½ �T1þ K1 Hþ½ � þ K1K2 Gal½ � Hþ½ � ð18Þ

Equation (18) is the rate law on the basis of the observed kinetic orders with

respect to each reactant of the reaction, which can be very easily explained.

Critical micellar concentration (CMC)

Surfactants spontaneously aggregate above a certain concentration called the critical

micelle concentration (CMC) to form micelle, whose determination has consider-

able practical importance, normally to understand the self-organizing behavior of

surfactants in exact ways. The concentration of CTAB at which the maximum

would be the kinetic CMC of the surfactant in the presence of the Gal and from the

graph the kinetic CMC value for Gal was found to be 8.5 9 10-4 mol l-1 in

reaction mixture [36]. The difference among the CMC values arises from the well-

known effect of added electrolyte, which lowers the CMC by causing a decrease in

the repulsion between the polar head groups at the micelle surface.

Effect of [CTAB]

To investigate the effect of ionic micelles on the reaction rate, the kinetic

experiments were performed in the presence of varying [CTAB] at constant values of

other reactants at 308 K. The Gal-NBP reaction is catalyzed by cationic surfactant

CTAB. A plot of kobs versus [CTAB] shows a maximum rate at [CTAB] =

8.5 9 10-4 mol l-1 and with further increase in [CTAB] ([8.5 9 10-4 mol l-1) the

rate of reaction decreased. It is a very common characteristic of bimolecular


123

reactions catalyzed by micelles (Table 2; Fig. 5). In Fig. 5, the pseudo-first-order

rate constant values are plotted against the [CTAB]. The rate constants rise rapidly

with [CTAB]. After reaching an optimal [CTAB], kobs falls. The increase in kobs was

observed even at [CTAB] \ CMC. This fact is usually interpreted as reactants

inducing micelle formation (surfactant molecules start aggregating below CMC).

The catalysis below CMC (i.e., submicellar catalysis) is not new, and confirms to

various available results.

Berezin’s model to explain the miceller effect

Berezin and coworkers treated the case of reaction of two uncharged organic

molecules. They developed the first general treatment based on the pseudo phase

model and successfully simulated spontaneous and bimolecular reactions between

neutral and organic reactants. Alternatively, the initial increase in rate up to the

kinetic CMC may also be explained with the idea of positive co-operatively in

enzymatic reaction. On the basis of the Piszkiewicz model, it may be thought that a

substrate molecule and a small number of detergent molecules aggregate to form

catalytic micelles, which are somewhat different from normal micelles consisting of

a larger number of detergent molecules [37]. These catalytic micelles react with the

oxidant to yield the products. As [CTAB] exceeds the CMC value, normal micelles

are formed and the rate of reaction begins to decrease. The inhibition of rates at

higher concentration (beyond CMC) of CTAB may be explained with the help of

Berezin’s model, which involves solubilization of both reactants in the micellar

phase [38]. According to Berezin’s et al.’s approach, a solution above the critical

micelle concentration (CMC) may be considered as a two-phase system, consisting

0

1

2

3

4

5

6

0 5 10 15

104 x [CTAB]/molL-1

104 X

k/s

-1

Fig. 5 Effect of variation of [CTAB] on rate constant. Reaction conditions with 308 K [NBP] =1 9 10-4 mol l-1, [Gal] = 5 9 10-2 mol l-1, [Hg??] = 2 9 10-4 mol l-1, [H?] = 2.5 9 10-3 mol l-1,CH3COOH = 50%

190 Y. Katre et al.

123

of an aqueous phase and a micellar pseudo phase. The reactants (S = substrate and

O = oxidant) may be distributed as shown in Scheme 2.

A quantitative rate expression for a bimolecular reaction occurring only in

aqueous (kW) and micellar (kM) phase for the pseudo-first-order rate constant is

given by Eq. (19)

kobs ¼kW þ k0MKSKO CSurf � CMCð Þ

1þ KSðCSurf � CMC½ � 1þKO CSurf � CMCð Þ½ � ð19Þ

where KS and KO are the association constant of Gal and NBP with CTAB,

respectively; CSurf is the analytical concentration of CTAB; k0M = (kM/V), V being

molar volume of the micelle; and kW and kM are the pseudo-first-order rate constant

in the absence and presence of micelles, respectively. Since the oxidant will be

charged species and the substrate is large molecules, the hydrophobic and

electrostatic interactions will be large and hence it may be expected that KS and

KO will be high. Since CSurf is small, it may be possible that kW � k0M KS KO

(Csurf - CMC). So that the Eq. (19) takes the form represented by Eq. (20)

(C6H12O6)W + (NBPH)+W (Product) W

(C6H12O6)M + (NBPH)+M

KO

(Product) M

kW

kM

KS

Scheme 2 Berezin’s model

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 1 2 3 4 5 6

10-4(CSurf-CMC)/mol.L-1

103k-1

ob

s/s

Fig. 6 Plot of k-1 versus (CSurf - CMC) at 308 K. Reaction condition [NBP] = 1 9 10-4 mol l-1,[Gal] = 5 9 10-2 mol l-1, [Hg??] = 2 9 10-4 mol l-1, [CTAB] = 9 9 10-4 mol l-1, [H?] = 2.5 910-3 mol l-1, CH3COOH = 50%


123

kobs ¼kW

1þ KSþKOð Þ CSurf � CMCð ÞþKSKO CSurf � CMCð Þ2ð20Þ

Again, since (CSurf - CMC) is very small, the terms containing (CSurf - CMC)2

may be neglected and Eq. (20) may be rearranged to Eq. (21).

1

kobs

¼ 1

kW

þ KS þ KO

kW

CSurf � CMCð Þ ð21Þ

And the value of kW and (KS ? KO) are 4.782 s-1 and 731 L mol-1,

respectively.

Plot of kobs-1 versus (CSurf - CMC) for Gal is linear (Fig. 6)

Conclusions

A Gal is oxidized in micellar system by NBP at 308 K. The cationic micelle of

CTAB accelerates the rate of reaction. (NBPH)? is considered to be the reactive

species of the oxidant. Oxidation products were identified and activation parameters

were evaluated for catalyzed reaction. A plausible mechanism and a related rate law

have been worked out. In conclusion, it can be said that cationic micelle of CTAB is

an efficient catalyst for the oxidation of Gal by NBP in acidic medium.

Acknowledgments One of the authors, YRK, thanks University Grants Commission New Delhi for the

minor research project.

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kinetic study of oxidation of galactose by n-bromo phthalimide in the presence of cationic micelle...

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