r

nuclear chlorination has not been reported earlier.The pH-rate profile shows a maximum around pH3.7. The stoichiometry under kinetic conditions{[CAT] : [SA] = 1:5 to 8) obtained as -d[COOH]!d [CAT], comes out to be 1:1 but when [CAT] is in-excess further reactions set in. The positive salt·effect in the absence of CI- may be due to the.separation of oppositely charged ions from the.activated state, as found" in the neutral decornpo-:sition of peroxydisulphate ion.

S. V. thanks the UGC, New Delhi for a teacherresearch fellowship.

References1. VIVEKANANDAN, S., VENKATARAO, K, SANTAPPA, M. &

SHANMUGANATHAN, SP., Indian J. Chem., 18A (1979),503.

2. JENNINGS, V. J., CRC c-u. Rev. Anal. Chem., 3 (1974).413.

3. AsHMORE, P. G., Catalysis and inhibition of chemical re-actions (Butterworths London), 1963, 81.

4. BALASUBRAMANIAN, V. & THIAGARAJAN, V., Int. J. chem.Kinet., 7 (1975), 605.

5. MUSHRAN, S. P., SHARMA, J. & PANDEY. L. J., Curro sa.,48 (1979), 629.

,6. CHANDRA SINGH, U. & VENKATARAO, K., J. inorg. nucl.Chem.• 38 (1976), 541.

Kinetics & Mechanism of Oxidation of Cellobiose &Melibiose by Hexacyanoferrate(III) in Ammoniacal

Medium']

K C. GUPTA* & ANITA KUMARI SHARMADepartment of Chemistry, Lucknow University,

Lucknow 226 007

Received 28 February 1979; revised 26 December 1979; accepted14 April 1980.

The rates of the title reactions are independent of [Fe(CN)~-]-and directly proportional to [melibiose] and [cellobiose] andsquare root of [ammonia]. Addition of NH.CI retards the rate.'The catalytic constants kOH- and kNH3 have been calculated.A mechanism involving the formation of an intermediate enediol.anion has been proposed.

OXIDATION of reducing sugars by Cu(II) andhexacyanoferrate(IlI) have been studied by

Singh et af.1-6 in strong alkaline medium who:suggested a mechanism based on kinetic data. Similarresults were reported by Marshall and Waters",Wiberg and Nigh" and Lambert and Jones". These.authors suggested a mechanism involving the forma-tion of intermediate enediol. The present paperdeals with the kinetics of oxidation of cellobiose andmelibiose by hexacyanoferrate(IlI) in the presenceof weak base ammonia and buffered medium.

Melibiose (AR, K. light) and cellobiose (Austranalpreparate) were used. All other chemicals used wereof AR grade. The total volume of the aqueousreaction mixture was kept 50 ml in each set. Theprogress of reaction carried out in dark-colouredbottles was followed by' estimating iodometrically

, tPaper presented at Indian Science Congress, ChemistrySection, Ahmedabad, 1978.

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NOTES

unreacted hexacyanoferrate(III) at different timeintervals.

One mol of reducing sugars required six mol ofhexacyanoferrate(III). The reaction showed a briefinduction period due to dissolved oxygen whichoxidised initially produced hexacyanoferrate (II).The reaction mixture was flushed with nitrogen whenthe induction period was no more noticed. Theorder of the reaction with respect to K3Fc(CN)6 wasdetermined at fixed concentrations of other reactantsand at constant ionic strength (by adding KCl). Thestandard zero-order rate constant, ks was calculatedusing Eq. (1).

ks = ko X S ( )V ... 1

6.xwhere ko = --z;;- ,S is the strength of hypo

solution and V is the volume of aliquot taken. Theresults are summarized in Table 1. The kg valuesdo not show any variation at different initial concen-trations of K3Fe(CN)6' indicating that the reactionis zero-order with respect to hexacyanoferrate(III).This was further supported by the linear plot of theconcentration of the unreacted hexacyanofcrrateiIll)versus time. The slopes of the linear plot gave ksvalues almost similar to those obtained by Eq. (1).

The data in Table 2 show that the reaction ratedecreases proportionately with the decrease in[disaccharide]. The ratio kg![ disaccharide] is practi-cally constant in each case, confirming the first-orderkinetics with respect to disaccharide.

The ks values increase with the increase in [ammo-nia]. The ratio ks/[NH3]1/2 is fairly constant (Table 3).The slight increase in this ratio at higher[ammonia] is due to catalytic activity of undisso-ciated ammonia which increases gradually. Thus therate of reaction is directly proportional to squareroot of ammonia at low concentrations.

TABLE 1- EFFECr' OF [K3Fe(CN).1 ON THE REACTION RATE

{[NH3]=20.0x lO-2M; 1J.=.035 M; [Cellobiose] = 1.00 x lO-2M;[Melibiose] = 1.66 x lo-2M; temp. 35°}

(K3F.(CN)6] X 103 k. X 10' (mol litre'< min-I)M

Cellobiose Melibiose

5.00 1.45 3.873.33 1.49 3.772.50 1.40 3.82

TABLE 2 - EFFECT OF VARYING [DISACCHARIDE] ON THEREACfION RATE .

{[K3Fe(CN)6] = 2.5 x lO-3M; [NH3] = 20.0 x lo-2M;'temp. = 35°}

[Cell 0- k. x 10' k« X .103 [Meli- k. x 10' k« X 103biose] (mol --- biose] (mol ----

102M Jitre=' [Cellobiose] 102M litre=" [Meli-min-I) min-I) biose]

2.00 2.00 4.61 2.301.66 2.30 1.38 1.66 3.82 2.311.25 1.77 1.41 1.25 2.84 2271.00 -1.40 1.40 1.00 2.34 2.34

87

INDIAN J. CHEM., VOL. 2oA, JANUARY 1981

TABLE 3 - EFFECT OF [NH3] ON THE REACTIONRATE

{[K3Fe(CN).] = 2.50 x lO-3M; [Cellobiose] = 1.00 x 10-2M;[Melibiose] 1.66 x lO-2M; temp = 35°}

Cellobiose Melibiose

[NH3]x102 k« X 10' k. X 10' k« X 105 k. X 10'M - (mol Iitre=) lN1-f,j"" (mol litre= [NH,j!'"

min-I) min-I)

50.00 2.400 3.39433.33 1.800 3.119 4.940 8.54325.00 4.240 8.48020.00 1.400 3.130 3.820 8.54710.00 0.977 3.089 2.692 8.5128.33 0.891 3.087 2.430 8.4206.66 0.800 3.099 2.180 8.447

On addition of NH4CI the reaction rate decreasesdue to common ion effect (Table 4). From theseobservations it appears that the oxidation processis generally base-catalysed in nature. The total rateis governed by Eq. (2).

ks = kOR- [OH-] + kNH3 [NH3] ... (2)

where KoH- and KNIT3 are the catalytic constants ofOH- and NH3 respectively. The rate of reactionhas been calculated at constant ionic strength anddifferent constant pH (Table 4). The plots of ksversus [ammonia] were found to be linear at lowconcentrations. From the intercepts and slopes ofthese linear plots the values of kosc: and kNH, werecalculated and their mean values are summarized inTable 5.

The activation parameters were evaluated fromlog k« versus liT plots. The values of energy of acti-vation are 6Et=80.5 and 96.2 kJ mol= and at35°C, kr=klk~ = 5.14 X 10-5 and 8.46 x 10-5 litresee'? and 6St = -74.1 and -20.4 JK-l mol'?for cellobiose and melibiose respectively. The nega-tive entropy of activation is due to solvation ofactivated complex which becomes more polar thanthe reactants leading to a decrease in entropy.

From the above experimental results it appearsthat the initiation involves the reaction between hy-droxyl ions and the disaccharide forming an activeintermediate enediol anion which is subsequentlyoxidisedby[Fe(CNH-]. -The latter being a fastreaction, therefore overall reaction becomes zero-order with respect to hexacyanoferrate(III). Thereaction sequence is shown in Scheme 1.

kbNH, + H20 ¢ NHt + OH-[OH-] = k ~, NH3P/2o 0-

1\ kl IH-C-CH-OH + OH- ¢ H-C=C-OH + H20 ...(Hi)

I LI IR R _disaccharide enediol anion (E)

k2[enediol anion] + [Fe(CN):-] ...• [Fe(CN)~-] + Products

fast

Scheme 1

Applying the steady state condition for -E' and in-

88

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... (iv)

TABLE4 - EFFECT OF VARYING [NH.Cl] ONRATE CoNSTANTAT FIXED [NH3]/[NH.CI] RATIO_

{[K.Fe(CN).] = 2.50 x 10-3M; [Cellobiose] = [Melibiose] =1.66 x lO-2M; IL = 0.035M}

k. x 105 (mol lltre'< min-I) for

Cellobiose Melibiose

35° 40° 35° 40°

[NH,]/[NH.Cl] 10; pH = 10.00.609 1.020 0.950 1.950.414 0.743 0.672 1.190.366 0.658 0.592 1.020.292 0.538 0.500 0.90

[NH.]![NH.Cl] - 20; pH = 10.20.828 1.410 1.258 2.130500 0.861 0.766 1.390.452 0.772 0.692 1.220.380 0.670 0.590 1.07

TABLE5 - VALUESOF -CATALYTICCONSTANTSAT DIFFERENTTEMPERATURES

{[K,Fe(CN).] = 2.50 x lo-3M; [Cellobiose] = [Melibiose] =1.66 x lO-2M; IL = 0.035M}

Temp. Cellobiose Melibiose("C)

kOH- x 10 kNH3 x 10'kOH- x 10 kNH,X 10'

35 0.0978 0.3840 0.1620 0.508840 0.1630 0.5971 0.2980 0.8950

equality of k2 [Fe(CN)~-] ). k_1[H20] the final rateexpression comes out to be _

d[Fe(CN)~-]dt k1[disaccharide] [OH-] .. (v)

= k1k1b[disaccharide] [NHz]1/2(vi)The rate law (vi) is in total agreement with ourexperimental findings.

It may be noted that reaction rate is not affectedby the initial presence of hexacyanoferrate(II).There is no resin formation and rapid transformationon treatment of cellobiose and melibiose withammonia'".

One of the authors (A. S.) is thankful to CST,U. P. for providing financial assistance and to Prof.S. S. Tiwari, for laboratory facilities.

... (i)

.. .(ii)

References1. SINGH, M. P., KRISHNA,B. & GHOSH,S., Z.physik. Chem.,

204 (1955), 1.2. SINGH,S. V. & SINGH,M. P., Z. physik: Chern. (Frankfurt),

50 (1966), 11.3. SINGH,S. V., SAXENA,O. C. & SINGH, M. P., J. Am. chem .

Soc., 92 (1970), 537.4. NATH, N. & SINGH, M. P., Z. physik: , Chem., 224 (1963),

419.5. NATH, N. & SINGH, M. P., J. phys. Chem., 69 (1965), 2038.6. SRIVASTAVA,R. K., NATH, N. & SINGH, M. P., Bull. chem.

Soc. Japan, 39 (1966), 833.7. MARSHALL,B. A & WATERS,W. A, J. chem. Soc., 1961,

1579.8. WIBERG,K. B. & NIGH, W. G., J. Arn. chem. Soc., 87

(1965), 3849.9. LAMBERT,D. G. & JONES,M. M., J. inorg. nuc1. Chem.,

29 (1967), 579.10. SPECK (Jr), C. JOHN, Cited in Advances in carbohydrate

chemistry, Vol. 13 (Academic Press, New York), 1958,63.