Indian Journal of Chemistry Vol. 43B, October 2004, pp. 2178-2188

Redox behaviour of chromium(VI) towards D-mannose in the presence and absence of micelles and inorganic salts

Kabir-ud-Din*, Abu Mohammad Azmal Morshed & Zaheer Khan t

Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, India

tDepartment of Chemistry , l amia Millia Islamia, New Delhi 110 025 , India

E-mail : kabir7@rediffmail.com

Received 23 April 2003; accepted (revised) 28 January 2004

The kinetics and mechanism of the oxidation of D-mannose by chromium(Vl) in the absence and presexe of sodium dodecyl sulfate(SDS) and polyoxylethylene t-octylphenol(TX-IOO) micelles have been investigated. Under pseudo- fir st­order conditions the reaction rate is of fractional- and first-order, respectively, in D-mannose and oxidant. The reaction is catalyzed by the micelles which is due to favourable electrostaticlthermodynamic/hydrophobicl hydrogen bonding between the reactants and anionic/nonionic micelles. From the observed kinetic data micelle-chromium(VI) binding constants(Ks) and micelle-D-mannose binding constants(KM) were calculated to be 86, 84 mol- I dm) and 58, 75 mol- I dm 1 for SDS and TX-lOO, respectively. The reaction is retarded by addition of inorganic salts (NaBr, LiBr, NH4Br) which is attributed to competition between salt cations and H+ from the reaction sites in the SDS micelles.

IPC: [nt.Ce C 01 G 37/00

Although kinetics of the oxidation of aldo sugars by transition metal ions have been studied' ·7, data in the presence of ionic- and non-ionic surfactants are scanty. As we know, micelles, microemulsions, liposomes, etc., are compartmentalized liquids which may show special performance toward reaction dynamics and reactiun equilibrias"o. The concentration of reactants into a small volume through electrostatic and/or hydrophobic interactions is the main factor involved in the kinetic micellar effects on bimolecular reactions"·13.

Chromium(VI), known to show mutagenic and carcinogenic effects'4, reduces to lower states with a wide variety of naturally occurring cellular reductants IS.16

• In case of oxidative degradation of carbohydrates by chromium(VI), the general conclusion is that the reactions exhibit a characteristic one-step two-electron oxidation. On the other hand, the works of SalaS and Sen Gupta6 suggest that the reduction involves formation of chromium(IV) as an intermediate.

As part of our program on the redox chemistry of organic acids 17 and carbohydrates'S in the presence of ionic and non-ionic surfactants we set out to elucidate the electron transfer behaviour of D-mannose by chromium(VI). This study may be of interest in the

biochemistry of carbohydrates related to metabolic problems. Therefore, the present work was under­taken to confirm the path of chromiulTI(VI) reduction by D-mannose in the presence and absence of manganese(Il) and ionic/non-ionic micelles. The use of manganese(II) is an useful tool :.o determine the involvement of chromium(IV) as an intermediate' 9.2o. It acts as a catalyst/inhibitor in many redox reactions of chromium(VI)2o.2'.

Experimental Section

D-Mannose (reductant, s.d.fine, India, 99%), potassium dichromate (oxidant, Merck, India, 99%), perchloric acid (Merck, India, 70% solution), manganese(II) sulfate-i-hydrate (Merck, India, 99%), TX-iOO (Fluka, Switzerland, 99%), cetyltrimethyl­ammonium bromide (CT AB, Fluka, Switzerland, >99%), SOS (Fluka, Switzerland, >98%), and inorganic salts (NaBr, LiBr, NH4Br) were used as received. The water used as solvent was subjected to deionization followed by distiHation (specific conductance: (i-2) x 10-6 Scm- I).

Kinetic measurements. The reaction was initiated by adding the requisite quantity of pre-equilibrated D­man nose solution to a thermally equilibrated mixture of chromium(VI), perchloric acid and other reagents

KABIR-UD-DIN el al.: REDOX BEHAVIOUR OF CHROMIUM(VI) TOWARDS D-MANNOSE 2179

(when required). The progress of the reaction was followed by measuring the absorbance of remaining chromium(VI) at known time intervals at 360 nm using a Bausch & Lomb Spectronic-20 spectro­photometer. At thi s wavelength neither chromium(lII ) nor the oxidized products has any appreciable absorbance. The pseudo-first order conditions were maintained with a large excess of reductant over oxidant concentration. The kinetic runs were carried out up to ca. 80% completion and the pseudo-first­order rate constants were obtained from the slopes of log (A)360 versus t plots. The results were reproducibl e

to within ±4% with average linear regress ion coeffic ient, r ~ 0.998 . The experimental second-order rate constants (k") were calculated from the relationship : k" = kobJ[D-mannose]. Other details are described elsewhere I7

.18

.

Stoichiometry and product analysis. Several reaction mixtures with [chro mium(VI)]>[D­man nose] (5 x 10-4 mol dm- 3 oxidant: 0.5 to 4.5 x 10-4 mol dm-3 reductant) at fixed [H+] (0.58 mol

dm-3) were prepared and kept for several days at room temperature . After completion of the reacti on, the unconsumed oxidant was determined spectro­photometrically. In all cases the reaction stoichiometry was fo und to be 1:2 (chro mium(VI):D­mannose). The exact stoichiometry equation is difficult to pred ict due to the autoacceleration nature of the reacti on (vide infra).

To characterize the reduction product of chromium(VI) (i.e., chromium(III)), [D-mannose] = 30 x 10-3 mol dm-3

, [chromium(VI)] = 4 x 10-4 mol

dm-3, and [HCI04] = 0.58 mol dm-3 were mixed at 60 dc. Spectra of the mixture were recorded at different time intervals. Figure 1 (Set A) , shows that, as the reaction prog resses, the 360 nm peak decreases. Under the experimental kinetic conditi ons and when all the chromium(VI) has reacted, the spectrum did not show any maximum (Figure 1, Set A). However, the spectra of the reacti on mixture, recorded with hi gher [chromium(VI)] (Figure 1, Set B), show two sharp peaks (410 and 570 nm) which are indicati ve of the presence of aqua-chromium(lII) species5c in the reaction mixture.

For organic products , qualitative analys is of the oxid ized reaction mi xture in the presence of HC l04

was performed. After ensuring the completion of reaction, the ox idi zed reaction mixture was treated with alkaline hydroxy lam ine solution. The presence

O J 1- - ---- - -------------,

~

" c

I SE T-B

~ 1 0 '- -- ----L------5~OLO--------1 a I 400

~ Ij G t <1 .r'

'lCr r) ., f - -

SE T-A

1. 00 500 600

Wavelength ( nm )

Figure 1 - (Set A) Spectra of the reaction product of the ox idati on of D-mannose (30.0 x 10-3 mol dm-3

) by chrom ium(VI) (4.0 x 10-4 mol dm- 3

) at 60 °C in presence of [HC I0 4 1 (0.58 mol dm-3

); (a) just after, (b) aftcr 30 min , and (c) after 60 min of mi xing.

(Set 8 ) Effects of SDS (26.0 x 10- .1 mol dm-3) and TX- IOO (50.0

x 10-3 mol dm-3) on the spectra of aquachromium(lIl ) ion (the

reac ti on product obtained after 120 min); (a) no surfactant, (b) TX-IOO, and (c) SDS . O ther conditions were the same as in Set A wi th [chromium(VI )] = 4.0 x 10-3 mol dm-3

of lactone in the reaction mi xture was tested by FeCl3 - HCI blue tes t22

. On the other hand, when barium carbonate was added into the reacti on mi xture to make it neutral3b, FeCl3 solution that had been coloured violet with phenol when added to the reaction mixture gave a bright-yellow colouration , indicating that aldonic ac id is formed in the oxidation of D-mannose6a

. Apparentl y, lactone, which is formed in the rate -determining step, is hydrolysed to aldonic acid in neutral med ium in a fast stepo. At higher pH, the [lactone] is reduced because of the formation of aldonic ac id ani on that shifts the equilibrium away from lactone23 .

Polymerization studies. Upon addition of acrylo­nitrile to a reaction mixture ([D-mannosel = 30 x 10-3

mol dm-3, [chromium(VI)] = 4 x 10-4 mol dm-3

,

[HCI04 ] = 0.58 mol dm-3, temp. = 60 °C), polymeriza­

tion (white precipitate formation ) started qui ckl y while bl ank experiments with either chrom ium(V l) or D­man nose gave no detectable precipitate. The observa­tions demonstrate that the reaction of D-mannose with chromium(VI) proceeds via free rad icals.

2180 INDIAN J. CHEM., SEC B, OCTOBER 2004

Results and Discussion

A. Kinetics in the absence of suljactants The results can be summarized as follows:

(i) Under the experimental conditions of [o-mannose] > [chromium (VI)], the first-order plots were non-linear indicating two step-oxidation behaviour (cf. Figure 2), i.e., induction and autoacceleration periods. The values of pseudo-first-order rate constants for the induction period are summarized in Table I. The independence of

0 2

-;;; 0 4 v c o

~ 0 6 ~ '" 1 g 1

'" ~ 0 8

I 1.0 r

OL_--~I----~I----~I----~I----LI __ ~I~--~I.~ 10 20 30 40 50 60 70

Ti m@ (min)

Figure 2 - Plots of log(absorbance) vs. time for the oxidative degradation of D-mannose (30.0 x 10-3 mol dm-3) by chromium(YI) (4.0 x 10-4 mol dm-3

) at 60 °C; [HCI04] = (a) 0.11 , (b) 0.23, (c) 0.34, (d) 0.46, (e) 0.58, (f) 0.69, (g) 0.93, and (h) 1.16 mol dm-3

.

kobs over a range of [oxidanth is in agreement with the first-order dependence on [Cr(VI)h.

(ii) The rate increased with increase in [HCI04] and it was also observed that the induction period decreased with increase in the acid content. At higher [HCI04] (~ 0.93 mol dm-3

), the induction period was completely eliminated (Figure 2). Therefore, all the kinetic runs were performed at fixed [HCI04] ( = 0.58 mol dm-3

). The plot of log kobs versus log[HCI04] was linear with slope = 1.7 (r = 0.970) indicating a complex dependence on the concentration of H+.

(iii) The reaction rate increased with increase in [o-mannose] (range: 5.0 x 10-3 to 50.0 x 10-3 mol dm-3

)

and the order in O-mannose was found to be fractional (Figure 3).

(iv) The rate increased with increase in [Mn(H)] (Table II). For example, under the conditions [D­mannoseh = 30 x 10-3 mol dm-3

, [Cr(VI)h = 4 x 10-4 mol dm-3 and [H+] = 0.58 mol dm-3 at 60°C, a 4-fold rate increase from 4.4 x 10-4 to 17.9 X 10-4 S-I was observed when [Mn(II)] was increased from 0.005 to 0.05 mol dm-3

. At lower [Mn(II)] (5 x 10-3 to 20 X 10-3

mol dm-3), the kobs increased marginally (from 4.3 x

10-4 to 5.3 X 10-4 S-I) but fast thereafter. The order with respect to [Mn(II)] was determined from the plot of log kobs against 10g[Mn(II)] only for the higher [Mn(II)] range which was found to be l.Ol.The

Table I - Effect of [Cr(YI)], [D-mannose], and [HCI04] on the pseudo-first-order rate constants (kobs or kllJ) for the reaction of chromium(YI) with D-mannose in the absence and

presence of [SOS] and [TX- 100] micelles at 60 °C

104[Cr(YI)] 103 [D-mannose] [HCI04] 104kobs or k!Jl." (S- I)

(mol dm-3) (mol dm-3) (mol dm-3) aqueous SOS TX-IOO

2 30 0.58 4.0 6.1 7.7 3 4.2 6.4 8.4 4 4.3 6.3 8.3 5 4.1 6.2 8.2 6 4.2 6.3 8.4 4 5 0.58 1.5 2.9 3.8

10 2.2 3.8 4.8 15 3.0 4.7 5.8 20 3.7 5.2 6.5 30 4.5 6.3 8.3 40 6.0 7.6 9.6 50 7.6 8.9 12.8

4 30 0.11 0.64 1.5 1.8 0.23 0.95 1.9 3. 1 0.34 2.0 3. 1 5.0 0.46 3. 1 5.0 6.8 0.58 4.5 6.3 8.3 0.69 9.2 9.4 9.6 0.93 very fast very fast very fast 1.16 very fast very fast very fast

"[SOS] = 26.0 x 10-3 mol dm-3, [TX-IOO] = 50.0 x 10-3 mol dm-3

KABIR-UD-DIN et al.: REDOX BEHAVIOUR OF CHROMIUM(VI) TOWARDS D-MANNOSE

'";

" -.. " n ....

or 'J

,:.:,: I : I ·' ~ '~ r ,

/ /

/

/

~ 5 / i

I .'. to

'to .:J • ,', / I

I.. - 1:-4 .-. co

/ /

O.Lt' ------7!':----------:':! ~ {) 1 5 ;0

1e)[ rr'O " ""Jst"l( """ 0 1 <1", - :) :

o O~----7I C!-:.O

I i ;~ ", a ""()S, ] I "'ol- I oj ,,,3 ,

~

~oo

2 181

Figure 3 - kobs vs. [mannose] (a) and IIkobs vs. I/[mannose] (b) plots; [Cr(VI)] = 4.0 x 10-4 mol dm-3, [HCl04] = 0.58 mol dm-3

,

and temp. = 60a C.

Table II - Effect of [Mn(H)] and temperature on the pseudo-fIrst-order rate constants (kobs or klJl) and the values of activation parameters (Eo, 11!-1' and 11S') for the reaction of chromium(VI) with D-mannose in the absence and presence of SDS

and TX-l 00 micellesa

103 [Mn(lI)] Temperature 104kobs or k)Jl (S-I)

(mol dm-3) (aC) aqueous SDS TX-IOO

0 60 4.3 6.3 8.3

5 4.4 5.4 10.7

10 4.8 6.4 12.3

15 5.0 6.9 13.8

20 5.3 7.7 15.3

25 6.5 8.4 16.9

30 8.3 9.4 36.4

35 11.9 12.5 42.2

40 13.6 15.3 57 ,6

45 15.4 16.6 65 .2

50 17.9 21.5 80.6

0 40 1.7 2.3 3.2

50 3.0 3.6 5.6

60 4.5 6.3 8.3

70 9.4 11.5 clouding

80 13.4 16.0 clouding

E, (kJ mol- I) 47 41 36

Mr (kJmol- ' ) 44 38 33

11S'(JK- l mol- l ) -299 -297 -296

'[Cr(Vlh = 4.0 x 10-4 mol dm-3, [D-mannoseh = 30.0 x 10-3 mol dm-3

, [HCI0 4h = 0.58 mol dm-3

, [SDS] = 26.0 x 10-3 mol dm-3, [TX-lOO] = 50.0 x 10-3 mol dm-3

2182 INDI AN J. CHEM., SEC B, OCTOBER 2004

positive catalytic effect is due to one-step three­electron reduction of ch romium(Vl), i.e. , Cr(Vl) -Cr(III), without passing through formation of Cr(IV)

. d" h fM (11)1920 as an lnterme late III t e presence 0 n '. (v) The rate constants, obtained at five different

temperatures in the range of 40 to 80°C, were used to calculate activation parameters (Table II).

Mechanism

The increase in the reaction rate with increase in [H+] may be explained by considering the equilibria between chromium(VI) species (Cr20 72-+H20 = 2HCr04-; HCr04-+H+ -~ H2Cr04) and that, under our experimental conditions of [HCI04] = 0.58 mol dm-3, the H2Cr04 species being the reactive one. Also, the aqueous solution of sugar is an equlibrium mixture of a - and ~-sugars . Mannose is polyhydroxyaldehyde which has the same configuration as that of glucose except for the configuration at C-2. In equilibrium, D­man nose mainly consists of a-monomer having the epimeric H in equatorial position. The ratio ~f the a ­and ~-pyranose forms has been estimated from NMR studies at 30°C to be 64:36 for D-mannose24.

It has been established in the oxidation of aldoses by metal ions that the anomer hav ing OH-l equatorial undergoes faster oxidation than the corresponding anomer having OH-J axiaI23.25.26", Therefore, the reaction is presumed to follow the 'chromate-ester' formation between the chromium(VI) and equatorial anomeric OH-l of ~-pyranose27 (a- anomer's ax ial OH-I is less exposed so less () accessible to chromium(VI)). The chromate-ester formation is similar to the oxidation of alcohols/a-hydroxy acids/carboxy lic acids by 'd;(·omium(Vl)7. 17. 18.28. In the rate-determining step, the chromate-ester breaks lead ing to the format ion of chromium(IV) and lacrone. After the slow two-electron step (3), reactions (4) to (6) may follow (thus the overall react ion becomes a three-electron process) (Scheme I).

The positive catalytic role of manganese(II) (vide supra) rules out the formation of chromium(IV) as an intermediate 19.20. Therefore, the ox idation of D­man nose by chromium(VI) is presumed to be via a one-step, three-electron transfer mechani sm in presence of Mn(lI) (Scheme II ); the third electron being supplied by Mn(H). In the mechanism, the first step is envisaged to be the formation of a complex between Mn(J!) and D-mannose (step 7) which is then oxidi sed by Cr(VJ) via the ester formation (steps (8) and (9)).

The possibility of Mn02 formation is ruled out as no turbidity appeared and the solution remained colourless at the end of the reaction . Furthermore, attempts to monitor the reaction uti lizing 470 11 m wavelength (characteristic of Mn(lIJ)29) fa iled indicating no build-up of Mn(III) (thi s is understand­able in view of the fact that disproportionation reaction (10) is fast29).

Considering Schemes I and II, the respective rate laws can be written as Eqns (13) and U S), - d[Cr(VI)h / dl =

kKcs Ka [H +] [0 - mannoseh [Cr(VI)h

(l+K" [H +]+Kcs Ku [H+][D-mannoseh)

or

kobs =

(l+K" [H +]+KcsKu [H +][D-mannosehJ

and

- d[Cr(VI)h / dl =

or

k obs =

kl Kcs i Ku [H +][D - mannoseh[Mn(Il)h

(l + K" [H +l + Kcsi K.,[H +][D - mannoseh)

... (12)

.. . (13)

. .. ( JS)

Eqn (13) explains all the experimemal observations and is coherent with the fractiona l- and first-order dependencies fo und for the rate of the reaction on the [D-mannose] (Figure 3) and [Mn(II)] (of course, at higher concentrations of Mn(II)), respectively. Taking its reciprocal , we get

(l + Ka [H+]) _~ __ ==----C:.:'-- +b

2 kobs b J [D - man nose ]T

... (16)

where b, = kKesKa[H+] and b2 = 11k. It is clear from Eqn (16) that the plot of lIkobs versus I/[D-mannoseh at constant [H+] should be linear; such Michaelis­Menten reciprocal plot was indeed obtained with an intercept on y-axis. From the intercept and slope va lues the rate constant (k = 8.7 x 10-4 s- ') and 'chromate ester' formation constant (Kcs = 431 mor' dm3) were evaluated.

Table III summarizes the values 0; second-order­rate constants (k", mor ' dlll3 s- ') for the reactivity of D-mannose towards different oxidants. We see that the ox idation by chloramine-T takes place only in

KABIR-UD-DIN el ai .: REDOX BEHAVIOUR OF CHROMIUM(VI) TOWARDS D-MANNOSE 2183

~OH + H2Cr04

H

(~-D-mannopyranose)

~R 00_ g,_OH

II o

(ester) H

~OH + Cr(lV)

H

(~-D-mannopyranose)

~. + Cr(VI)

H (radica l)

~OH+C'(V) H

(~-D-mannopyranose)

(R = CH20H)

k

fast ~

fast ~

fa st ~

..... (1)

(ester)

R 0

~ .... (1 ) + Cr(IV)

(lactone) 0

~. + Cr(IlI) .... (4)

H

(radical )

R 0

~ + C,(V) . . ... (5)

~ o

(lactone)

R

+ Cr(Ill) .... (6)

~ (lactone) 0

Scheme I

highly alkaline medium. Therefore, the reactivity of D-mannose can not be compared with chloramine-T. Though a ll other va lues of kll are for acidic

conditi ons, comparison is not poss ibl e as well because different mineral ac ids (H2S04 , HCI04 , HCI ) or buffe r (sodium picolinate - picolinic ac id) were used; th at too at different te mperatures . However, a compari so n

of kll under identica l conditions using Cr(Vl ) as ox idant can be made which reveals the sequence: L­

arabinose > D-fructose > D-mannose > D-xylose towards their reacti vity (the assoc iated rate constan ts being 8.3, 6.3 , 5.6 and 2. 1 x 1O-3 mol- I dm3 S- I)17 . Th is

trend shows that the ox idation by Cr(VI ) seeniingly depends on the number of ava il ab le primary -O H

2184 INDIAN J. CHEM ., SEC B, OCTOBER 2004

~OH fast R 0

~OH ... .. . Mn(n) l +

I

+ Mn(lI)

H H

(f3-D-mannopyranose)

A + Cr(VI)

Al

2Mn(III) fast ..

A - chromiurn(VI)

~ ~

+ Cr(lll) + Mn(I1I)

(lactone) 0

MnCH) + Mn(IV)

~OH fast R 0

~\ + Mn(" )

~ + Mn(IV)

H o

(f3-D-mannopyranose) (lactone)

Scheme II

Table III - Values of second-order rate constants (kll) for the oxidation of D-mannosc by different oxidants

Oxidant Medium Temperature 103 kll Ref. (mol dm-3) CC) (mor' dm3 s-')

Ce(IV)· H2SO4 25 3.0 3a (0.5)

V(V)b HCI04 45 4.1 30 (5 .7)

Mn(lII)" Buffer 40 0.23 31 (pH 6.1)

ch1oramine-r NaOH 45 15.0 32 (0.10)

bromamine-1"" HC1 25 0.38 33 (0.51)

Cr(VI)f HCI04 33 12.6 5b (0.75) (0.58) 40 5.6 Present work

•. f[oxidant] = 2.5 (a), 2.0 (b), 0.25 (c), 2.0 (d), 2.2 (e), 0.8 & 0.4 (f) x 10-3 mol dm-3;

[D-mannoseh = not specified (a), 10 (b), 30 (c), 2 (d), 4 (e), 1.6 & 3 (f) X 10-2 mol dm-3

... (7)

.. . (8)

... (9)

.. . (1 0)

... (11)

groups, the stereochemistry, and the chelating ability of the monosaccharide.

Kinetics in the presence of suljactants It was observed that a reaction mixture containing

CTAB (=1.4 x 10-4 mol dm-3) and HCI04 (=0.58 mol

dm -3) became intense turbid at room temperature (-30 DC); it did not become clear even after raising the temperature. Obviously, CTA perchlorate is formed which is known to be very insoluble. However, no precipitation occurred with anionic SDS or nonionic TX-100 surfactants; therefore, the kine~i c experiments

KABIR-UD-DIN el al.: REDOX BEHAVIOUR OF CHROMIUM(VI) TOWARDS D-MANNOSE 2185

Table IV - Effect of [SDS] and [TX- 100] on the pseudo-fIrst-order rate constants (kljl)

for the reaction of chromium(VI) with D-mannosea

103[surfactant] 104 ky! (S-I) 104 ky!cal (S -I)

(mol dm-3) SDS TX-100 SDS TX-100

2.5 4.5 4.9 5.2

5 4.8 5.3 6.2

10 5.3 5.9 5.3 6.4

IS 5.6 6.3 5.7 6.9

20 5.9 6.7 6.0 7.2

26 6.3 7.0 6.6 7.4

30 6.4 7.2 6.4 7.5

35 6.7 7.4 6.7 7.7

40 6.9 7.7 7.2 8.0

45 7.3 8.0 7.3 8.2

50 7.4 8.3 7.5 8.5

60 7.7 9.6 7.8 10.1

67 11. 1 11.2

a[Cr(Vl)h = 4.0 X 10-4 mol dm-3; [D-mannoseh = 30.0 x 10-3 mol dm-\ [HCI0 4h = 0.58 mol dm-3; temp. = 60 °C

were carried out only with SDS and TX-100 surfactants.

Effect of {SDSj and (TX-JOOj. The effect of varying the surfactant concentrations on the reaction rate (kljl) was studied keeping other variables constant ([Cr(VI)] = 4 x 10-4 mol dm-3

, [D-mannose] = 30 x 10-3 mol dm-3

, [H+] = 0.58 mol dm-3, temperature =

60 DC) . The results (Table IV) are illustrated in Figure 4 as kljl - [surfactant] profiles, which clearly demonstrate the surfactants ' catalytic effects not only above but even below cmc, i. e., micellar as well as submicellar catalyses are observed . Micelles are not fixed entities but have a transient character34

.

Surfactant monomers rapidly join and leave micelles and the aggregation number represents only an average over time. According to the multiple equilibrium model, therefore, the distribution of sUlfactant (D 1) between various states of aggregation is controlled by a series of dynamic association­dissociation equilibria:

8. 5 T X-IOO

7 5

I I '"

" 5 ~ :r x

oJ

<:>

'; 5

" 5

[) 10 20

103 [ surf oc t ant ] ( m old m - 3 )

Figure 4 - Effect of [surfactant] (SDS-~. TX-I 00- . ) on the kljl. Other conditions were the same as in Figure 1 (Set A).

The catalysis below cmc (i.e., submicellar catalysis) is not new and conforms to various available results9

.35

. The feasibility can be sought in the fact that small aggregates of the surfactant (dimers, trimers, tetramers, etc.) exist below the cmc; these small submicellar aggregates can interact physically with the reactants forming catalytically active entities. Effect of other variables

In order to confirm whether or not the same mechanism operates in micellar medium, the kljl

2186 INDIAN J. C1-I EM ., SEC S, OCTOBER 2004

values were determined as functions of variations in [D-mannose], [Cr(YI)] , [H+], [Mn(II)] and tempera­ture at fixed [SDS] or [TX-lOO] (Tables I, II and IV), we can see that the same pattern is being followed , i. e., fractional-order in [D-mannose] , complex-order in [W] , and first-order in [Cr(YI)]. The effect of vary ing [Mn(II)] on the rate was also the same,

namely, the k",-values increased with increase in fMn(II)]. These observations estab li sh that the mechanism of redox behaviour of chromium(YI) towards D-mannose in presence of surfactant micelles of SDS and TX-IOO remains the same as in the absence of surfactants.

Binding constants and rate constants in the micellar media

The binding (Ks and KM ) and the rate constants (km)

in the micellar media can be determined by considering the micellar pseudo-phase kinetic model proposed by Menger and Portno/ as modified by others " .36. This model considers the micelles and aqueous as two separate phases and that the reac tion occurs in both the phases (Scheme III).

(Cr( VI))w + Dn "Ks .. (C r(VI))m

+ + K,

(mannose)w + Dn ~ (mannose}m

I k w k m I ~ products ~

Scheme III

The method for calcul ation of constants by the use of non-linear least squares technique has been deta iled

earlier' 8. The best fit values are summarized in

Table V. For comparison, the k",ca' (Table IV) va lues were obtained by substituting the kw. Ks, krn, KM and [On] in the relevant equation. The agreement between the calcul ated and observed values provides supporting evidence for the method of calcul ation .

The reaction site. Despite uncerta inty of the exact reaction site of the micel lar-medi a ted reaction s, locations of the reactant mo lecules can at leas t be ascerta ined with some degree of reliability . T he chromi um (YI) (H2Cr04) has ne ither hydrophobic nor electrostatic inte ract io ns with the anionic headg roups of SDS mi cell es but the catalytic effect of th is surfactant clearly indicates the localization of H2Cr04 into th e Stern layer ([he s ite of most ionic micelle medi ated reac tion s) . In acidic medium, the pH of the an io nic micellar surface is decreased by ca. 2 uni ts due to the e lectrosta tic interaction/excl usion of Na+ by H+37 .

Under our experimental conditi o ns of [HCI04] ~ 0.58 mol dm-3

, chromium(YI) (H2Cr04) gets converted to HCrO) + (H2Cr04 + H+ ~ HCr03 + + H20)38. Thus , anionic headg ro ups (-OS03-) of SDS micelles fo rm ion-pai rs with HCrO) +. On the other hand, association of D-mannose molecul es (due to possessing 5-0H g roups) cannot be ru led o ut)9. The micelle th us he lps in bring ing the reac tants together which may now o rie nt in a manner suitable for th e es ter format io n (a characteri stic fea ture of the mechanism of chro mium(YI)-organ ic substrate reactions28).

Regarding catalys is in presence of TX-lOO, hydrogen bondin g between oxyethy lene and/or -OH of non-ionic TX-lOO mi cell es an d th e reac tants

Table V - Values of binding constants (Ks and KM ) and rate constants (kw. kIll and k2"') for the reacti on of chromium(V I) wi th D-mannose"

Paramctcrs Aqueous SDS TX- IOO I 03cmc (mol d l11 - .l) 9.8 (8.2)b (0.3)b

103 kw (S- I) 15.0 103k",(S- I) 7.0 3. 15

Ks (mol- I dm3) 86 58 KM (mol - I dm)) 84 75 104k2'" (mol- I dm) S- I)" 9.8 2 1.4

"[Cr(V lh = 4.0 x 10--1 mol dm-), rD-mannoseh = 30.0 x 10-3 mol dm-3, IHCI04h =

0.58 mol dm -), rSDSh = 26.0 x 10-3 mol dm- 3, [TX-I 001 = 50.0 x 10-3 mol dm-3

bLi terature values (quoted in parentheses) are taken from Mukerj ee P & Myscls K J, Critical Micelle COllcelltrations of Aqueous SIII.factant Systems, NSRDS-NSS 36, Superintendent of Documents, Washington , D.C., 1971 "Second-order rate constants (k2"') ~ re based on the relation: k2

0n = kIll . Von; VIlI = 0. 14 dill) 11101- 1 for SDS II and 0.68 dm) mor l for TX- I OO I3"

KA BIR-U D-D IN t'l a/.: REDOX Bl~ II A VJOl JR OF C' l !ROM IL/IW\ ' 1) TOWA RDS D-M A 1 OSE ~In

concc ntrZt ti,,p p: r•<ll'l t! : JL';,·: ,: .. ' ,,'(, :11. '!!•:!\! \ Oillll' t ' o r 11 i: •" · 11 , ,I c I'·,, '•.· ti 1 l' 'i'X-: ~)() llllCelie\ ta: .. :l\. r·h~!i.l.! ~ 1 C' t s. • ,) · ·""J ·;..' ,.:. ' \.\r C~~~~

Lnat nn: 0'''' t11 o\e, ,dl IC. t~·~~ L"lllial ,'" "-' !t l ...! l •: 1': l,;·~·1c:

., T\-1 (J. · " ... OJn n :p·j,, .. , i ' \ I ·· the 1': .. ·: •·

lllUlC" l:nL'I..1i\l e\'2!l d. ·(~\, l011Lt:l1l!"a:l\J'',) \\ :1: ... 1

lltlcr<IC<~l'i . c iectros ta tic J l l lel'<tct i on~ wit h lllc· pu l~..- hca J 21 <''lil\ , :· TX-1 OU, l>clt thi ~ no• .-inn1 ..: . Ul'! .ac l~l i!l U 1U!G

:;tahilizc an un dissociatc d H;CrU 1 rcla1ive t< ' the more h dropnil ic HC rO,< I an d gt ucosc thmug.l ilyuroge n bond 111 g . Thereafkr. t tl ~ a~'>oc iated D··

man nose an L! 1-l cC rOJ wit h the nnn -1 nn ic TX-1 00 trncelles (through hydrogen bo ndi ng ) seeming ly fac ilit ate fo rm at ion o r the ch romate-ester.

Fire effect o{.m.'r ..

Addition of electroly tes, in general. is rcspunsibk for ra t ' inhibit! I I'• Of' micellar rncdiateci rCa 'liOnS due to the excl usion or the rcagent(s ) !'rom the mi cellar p.l£'1{(/o-phase. Figun · ~ show,. that the kindic electrolyte efk ·t ts negative for all the added electroly tes ( 'aBr, LiB r. NH 18 r) !'or the ox idation of IHnannose b) chrom iumf\' !) in presence of SDS micelles . A l~u. the obsen ed rate constants depend on th na!llrc uf the cation. Thus. the excess monopositive cati ons seem to exc lude 1-t !'rom the rea ·tion site (which. in turn, mean ~; red uct ion in the effect ive IH Cr0/1) and the inhibitory power dec reases in the order 1-L/ <Li +<Na+. The behaviow is a:--, expected according to thei- increasi ng

hydrophob icity (N ;./> Li< >NH/) and polarising power.

Aclii 'Oiion parwneters

Comparison between the activation parameters in SDS and TX- 1 00 wit h that in aqueous medi um (Table II) clearly indicates tha t th e mice ll es act as catal ys ts and prov ide a new reac ti on path with lower val ues of £ ,, . Furthermore, though a change in temperature is known to prod uce changes in the size, shape, surface charge, etc., of the micelles, equall y good !'it of the observed data U'-nh' and /.:. ~) to the Ey ring eq uat ion (both in the absence as well as pre. ence of surfacta nts) shows that the mi celle structural sensitivity to temperat ure is kineti ca ll y unimportant. Simil ar conclusions had been drawn earlier also 12

hA 1

.

---- _,... .__ - ...__. ------~-c--.-~--~-~----_:~~ _I~~~ r r

0 2 ~.

LsaltJ(mol d;n-l)

--~hi1Rr

-- L c :Jn

Figure 5- E!'tccl ol ~all ~ ( I l ~ Br-e Lif1 :·-o al3r- '-l ! on tl,_· /,,,_ Oilie r condnion o; \\c l c the >amc as 111 l•igu re I (Set \) " '1

!SDSI=2(,_0 x 10 mold1 11'

Acl<nowlcdgcmcn t ~

One ol' the authors (AMAM ) is grateful to the HR D Min is try. Govt of Ind ia. for ailowt ng a research

.O.C. and ah' to the EclucatioP ·1i:listry of Bangi<Jdesh l'or acco rdin g permtssion to pur ... u.:: research in lndi .L

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