Kinetics and Mechanism of Oxidation of Erythritol by Permanganate Ions in an Alkaline Solution

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Kinetics and Mechanism of Oxidation of Erythritol by Permanganate Ions in an Alkaline Solution Manjanath B. Patgar, Sharanappa T. Nandibewoor, Shivamurti A. Chimatadar* P. G. Department of Studies in Chemistry, Karnatak University, Pavate Nagar, Dharwad 580003, India Corresponding author: Tel.: +91‐836‐2771606; Fax (Off): 91‐836‐2771275 *E‐mail address: schimatadar@gmail.com

Abstract The oxidation of erythritol (ERT) by permanganate ion ( MnO-4 ) in aqueous alkaline medium and a constant ionic strength of 6.0×10‐2 mol dm‐3 were studied spectrophotometrically at 25oC. The reaction between erythritol and MnO-4 in alkaline medium exhibits 1:4 stoichiometry. The product, 2, 3, 4‐ trihydroxybutanoic acid, was isolated and identified with the help of TLC and characterized by FT‐IR and GCMS. The other product MnO42‐ was confirmed by UV‐Vis spectral studies. The order of the reaction with respect to MnO-4 and ERT concentrations were found to be unity and less than unity respectively. The rate of the reaction increased with increase in []. The effect of ionic strength, dielectric constant and added product on the rate of reaction was also studied. Based on the experimental results, the probable mechanism was proposed. The activation parameters with respect to slow step of the mechanism were computed and discussed. Thermodynamic quantities were also calculated.

Keywords Kinetics; Oxidation; Mechanism; Erythritol; Alkaline Permanganate; Thermodynamic Parameters

Introduction Potassium permanganate is widely used as an oxidizing agent as well as an analytical reagent and also as disinfectant. These reactions are governed by the pH of the medium. Among six oxidation states of manganese from +2 to +7, permanganate, Mn(VII), is the most potent oxidation state in acid as well as in alkaline media. The oxidation by permanganate ion finds extensive application in organic synthesis [1, 2]. During oxidation by permanganate, it is evident that permanganate is reduced to various oxidation states in acidic, alkaline, and neutral media. In a strongly alkaline medium, the stable reduction product [3, 4] of permanganate ion is magnate ion, MnO42-. The process can be divided into a number of partial steps and examined separately. The MnO2 appears only after a long time, that is, after the complete consumption of MnO -4 .

The kinetics of oxidation of sugars is subject of extensive research in recent years. This is attributed to the economic and biological importance of carbohydrates to living organisms. The oxidations have been reported out in both acidic and alkaline media using such oxidants as transition metal ions, inorganic acids, organometallic complexes and enzymes [5‐12]. Despite much work already done on the oxidation of sugars, very little attention was given to the use of permanganate anion [6]. The present study is therefore undertaken to clarify the mechanism of oxidation of erythritol by permanganate anion in alkaline medium as a follow up of the previous studies in the literature on the oxidation of sugars with chromium(VI) and iridium(IV) ions [13, 14].

OH HO

OH OH

Erythritol

Erythritol is a four‐carbon sugar alcohol with sweetness intensity varying from 0.6 to 0.8 [15]. Erythritol is the only sugar alcohol produced commercially by fermentation [16]. Blood glucose and insulin levels do not increase when erythritol is administered orally to normal male subjects [17]. From the results of acute and subchronic studies in animals, erythritol can be classified as non‐toxic [18]. In addition, erythritol is also noncariogenic [19]. The Food

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and Drug Administration (FDA) has declared erythritol generally recognized as safe. Studies had shown that the consumption of 20 and 35 g of erythritol by healthy volunteers, in liquid, is tolerated well without any symptoms.

Because erythritol has a high digestive tolerance and is noncaloric, noncariogenic and nonglycaemic and has antioxidant properties, it fits well into the functional food concepts to which it confers a healthy image [20].

I assumed there were no reports on the mechanism of the oxidation of erythritol by permanganate in alkaline medium. Hence, in view of the pharmaceutical importance of erythritol and in order to understand the active species of oxidant and to identify a suitable mechanism, the title reaction was investigated. An understanding of the mechanism allows the chemistry to be interpreted, understood, and predicted.

Experimental Materials and Reagents Reagent‐grade chemicals and double distilled water were used throughout the work. A solution of erythritol (HIMEDIA) was prepared by dissolving an appropriate amount of recrystallized sample in double distilled water.

The purity of the erythritol was checked by its m.p 120C (literature m.p 121C). The required concentrations of erythritol were prepared from the stock solution. The stock solution of Mn(VII) was prepared by dissolving potassium permanganate (s. d. fine Chem. Ltd) in water and concentrations were ascertained by titrating against oxalic acid [21]. The KNO3 (BDH) and KOH (BDH) were used to maintain the ionic strength and alkalinity of the reaction respectively.

Instruments Used For kinetic measurements, a Peltier accessory (temperature‐controlled) attached to a Varian CARY 50 Bio UV−vis spectrophotometer (Varian, Victoria‐1370 Australia) connected to a rapid kinetic accessory (HI‐TECHSFA‐12) was used. For product analysis, Shimadzu 17A gas chromatograph with a Shimadzu QP‐5050A mass spectrometer with electron impact (EI) ionization technique, a Nicolet 5700 FT‐IR spectrometer (Thermo Electron Corporation, Madison, WI) were used.

Kinetic Studies The kinetic measurements were performed on a Varian CARY 50 Bio UV‐vis Spectrophotometer (Australia) under pseudo‐first‐order condition where [erythritol] > [ MnO -4 ] at 25oC, unless otherwise specified. The reaction was initiated by adding the MnO -4 to the erythritol solution, which also contained the required concentration of KNO3 and KOH. Progress of reaction was followed spectrophotometrically at 526 nm by monitoring the decrease in absorbance due to MnO -4 (ε = 2100±50 dm3 mol‐1 cm‐1). The spectral changes during the chemical reaction for the standard condition at 25oC are shown in Fig. 1. It is evident from the figure that the concentration of MnO-4 decreases at 526 nm. It was verified that there is negligible interference from other species present in the reaction mixture at this wavelength. The pseudo‐first order rate constants, kobs, were determined from the log[ MnO-4 ] versus time plots. The plots were linear up to 80% completion of reaction under the range of [] used (Fig. 2). The rate constants were the average of three independent sets and reproducible to within ±5% (Table 1).

FIG. 1 SPECTRAL CHANGES DURING THE OXIDATION OF ERYTHRITOL BY PERMANGANATE IN ALKALINE MEDIUM AT 25 OC. CONDITIONS: [ MnO-4 ] = 2.0 ×10‐4, [ERT] = 2.0 × 10‐3, [] = 2.0×10‐2, AND I = 0.06 mol dm‐3 (SCANNING TIME INTERVAL IS 1 MIN).

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2.4

(6)

5+log[MnO4-]

2 1.6

(1)

1.2 0.8 0.4 0 0

1

2

3 Time (min)

4

5

6

FIG. 2 FIRST ORDER PLOT OF OXIDATION OF ERYTHRITOL BY PERMANGANATE IN ALKALINE MEDIA AT 25̊C, [ERYTHRITOL] = 2.0×10‐3 mol dm‐3, [] = 0.02 mol dm‐3; I = 0.06 mol dm‐3, [ MnO-4 ] ×104 mol dm‐3 = (1) 0.50 (2) 1.0 (3) 2.0 (4) 3.0 (5) 4.0 AND (6) 5.0. 

TABLE 1 EFFECT OF MnO 4 , [ERYTHRITOL], AND  CONCENTRATION ON THE OXIDATION OF ERYTHRITOL BY ALKALINE PERMANGANATE ION AT 25˚C, -

I=0.06 MOL DM‐3.

104[ MnO-4 ] (mol dm‐3) 0.5 1.0 2.0 3.0 4.0 5.0

2.0 2.0 2.0 2.0 2.0

2.0 2.0 2.0 2.0 2.0

103[ERT] (mol dm‐3)

102[] (mol dm‐3)

103kobs (s‐1)

103kcal (s‐1)

2.0 2.0 2.0 2.0 2.0 2.0

0.5 1.0 2.0 3.0 5.0

2.0 2.0 2.0 2.0 2.0

2.0 2.0 2.0 2.0 2.0 2.0

2.0 2.0 2.0 2.0 2.0

0.5 1.0 2.0 3.0 5.0

3.41 3.44 3.44 3.58 3.60 3.65

1.07 2.08 3.44 5.35 7.46

0.87 1.97 3.25 5.27 7.53

3.71 3.71 3.71 3.71 3.71 3.71

1.07 2.03 3.71 5.11 7.31

0.90 1.80 3.58 5.32 7.89

Results and Discussion Stoichiometry and Product Analysis Different sets of reaction mixtures that contain varying ratios of MnO -4 to erythritol in presence of constant amount of  and KNO3 were kept for 3 h in a closed vessel under nitrogen atmosphere. The remaining concentration of MnO-4 was estimated spectrophotometrically at 526 nm. The results indicated 1:4 stoichiometry (Erythritol: MnO-4 ) as given in Eq. 1. The oxidation product was identified as 2, 3, 4‐ tetrahydroxybutanoic acid, which was confirmed

by IR and GC‐MS spectra. The nature of the carboxylic acid was confirmed by the IR spectrum (Fig. 3), which has shown a carbonyl (>C= O) stretch at the range 1720 to 1640 cm‐1 and OH stretching of the acid at 3363 cm‐1. The product was also confirmed by GCMS analysis data. The mass spectrum showed molecular ion peak at 136 amu, confirming the presence of product, 2, 3, 4‐ trihydroxybutanoic acid (Fig.4). The other product, MnO42‐ was confirmed by UV‐Vis spectrum.

O

CH2OH H

OH

HO

H CH2OH

Erythritol

54

+ 4[MnO4 OH]2-

H HO

C

OH OH H

2+ 4MnO4 + 3H2O

CH2OH 2,3,4-trihydroxybutanoic acid

(1)

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FIG. 3 FT‐IR SPECTRUM OF 2, 3, 4‐TRIHYDROXYBUTANOIC ACID, THE PRODUCT OBTAINED DURING THE OXIDATION OF ERYTHRITOL BY PERMANGANATE IN ALKALINE MEDIUM.

FIG. 4 GC ‐ MS SPECTRA OF THE PRODUCT OF OXIDATION 2, 3, 4‐TRIHYDROXYBUTANOIC ACID.

Reaction Orders The reaction orders were determined from the slope of log kobs versus log (concentration) plots by varying the concentrations of erythritol and potassium hydroxide in turn, keeping all other concentrations and conditions constant.

Effect of [Permanganate (VII)] The oxidant MnO -4 concentration was varied in the range of 5.0×10−5 to 5.0×10−4 mol dm‐3 under constant concentration of erythritol, , and constant ionic strength. The linearity of the plots of log [ MnO -4 ] versus time up to 80% completion of the reaction indicates that order with respect to [ MnO-4 ] was unity (Fig. 2). This was also

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confirmed by the almost constant kobs values recorded in Table 1.

Effect of [Erythritol] The erythritol concentration was varied in the range 5.0 × 10‐4 to 5.0 × 10‐3 mol dm‐3 at 25C while keeping other reactant concentrations and conditions constant. The kobs values increased with increase in concentration of erythritol (Table 1). The order with respect to [ERT] was found to be less than unity under the experimental conditions. This less than unit order in the [ERT] was also confirmed by the linear plot of kobs versus [ERT] 0.86 and the nonlinearity of the plot of kobs versus [ERT] (Fig. 5).

0

3

103 [ERT]0.86 mol dm-3 6 9 12

15

18 8

8 (a)

6

(b)

4

4

2

2

0

103 kobs s-1

103 kobs s-1

6

0 0

1

2 3 4 103 [ERT] mol dm-3

5

6

FIG. 5 PLOT OF (a) kobs VERSUS [ERT] AND (b) kobs VERSUS [ERT] 0.86.

Effect of [Alkali] The effect of alkali concentration on the rate of the oxidation reaction was studied in the range of 5.0×10 to 5.0×10 mol dm‐3 at constant concentration of erythritol, MnO-4 and ionic strength of 0.06 mol dm‐3. The rate constant increased with increase in [alkali] and the order was found to be a positive fractional (i.e., 0.82).

Effect of Ionic Strength (I) and Dielectric Constant (D) The effect of varying [KNO3] at constant [ MnO-4 ], [ERT], and [] was found that increase in ionic strength had no significant effect on the rate of the reaction. In order to elucidate the nature of reactive species, the dielectric constant (D) of the solvent system was varied by adding t‐butylalcohol (0–40% v/v) to reaction mixture, keeping other experimental conditions constant. The D values were calculated from the equation D = DWVW +DBVB, where DW and DB are dielectric constant of pure water and t‐butyl alcohol, respectively. VW and VB are the volume fractions of components water and t‐butyl alcohol, respectively, in the total mixture. It was observed that the rate constant increased with decrease in the dielectric constant of the reaction medium and the plot of log kobs versus 1/D was linear with positive slope (Fig. 6). It was also tested for the possible oxidation of solvent used by the MnO-4 under experimental conditions and found no oxidation of solvent by MnO-4 .

2.2 2.0

4+logkobs

1.8 1.6 1.4 1.2 1.0 1.0

1.2

1.4

1.6 1/D×102

1.8

2.0

2.2

FIG. 6 EFFECT OF DIELECTRIC CONSTANT ON THE OXIDATION OF ERT BY ALKALINE PERMANGANATE AT 25C.

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Effect of Temperature (T) The influence of temperature on rate of the reaction was studied at 15, 25, 35, and 45oC under varying concentrations of ERT, and alkali, keeping other conditions constant. The rate constant, k with respect to slow step of Scheme 1 was found to increase with increase in temperature. The rate constants, k, and equilibrium constants K and K of Scheme 1 were obtained from intercepts and slopes of 1/kobs versus 1/ [ERT] and 1/[] plots at four different temperatures and used to calculate the activation parameters. The energy of activation corresponding to these constants was evaluated from the Arrhenius plot of log k versus 1/T, and the other activation parameters so obtained are tabulated in Table 2.

TABLE 2 ACTIVATION PARAMETERS AND THERMODYNAMIC QUANTITIES FOR THE OXIDATION OF ERT BY ALKALINE PERMANGANATE.

(A) EFFECT OF TEMPERATURE WITH RESPECT TO SLOW STEP OF THE SCHEME 1 AND ACTIVATION PARAMETERS. Temperature (K)

k (s‐1)

Parameter

Values

0.595 1.311 2.407 4.472

Ea (kJ mol ) H≠ (kJ mol‐1) S≠ (J K‐1mol‐1) G≠ (kJ mol‐1) log A

50.0±3 48.2±3 ‐81.0 ±4 54.14±3 8.9 ± 0.1

‐1

288 298 308 318

(B) EQUILIBRIUM CONSTANTS K1 AND K2 AT DIFFERENT TEMPERATURES. Temperature (K)

K1 (dm3 mol‐1)

10‐2K2 (dm3 mol‐1)

288 298 308 318

0.731 0.820 0.904 1.261

1.03 0.85 0.77 0.62

(C) THERMODYNAMIC QUANTITIES WITH RESPECT TO K1 AND K2.

Quantities ∆H (kJ mol-1) ∆S (JK-1 mol-1) ∆G (kJ mol-1)

Using K1 values 13.07 42.40 0.22

Using K2 values ‐12.28 ‐4.0 ‐11.05

Test for Free Radicals (Polymerization Study) To test the intervention of free radicals, the reaction mixture was mixed with acrylonitrile monomer and kept for 2h under nitrogen atmosphere. On dilution with methanol, a white precipitate was formed, indicating the intervention of free radicals in the reaction. The blank experiments of either MnO -4 or ERT alone with acrylonitrile did not induce any polymerization under the same condition as those induced for the reaction mixture. Initially added acrylonitrile decreased the rate indicating the free radical intervention, as in earlier work [22].

Effect of Initially Added Products The initially added product MnO42‐ did not have any significant effect on the rate of the reaction. Thus, from the observed experimental results, the rate law for reaction varies with [ERT], [ MnO-4 ] and [] respectively as follows: Rate = kobs [ERT] 0.86, [ MnO-4 ] 1.0, and [OH−] 0.82 Reaction Scheme and Deduction of Rate Law Permanganate ion, MnO-4 in an aqueous alkaline medium exhibits many oxidation states, the stoichiometric results and pH of reaction media play an important role. Under the prevailing experimental conditions at pH > 12, the reduction product of Mn(VII) is Mn(VI) and is stable [4].4 But during this study, colour of the solution changed from violet to blue and then to green. The spectrum of green solution was identical to that of MnO42‐. It is probable that blue colour originated from the violet of permanganate and the green from manganate, excluding the accumulation of hypomanganate.

The results imply that first the alkali combines with permanganate to give an alkali‐permanganate species

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[MnO4.OH]2‐ in a prior equilibrium step, which is in accordance with literature [23, 24] and also to the experimentally observed order in  ion concentration. In the next equilibrium step, [MnO4.OH] 2‐ combines with a molecules of erythritol to give a complex, which decomposes in a slow step to give the free radical. This free radical of ERT combines with a molecule of [MnO4.OH]2‐ species in a fast step to give an aldehyde, which reacts with another molecule of [MnO4.OH]2‐ in a fast step to give the free radical of aldehyde, which reacts with further a molecule of [MnO4.OH]2‐ species in a fast step to give products, 2, 3, 4‐ tetrahydroxybutanoic acid and MnO42‐.

Thus, all these results indicate a mechanism of the type as in Scheme 1.

K1

MnO4- + OHCH2OH H

OH

HO

H CH2OH

+

[MnO4 OH]2-

[MnO4 OH] 2

K2

Complex(C)

CHOH

Complex(C)

H

k

OH

HO

H

+ MnO42- + H2O

CH2OH

O

CHOH H

OH

HO

+ [MnO4 OH]2-

H

fast

H

OH

HO

HO

CH2OH

CH

H

fast

+ [MnO4 OH]2-

HO

C

H

OH

HO

H

+ MnO42- + H2O

CH2OH

CH2OH O C

H

+ MnO42- + H2O

H O

OH

CH

H

CH2OH O

O OH H

fast

+ [MnO4 OH]2-

C

H

OH

HO

CH2OH

OH

H

+ MnO42-

CH2OH

SCHEME 1 PROPOSED MECHANISM FOR THE OXIDATION OF ERYTHRITOL BY ALKALINE MnO-4

Spectroscopic evidence for the complex formation between oxidant and substrate was obtained from UV‐visible spectra of erythritol (2.0 x 10‐3 mol dm‐3), MnO -4 (2.0 x 10‐4 mol dm‐3), [] = 0.02 mol dm‐3 and mixture of both. A hypsochromic shift of about 13.0 nm from 298.0 to 311.0 nm was observed (Fig. 7).

The probable structure of the complex is given below:

H CH2O H HO

OH H CH2OH

58

O O Mn OH O O

2-

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Frome the Scheme1, the rate law (7) can be derived as follows:

Rate =

-d[MnO4-] dt

=

k[Complex]

2= kK2[MnO4OH] [ERT] = kK1K2[MnO4 ]f[OH ]f[ERT]f

Therefore

Rate = kK1K2 [MnO4-]f [OH-]f [ERT]f

(2)

The total [ MnO-4 ] can be written as [MnO4-]t

= [MnO4-]f + [MnO4OH]2- + [Complex] = [MnO4 ]f + K1[MnO4-]f[OH-]f + K2[MnO4OH]2- [ERT] = [MnO4 ]f + K1[MnO4-]f[OH-]f + K1K2[MnO4-]f[OH-]f[ERT] = [MnO4 ]f {1+K1[OH-]f+K1K2[OH-]f{ERT]}

Therefore, [MnO4-]f =

[MnO4-]t {1+K1[OH-]f+K1K2[OH-]f{ERT]} (3)

Where subscripts ‘t’ and ‘f’ stands for total and free permanganate ion concentrations respectively.

Simillarly, [ERT]t = [ERT]f + [Complex] = [ERT]f + K2[ERT]f [MnO4OH]22= [ERT]f {1+K2[MnO4OH] }

Therefore, [ERT]f

=

[ERT]t 1+K2[MnO4OH]2-

In view of low concentration of [ MnO-4 ] used in the experiment, the term K2[MnO4OH]2‐ is neglected.

[ERT]f

= [ERT]t (4)

And similarly, [OH-]t= [OH-]f +[MnO4OH]2- + [Complex] = [OH-]f + K1[OH-]f [MnO4 ] + K1K2[MnO4 ]f[OH ]f[ERT]

=

[OH-]f {1+K1[MnO-4]f + K1K2[MnO4-]f[ERT]}

(5)

In view of low concentration of [ MnO-4 ] and [ERT] used in the experiment, in Eq. (5), the terms K1[ MnO-4 ] and K1K2[ MnO-4 ][ERT] are neglected in comparison with unity.

Therefore, [OH-]t= [OH-]f (6)

Substituting Eqs. (3), (4) and (6) in Eq. (2) and omitting the subscripts, we get,

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Rate

=

-d[MnO4-] dt

=

kK1K2[MnO4-][OH-][ERT] 1+K1[OH-]+K1K2[OH-][ERT] (7)

Or Rate [MnO4-]

= kobs

=

kK1K2[OH-][ERT] 1+K1[OH-]+K1K2[OH-][ERT] (7)

Eq. (7) confirms all the observed orders with respect to different species, which can be verified by rearranging to Eq. (8).

1 1 1 1 = + + k (8) k kK1K2[OH-][ERT] kK2[ERT] obs According to Eq. 8, other conditions being constant, the plots of 1/kobs versus 1/[OH−] and 1/kobs versus 1/[ERT] should be linear, and are found to be so (Fig. 8). From the intercepts and slopes of such plots, the equilibrium constants K1, K2, and rate constant k were calculated as 0.82 dm3 mol−1, 0.85 dm3 mol−1, and 1.31 s−1, respectively. The value of K1 is in agreement with earlier literature [25]. Using these constants, the rate constants were calculated over different experimental conditions and there is a reasonable agreement between the calculated and experimental values, which fortifies the proposed mechanism (Table 1). This may be attributed to the greater tendency of permanganate to undergo deprotonation compared with the formation of hydrolyzed species in alkaline medium. The negligible effect of ionic strength on the rate of reaction explains qualitatively the involvement of neutral molecule, as seen in Scheme 1. The effect of solvent on the reaction rate has been described in detail in the literature [26]. In the present study, the plot of log kobs versus 1/D is linear with a positive slope, which seems to be contrary to the expected reaction between neutral and anionic species in media of lower dielectric constant. However, an increase in the rate of the reaction with decreasing dielectric constant may be due to the stabilization of the complex at low dielectric constant.

The thermodynamic parameters for the different equilibrium steps of Scheme 1 can be evaluated as the following.

The [ERT], and [] (Table 1) were varied at four different temperatures. The plots of 1/kobs versus 1/ [ERT], and 1/kobs versus 1/[] were linear (Fig.7). From the slopes and intercepts, the values of K1 were calculated at different temperatures. A van’t Hoff plot was made for the variation of K1 with temperature (i.e., log K1 versus 1/T) and the thermodynamic parameters calculated. These values are given in Table 2. A comparison of the latter values with those obtained for the slow step of the reaction shows that these values mainly refer to the rate limiting step, supporting the fact that the reaction before the rate‐determining step is fairly fast and involves high activation energy [27]. In the same manner, K2 was calculated at different temperatures and the corresponding thermodynamic quantities are given in Table 2.

The values of ∆S# and ∆H# were both favourable for electron transfer processes. The low value of enthalpy of activation obtained might be due to the involvement of prior equilibrium steps as given in Scheme 1 [28]. A negative value of ∆S# (–81 JK‐1 mol‐1) suggests that the intermediate complex is more ordered than the reactants.

The observed modest enthalpy of activation and higher rate constant for the slow step suggest that the oxidation presumably occurs via an inner sphere mechanism. This conclusion is supported by literature [29, 30].

FIG. 7 SPECTROSCOPIC EVIDENCE FOR COMPLEX FORMATION BETWEEN PERMANGANATE AND ERYTHRITOL: UV‐VIS SPECTRA OF (a) ERYTHRITOL, (b) PERMANGANATE, AND (c) A MIXTURE OF PERMANGANATE AND ERYTHRITOL. CONDITION: [ERT] = 2.0×10‐3, [ MnO-4 ] = 2.0×10‐4.

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3.0

10-3/k10 obs-3(s) /kobs s

2.5 288

2.0 1.5

298

1.0

308

0.5

318

0.0 0.0

50.0

100.0 150.0 3 mol 1/[OH - - ] dm -1-1 1/[OH ] (dm3 mol )

200.0

250.0

(a)

2.4

10-3/kobs-3(s) 10 /kobs s

2.0

288

1.6 1.2 298

0.8

308

0.4

318

0.0 0.0

0.5

1.0 1.5 -3 -3 /[ERT] dm 3 3 mol -1 -1 10 10 /[ERT] (dm mol )

2.0

2.5

(b)

FIG. 8 VERIFICATION OF RATE LAW (7) IN THE FORM OF Eq. (8) FOR THE OXIDATION OF ERYTHRITOL BY PERMANGANATE ION IN ALKALINE MEDIUM. PLOT OF 1/kobs VERSUS (a) 1/ [] AND (b) 1/ [ERT] AT DIFFERENT TEMPERATURE (CONDITION AS IN TABLE1).

Conclusion The oxidant species MnO-4 requires a pH > 12, below which the system becomes disturbed and the reaction proceeds further to give a reduced product of the oxidant as Mn(IV). Hence, it becomes apparent that the role of pH in the reaction medium is crucial. It is also noteworthy that under the conditions studied, the reaction occurs in successive one electron reduction in a single step. The active species of MnO-4 is understood to be [MnO4∙OH]2‐. The rate constants and equilibrium constants were evaluated and the activation parameters with respect to slow step of the reaction were computed. The description of the mechanism is consistent with all the experimental evidence including kinetic, spectral, and product studies.

ACKNOWLEDGEMENT

One of the authors (Manjanath B. Patgar) thanks to Karnatak University, Dharwad, for the research fellowship (KU/Sch/UGC‐UPE/2013‐14/1118) under the UGC‐UPE programme (2013‐16).

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[4]

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