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ISSN 0036 0244, Russian Journal of Physical Chemistry A, 2014, Vol. 88, No. 5, pp. 780784.

Pleiades Publishing, Ltd., 2014.


Substrate Inhibition: Oxidation of D Sorbitol and D Mannitol by Potassium Periodate in Alkaline Medium1
Y. Lakshman Kumara, R. Venkata Nadhb and P. S. Radhakrishnamurtia
a b

Department of Chemistry, TJPS (PG) College, Guntur 522006, India School of Biotechnology, Vignan University, Vadlamudi 522213, India e mail:
Received April 01, 2013

AbstractIn the oxidation of D sorbitol and D mannitol by potassium periodate in alkaline mediam, sub strate inhibition was observed with both the substrates, i.e., a decrease in the rate of the reaction was observed with an increase in the concentration of substrate. The substrate inhibition was attributed to the formation of stable complex between the substrate and periodate. The reactions were found to be first order in case of peri odate and a positive fractional order with hydroxide ions. Arrhenius parameters were calculated for the oxi dation of sorbitol and mannitol by potassium periodate in alkali media. Keywords: kinetics, substrate inhibition, D sorbitol, D mannitol, potassium periodate, alkaline medium. DOI: 10.1134/S003602441405015X

INTRODUCTION Studying the oxidation of sugars is very significant because in most of the living organisms, the major amount of energy is obtained from carbohydrates [1]. Polyols (viz., inositol, mannitol, sorbitol) are impor tant osmolytes in various animals, plants and procary ote organisms [24]. Sorbitol and inositol are signifi cant osmolytes in the renal medulla of mammals [57]. D mannitol is abundant in nature, such as in plants, algae, yeasts, fungi, and bacteria. Mannitol is a major storage polysaccharide in fungi that also plays an important role as a metabolite in stress tolerance [8, 9] from the physiological functions fulfilled by mannitol in lower and higher eukaryotes [10]. Sorbitol finds many applications in food, pharmaceutical and cos metic industries and as additives in many end prod ucts. It was used as a key intermediate in the synthesis of ascorbic acid [11]. 1 Acid catalysed oxidation of sugars and sugar alco hols with different oxidants was reported by earlier researchers [1214]. Most of the studies carried out in perchloric acid media reported that the reaction was 1 acid catalysed [15, 16]. The kinetics and mechanism 2 of cerium (IV) oxidation of hexitols (D sorbitol and D mannitol) in aqueous sulfuric acid media have been studied in the presence and absence of surfactants 1 [17]. The ruthenium (III) catalysed oxidation of D mannitol by cerium (IV) was studied spectrophoto metrically in aqueous sulfuric acid medium [18]. Ode 3 bunmi and marufu [19] studied oxidation of D man nitol and D sorbitol by KMnO4 in NaHCO3/NaOH
1 The article is published in the original.

buffer and IrCl 6 in sodium acetate/acetic acid buffer. Vanadium (V) oxidation of D sorbitol was studied in the presence of externally added surfactants [20]. The literature survey shows that the oxidation of sugar alcohols has received modest thought compared to studies on the oxidation of sugars. Taking these facts into consideration, recently the present authors have carried out periodate oxidation of cyclic polyol inositol in alkaline medium and reported substrate inhibition due to stable complex formation between inositol and periodate [21]. In continuation, this paper describes the kinetics and mechanism of periodate oxidation of sugar alcohols like D sorbitol and D mannitol in alkaline medium. Authors are interested to check the possibility of exhibition of substrate inhi bition by these acyclic polyols. EXPERIMENTAL All the reagents used in these experiments were of analytical reagent grade. Requisite volumes of oxidant and substrate solutions were thermostated at 35 0.1C to attain equilibrium. After rapid mixing an equal volume of oxidant solution to the substrate solu tion, progress of the reaction was followed by assaying aliquots of the reaction mixture for periodate, iodo metrically using starch as an indicator after suitable time intervals. In the oxidation of sorbitol and manni tol, one oxygen loss or two electrons transfer per peri odate molecule was observed i.e., oxidation capacity of oxidant was observed till the conversion of periodate


SUBSTRATE INHIBITION: OXIDATION OF D SORBITOL AND D MANNITOL Table 1. The rate constants (k1 104, min1) of oxidation of sorbitol and mannitol by periodate in alkaline medium Variant Periodate c, M 0.00025 0.0005 0.001 0.005 0.025 0.05 0.1 0.05 0.1 0.2 0.5 1.0 35 45 55 0 0.01 0.025 0.05 0.1 0.1 0.1 0.1 D Sorbitol 3.881 2.854 2.897 3.608 2.854 1.677 1.909 2.246 2.854 3.182 4.182 9.355 2.854 9.197 43.127 2.854 3.012 3.677 4.629 2.854 2.217 2.747 3.192 3.250 D Mannitol 18.810 19.000 19.370 46.800 19.000 12.730 12.350 19.000 24.880 33.890 19.000 71.460 240.250 19.000 27.03 33.130 49.268 19.000 17.353 21.854 23.374 69.999




substrate(s)/alkali were determined from the slope of log k1 vs. log [variant] plots while maintaining all other concentrations and conditions constant. The rate constants increased with an increase in alkali concentration and a fractional slope for log k1 versus log[alkali] indicates a positive fractional order dependence of rate on alkali. In the earlier studies by Okoro and Odebunmi, an increase in the rate of oxi dation of sorbitol and mannitol by KMnO4 was observed with an increase in the pH (range 9.60 to 11.00) of the reaction medium and was attributed to base catalyzation [22]. The three dissociative equilibria of the periodate ion in alkaline medium and the corresponding equilibrium constants at 298.2 K were given by Aveston [23] 2IO 4 + 2OH
IO 4

H 2 I 2 O 10 ,
2 H 3 IO 6 , 3

log 1 = 15.05 , (1) log 2 = 6.21 , (2)

+ OH + H 2 O


Boric Acid

No Salt KNO3 KCl KI KBr

(3) IO 4 + 2OH H 2 IO 6 , log 3 = 8.67 . The composition of different species of periodate in aqueous alkaline solution can be obtained from the equilibrium constants K1, K2 and K3. Within the used alkali concentrations, the amounts of both the species 4 of periodate (dimmerH2I2 O 10 and IO 4 ) can be neglected. From the give below calculations, it is clear 3 that the key species of periodate are H2 IO 6 and H2 IO 6 , which is corroborating with the earlier reports of Crouthamels data [24] and other recent results reported in the literature [25, 26].
3 [ H 2 IO 6 ] 2

Note: [Periodate] = 0.0005 M, [Substrate] = 0.025 M, [OH] = 0.1 M, temperature 35C, c is concentration of variant.

3 [ OH ]

2 2

1 + 2 [ OH ] + 3 [ OH ] = f ( [ OH ] ) [ IO 4 ] cx ,

[ IO 4 ] ex

Table 2. Concentrations of periodate species at different concentrations of alkali [OH ] M 0.05 0.1 1.0

2 [H3I O 6 ]

3 [H2IO 6 ]

[ H 3 IO 6 ] =

2 [ OH ] 1 + 2 [ OH ] + 3 [ OH ]

[ IO 4 ] ex

0.000307612 0.000228251 0.0000393489

0.000180027 0.000267163 0.000460572

into iodate. This was also confirmed by non oxidation of sorbitol and mannitol by iodate. RESULTS AND DISCUSSION The kinetic studies were carried out under pseudo first order conditions with the concentration of the substrate in large excess compared to that of the oxi dant. The pseudo first order with respect to [perio date] was almost constant in the concentration range 0.000250.001 M (Table 1). The reaction orders of

= ( [ OH ] ) [ IO 4 ] cx . The original overall periodate ion concentration is represented by [ IO 4 ] ex , which is approximately equal to the sum of the concentrations of two main species of 3 2 periodate (H2 IO 6 and H2 IO 6 ). At different alkali concentrations, [H2 IO 6 ] and [H3 IO 6 ] are calcu lated (Table 2). The predominant species are H3 IO 6
3 2 3 2

and H2 IO 6 at lower (0.05 M) and higher (1.0 M) concentrations of alkali respectively. At 0.1 M alkali (where most of the reactions are carried out) both ionic species exist equally and hence the sugar alco 2 3 hols complex with either H3 IO 6 or H2 IO 6 .
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782 Table 3. Arrhenius parameters at 308K Substrate D Sorbitol D Mannitol E , kJ/mol 113.8336 106.5674


H , kJ/mol 111.2728 104.0066

S , J K1 mol1 13.31927 6.150066

log PZ 13.9511 13.57661

G , kJ/mol 115.3752324 105.900888

Two or more molecules of water are displaced from the trimester sphere of periodate ion when three or more hydroxyl groups in a polyol are in a sterically favourable arrangement. Hydroxyl groups lose a pro ton in alkaline medium and form much stronger com plexes. Barker [27] described the existence of trimes ters of periodate ion with inositol and suggested that the iodine atom is attached to an axeqax sequence of oxygen atoms which was confirmed by N. M. R. spectrum. Out of the three hydroxyls which have com plex one is axial hydroxyl second equatorial third axial hydroxyl these three OHs have complexed with the 2 periodate species H3 IO 6 in alkaline medium. Relative Oxidation of Sorbitol and Mannitol by Periodate The kinetic data (Table 1) clearly indicates that in alkaline medium, rates of oxidation of mannitol are very much higher as compared to those of sorbitol. This report is quite remarkable because the reports demonstrate that sorbitol is the most reactive while mannitol is the least reactive in their oxidation by alka line KMnO4 [22] and by vanadium pentoxide (V2O5) in acidic medium [28]. Current observations can be explained based on the geometry of 5, 6 hydroxyls in these compounds. No stable complex formation takes place between perio date and mannitol due to the trans geometry of 5, 6 hydroxyls in mannitol. Whereas, in the case of sorbi tol, due to the cis geometry of 5, 6 hydroxyls a stable complex forms which is responsible for the lower oxi dation rate in sorbitol. In sorbitol oxidation, the breakdown of the complex leads to cc fission and gives two aldehydes (a) formaldehyde and (b) arabi nose. In the case of mannitol, there is no stable com plex formation because of the trans geometry of 5, 6 hydroxyls. Hence oxidation rates are quite higher for mannitol as compared to sorbitol. We envisage differential mechanism for mannitol which may be an acyclic pathway leading to the corre sponding aldehyde as the primary major product. The inhibition observed with increasing concentration of mannitol can be traced to competing cyclic mecha nism operating to a lesser extent, because of unstable complex formation with mannitol along with acyclic mechanism which is the cause the higher rate observed with mannitol. Oxidation of sorbitol yields by cyclic mechanism as primary products D arabinose and formaldehyde, whereas oxidation of mannitol yields

D mannose as a primary product. The products for mation was confirmed by spot tests [29]. Substrate Inhibition A decrease in k1 values with an increase in concen tration of sorbitol/mannitol was observed and an inverse fractional with respect to substrate was found (Table 1). Much attention was not paid by the earlier workers who observed similar effects with hexacyano ferrate (III) oxidation of carbohydrates [30] and ceric sulphate oxidation of alcohols [31]. Olusanya and Odebunmi studied copper (II) inhibition in the oxida tion of maltose and xylose by hexacyanoferrate (III) in alkaline medium and reported that the order of reac tion in sugar decreased from one to zero at higher sugar concentration, but actually, their data shows that a 12% decrease in rate of reaction at higher concentra tion of maltose can be observed [32]. Substrate inhibition, i.e., a decrease in rate of reac tion with an increase in substrate concentration can be contributed to the formation of periodate sugar alco hol complex which resists oxidation. The inert com plexes must contain a higher proportion of substrate molecules because of 1:1 complexes, which take part in the oxidation. Similar substrate inhibition was reported by us in the oxidation of myoinositol by peri odate in alkaline medium [21]. Effect of Temperature The rate constants (k1) increased with an increase in temperature with both substrates. The plot of ln k vs. 1/T yielded a straight line and the values of activation parameters viz. enthalpy of activation (H), entropy of activation (S) and Gibbs free energy (G) were evaluated from the slope of the plot by using the Eyring equation [33, 34] and tabulated (Table 3). The high +ve value of free energy of activation (G) indi cates highly solvated transition state, while ve value of entropy of activation (S) suggest the formation of an activated complex with reduction in the degree of freedom of molecules. Effect of Boric Acid Increase in rate was observed with an addition of boric acid due to favouring conditions for complex formation between substrates and highly dissociating potassium borate, which competes with the stable complex formation between sorbitol/mannitol and
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periodate. Such observations were reported earlier by other authors in sodium borate solution with other hexose like inositol. One of the first reports on the for mation of boronic esters from diols and polyols, described the preparation of several esters of phenyl boronic acid by reaction of the latter, in warm water, with sugars like mannitol and sorbitol, and 1,2 diols like catechol and pinacol [35]. Similar formation of tridentate borate ester anions (I) in sodium borate solution by cyclic polyols was reminiscent by their strong mobility towards the anode in electrophoresis [36] and changes in NMR spectrum [37, 38]. Effect of Salts Salt effect was studied on the oxidation of sorbitol and mannitol by periodate. Ions like chloride, iodide and nitrate didnt show an appreciable effect on the rate of reaction, where as bromide increased the rate. Earlier, a positive catalytic effect of bromide ion was reported in the oxidation of dextrose [39] and fructose [40] by Ce (IV) in aqueous sulphuric acid medium. Product Analysis Sugar oxidation occurs under different conditions of pH, temperature and ionic strength giving product that depend on the reactions used [14]. The kinetics and mechanism of oxidation of monosacharides and disaccharides have been studied in both acidic and alkaline media, employing different transition metal ions, inorganic acids, complex ions and hydrogen per oxide as oxidants. The results showed that the mecha nism may depend on the nature of the substrates, in some cases it involves the formation of intermediate complex, free radical or transition states [12]. In the present case, the spectral analysis of hydrazone deriv
2 3

ative of the product indicates the products to be an aldehydes as indicated above. Agarwal et al. [1] also reported the aldehydes in the oxidation of mannitol by Ce(IV) in acid medium. Faster Reactions in Acid Medium The rates of reaction between periodate and sorbi tol/mannitol were instantaneous in acid medium and hence were unable to follow kinetics as per the method described above in the experimental section. However, these reactions were slow in alkaline medium. This observation was in parallel with our earlier studies of oxidation of inositol by periodate [21]. The lower rates of oxidation of sorbitol/mannitol in alkaline medium can be attributed to the slow scission of substrate as well as slow cleavage of the complex between oxidant and substrate. Such reports also are available in the lit erature in the case of reaction between alcohols and periodate. According to Bunton and colleagues [42, 43] the monocyclic type formed as a transient inter mediate in glycol scission reactions is a dianion at pH 9. They regard dianion as highly stable because it cannot likewise suffer dehydration thus accounting for the rel atively slow rate of periodate oxidations at high pH. Perlin and Von Rudolff [44] reported that the cleavage of complex between diols and periodate is slower in weakly alkaline compared to that in acid medium. Rate Law in Alkali Medium H3 IO 6 + S H2 IO 6 + S
3 2 K4 K5

Complex C1 Complex C2

k1 k2

Products, Products.

Since, the [ IO 4 ] and [H2I2 O 10 ] are negligible, the total concentration of periodate can be written as

[ IO 4 ] T = [ H 3 IO 6 ] + [ H 2 IO 6 ] + [ Complex C 1 ] + [ Complex C 2 ] = K 2 [ IO 4 ] [ OH ] + K 3 [ IO 4 ] [ OH ]
2 2

+ k 1 K 2 K 4 [ S ] [ IO 4 ] [ OH ] + k 2 K 3 K 5 [ S ] [ IO 4 ] [ OH ] ,
[ IO 4 ]

[ IO 4 ] T [ OH ] { K 2 + K 3 [ OH ] + k 1 K 2 K 4 [ S ] + k 2 K 3 K 5 [ S ] [ OH ] } Rate = k 1 [ Complex C 1 ] + k 2 [ Complex C 2 ] = k 1 K 4 [ H 3 IO 6 ] [ S ] + k 2 K 5 [ H 2 IO 6 ] [ S ]

2 3 2

= k 1 K 2 K 4 [ IO 4 ] [ OH ] [ S ] + k 2 K 3 K 5 [ IO 4 ] [ OH ] [ S ] = [ IO 4 ] [ OH ] [ S ] { k 1 K 2 K 4 + k 2 K 3 K 5 [ OH ] } = [ IO 4 ] T [ S ] { k 1 K 2 K 4 + k 2 K 3 K 5 [ OH ] } K 2 + K 3 [ OH ] + k 1 K 2 K 4 [ S ] + k 2 K 3 K 5 [ S ] [ OH ]
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CONCLUSION Substrate inhibition was observed in the oxidation of D sorbitol/D mannitol in alkaline medium due to stable complex formation between the substrate and periodate. REFERENCES
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SPELL: 1. catalysed, 2. hexitols, 3. marufu