Kinetic analysis of Induced Electron Transfer Reactions in Pentaamminecobalt(III) complexes of α-Hydroxy Acids and α-Amino Acids by Permonosulphuric acid in Anionic, Cationic and Neutral Micellar medium

 

P. Rajkumar1.2* and K. Subramani1

1Department of Chemistry, Priyadarshini Engineering College, Vaniyambadi-635751, TamilNadu, India

2PG and Research Department of Chemistry, Islamiah College, Vaniyambadi-635752, TamilNadu, India

*Corresponding Author E-mail: rajkum.phd@gmail.com

 

 

ABSTRACT:

The Oxidation of Pentaamminecobalt(III) complexes of α-Hydroxy acids and α-Amino acids by PMS has been studied in different Micellar medium. The Oxidation of free α-hydroxy acids and α-amino acids were also carried out under the same conditions for comparative purpose.  On comparing these two substrates with PMS oxidation of  α-hydroxy acids react faster than α-amino acids.  In these reaction the rate of oxidation shows first order kinetics each in [Cobalt (III)] and [PMS].The reaction has followed by observing the decrease in the absorbance at 502 nm for Co(III) complex in a UV-visible spectrophotometer. Product and Stoichiometric analysis were carried out for the oxidation of complexes and free ligands in three different (Anionic, Cationic and Neutral) micellar medium with increasing micellar concentration an increase in the rate is observed. Among three different micelles Cationic micelle of Cetyltrimethylammonium bromide (CTAB) react faster than Anionic micelle of Sodium laurylsulphate(NaLS) and Neutral micelle of Triton.  A Mechanism involving the one electron transfer for the complex and two electron transfer for the ligand was proposed.

 

KEYWORDS: Pentaamminecobalt(III) complexes, Triton X-100, Cetyltrimethylammonium bromide (CTAB), Oxidation  reaction Mechanism, Permonosulphuric acid(PMS), Sodium  laurylsulphate (NaLS).

 

INTRODUCTION:

Chemical reactions are the deciding factors of the life cycle as it is known that human life is mainly depending upon chemicals is different forms. A Chemical reaction may have several aspects, one of which being oxidation. Oxidation process are commonly been in everyday life. Oxidation reactions are not less common in general Chemistry and their impatience has resulted in the accumulation of a large number of empirical observation and conclusions. The study of such oxidation reactions through elucidation of their mechanisms and the investigation of reaction kinetics deals with the study of change in concentration of the components of the reaction system in gas phase, as well as Liquid phase, with the passage of time and the results are summarized in the form of rate expressions.

 

The induced electron transfer reactions become particularly interesting when the oxidisable group in the bound ligand  is separated from the Carbonyl – bound Cobalt(III) by a conjugated fragment as the electron transfer can be induced from  remote  positions. Such an Induced electron transfer reaction has been studied by Robson and Taube with PMS and Pentaamminecobalt(III) complex in Micellar Medium.

 

The Kinetics studies employing Permonosulphuric acid (PMS) is an efficient reagent for oxidation of primary and secondary alcohols to carbonyl compounds1-2. Introduction of PMS is economic and effective reagents for oxidation under mild and anhydrous conditions constitute a standing challenge3. The anhydrous salt of PMS is white crystalline solid (m.pt - 45°C). It is stable for some days but slowly loses ozonized oxygen. It liberates Iodine almost instantaneously from KI. The little work has been done on PMS as oxidant in micellar medium4-5.A large class of organic compounds was oxidized by PMS has been reported. Since Induced electron transfer in Pentaamminecobalt (III) complexes of α-hydroxy acids and α-amino acids with various oxidants have been studied. The extent of PMS oxidation of Cobalt (III) complexes of α-hydroxy acids and α-amino acids is micellar medium as an  oxidisable  hydroxyl group is separated from Carboxyl bound to Co(III) center by a saturated fragment namely C-C bond. The cation  radical is formed due to the oxidation of hydroxyl group by PMS is nearly in a C-C, O-H and N-H bonds fission and reduction at Cobalt(III) center. The Partitioning of reaction paths proceeding by one and two electron transfer have been estimated6-7.

 

Evolution 60 thermo spectrophotometer has been employed to study the oxidation of α-hdroxy acids and α-amino acids and their Cobalt (III) complexes using Permonosulphuric acid as an oxidant in the presence of Anionic, Cationic and Neutral micelles. One equivalent oxidant like Ce (IV) induced electron transfer in Pentaamminecobalt(III) complexes of α-hydroxy acids and α-amino acids  results  in nearly 100% reduction at Cobalt(III) centre with synchronous Carbon-Carbon bond fission and decarboxylation.  Such an electron transfer route seems to be unavailable for Permonosulphuric acid in its reaction with Cobalt(III) bound and unbound α-hydroxy acids and α-amino acids to respective keto acid Cobalt (III) complexes in Sodium laurylsulphate (NaLS), Cetyltrimethylammonium bromide(CTAB) and Triton X-100. Possibly the transition state is more electron deficient. Such a transition state can be envisaged only when the C-H bond fission occurs in the slow step with hydride ion transfer8-9. But for α-amino acid complexes PMS attacks the -NH2 or -NH center in the slow step of the reaction leading to the formation of a radical -NH or - N-.

 

MATERIALS AND METHODS:

All the glass apparatus were made of Pyrex glass and stoppers were well ground. The Loss of solvent, tested in standard flask and in reaction bottles, was found to be negligible. Burettes, Pipettes and standard flask were standardized by usual procedure. The temperature was controlled by an electrical operated thermostat. It was provided with sufficient thermal lagging, suitable heaters, stirrer and proper cooling arrangements for continuous work. The temperature accurately the bath liquid, water was covered with a layer of thermocol bits to minimize heat loss due to radiation and also water loss due to evaporation10.

 

The kinetics studies were carried out by allowing reactions in glass stoppered corning glass vessels. All ingredients of the reactions mixture were taken in separate flasks and the latter were suspended in a temperature controlled water bath. The solution of temperature pre-equilibrated Permonosulphuric acid of desired concentration was withdrawn and then immediately discharged. Permonosulphuric acid of desired concentration was withdrawn and then immediately discharged into the reaction mixture. From this mixture 5ml of solution was titrated against sodium thiosulphate using starch as an indicator. Then this titration was continued at regular time intervals for about 75% of the reaction. The rate of the reaction (-dc/dt) in each kinetic run was determined by the slope of the tangent drawn at fixed concentration of  Permonosulphuric acid  which is written as [PMS]. The order of the reaction with respect to each reactant was determined by the relation between initial rate, i.e., (-dc/dt) and initial [reactant].

 

The α-hydroxy acids and α-amino acids were commercial products (Merck, Ltd., Mumbai, India) of the highest purity available and were used as such. The surfactants used in the present work are Sodium laurylsulphate, (NaLS) 11, Cetyltrimethyl ammonium bromide (CTAB) 12 and Triton X-10013. The surfactants were purified by adopting earlier procedure 14. The chemicals were purchased from BDH (UK) and SD Fine chemicals (INDIA), E.Merck (INDIA).  Pentaamminecobalt (III) complexes of α-hydroxy acids were prepared by reported methods. Double distilled water was used as a solvent. H2SO4was standardized by using standard sodium bicarbonate solution with methyl orange as an indicator.  For the oxidation of Co (III) complexes of α-hydroxy acids, α-amino acids and unbound ligands the PMS was used.  The rate measurement were carried out on 60 ± 0.2° C  in100% aqueous medium for α-hydroxy acids .The Temperature was controlled by electrically operated thermostat.  The total volume of reaction mixture in the spectrophotometric cell was kept as 2.5ml in each kinetic run. But the rate measurement was carried out at room temperature (27 ± 0.2 °C) in 100% aqueous medium for α-amino acids. An Evolution 60 Thermo spectrophotometer fitted with recording and thermo stating arrangement was used to follow the rate of the reaction. Rate of this Permonosulphuric acid oxidant with unbound ligand and Cobalt (III) bound complexes were calculated from observed decrease in absorbance at 502nm. Thus the reaction is first order with respect to the α-hydroxy acids and α-amino acids .The excess of the reluctant was used in kinetic runs.  It gives pseudo first order rate constant.  The pseudo first order rate constants calculated using the following integrated rate equations,

 

K = 2.303/t log [a/a-x]

Where a initial concentration of oxidant and [a-x] concentration of oxidant at time t, are expressed in sec-1.

 

The stiochiometric studies for the PMS oxidation of Pentaamminecobalt(III) complexes of α-hydroxy acids and unbound ligand in the presence of micelles were carried out at 60 ± 0.2 °C.  It was observed that the Cobalt (II) formation was negligibly small. 

 

RESULT AND DISCUSSION:

The oxidation kinetics was carried out at different initial concentration of reactants at 60 ± 0.2°C for α-hydroxy acids and   27 ± 0.2°C for α-amino acids. The concentration of α-hydroxy acids and α-amino acids was varied in the range [0.5- 2.5] x 102 mol   dm -3 at fixed concentrations of other reaction ingredients. A plot of initial rate versus [α-hydroxy acids and α-amino acids] yielded a straight line passing through the origin confirming first order dependence. The concentration of Nals, CTAB and Triton X-100 was varied in the range of [1 x10-3, 10 x 10-3, 1 x 10-4, 5 x 10-3, 5 x 10-4] mol d m-3 at concentrations of other reaction ingredients. A plot of initial rate versus [Micelles] yielded a straight line passing through the origin confirming first order dependence. The second order plots were also made for comparable concentrations of α-hydroxy acids, α-amino acids and micelles15.

 

The absence of formation of Cobalt(II) rules out the synchronous C-C bond fission and electron transfer to Cobalt(III). The thermodynamic parameters are in consistent with bimolecular reaction. The rate of PMS oxidation of Cobalt(III) Mandelato, Lactato and Glycolato complexes depends on the first power of PMS concentration. Similarly the reaction between PMS and unbound          α-hydroxy acids exhibits first order kinetics with respect to concentration of PMS.  The rate of the  reaction for PMS Oxidation of Co(III) complexes of alpha-hydroxy acids are arranged  in the following order

Mandelato < Glycolato < Lactato

The similar trends are followed in the unbound  ligand also.

 

For both α-hydroxy acids and α-amino acids the rate of the reaction is increased by the addition of micelles namely NaLS, CTAB and Triton.  A plot of specific rate constant versus micellar concentration is sigmoidal in shape the catalytic effect is more in CTAB than NaLS and Triton.

 

Table 1 Summarizes the Kinetic data for the Permonosulphuric acid oxidation of free α-hydroxy acids with 2N H2SO4 in presence of anionic, cationic and neutral micelles at 60 ± 0.2 °C.  Though the reaction exhibits total second – order dependence on [Cobalt (II)] as well as [α-hydroxy acids].  Based on the oxidation of PMS with α-hydroxy acids the following rate law has been deduced.

Rate= k2 [α-hydroxy acids] [PMS]

 

Table 2 lists the formation constants for PMS Co-complexes of α-hydroxy acids along with the specific rates.  Such complex formation seems to be absent when the carboxyl and it is tied up by Co(III) and the reaction between PMS and Co(III) complexes of α-hydroxy acids exhibit uncomplicated second order kinetics.

 

Table 3 and Table 4 Summarizes the kinetic data for the PMS oxidation of Co (III) bound and  unbound α-amino acids in the presence of three different micellar medium. Though the reaction exhibits first order dependence on PMS, kinetic saturation has been observed with respect to substrate concentration, suggestion a complex formation between PMS and α-amino acids. i.e., stabilization of the Cobalt-substrate in the micelles.

 

The rate of PMS oxidation of Cobalt (III) Glycinato, Alaninato, Isoleucinato, N – acetyl glycinato and N – benzoylglycinato Complexes depend on the first power of PMS concentration. Similarly the reaction between PMS and unbound   α-amino acids exhibits first order kinetics with respect to concentration of PMS.  Of the five complexes N – benzoylglycinato cobalt(III) complexes react faster than other four complexes, where as in the unbound ligand similar trends follows.`

 

Table 1: First order rate constants for PMS oxidation of α-Hydroxy acids in the presence of NaLS, CTAB and Triton 

102[α-Hydroxy acids] mol dm-3

104k1 (s-1)

NaLS

102k2
dm3mol-1s-1 NaLS

104 k1 (s-1)

CTAB

102k2 dm3mol-1s-1

CTAB

104k1 (s-1)

Triton

102k2 dm3mol-1s-1

Triton

Mandelic acid

0.5

1.5

2.5

Lactic acid

0.5

1.5

2.5

Glycolic acid

0.5

1.5

2.5


1.244

3.601

6.079

2.106

6.414

10.571

1.133

3.340

5.561


2.488

2.403

2.431

4.213

4.276

4.228

2.270

2.226

2.224


1.828

5.484

9.140

2.286

6.838

11.031

1.18 0

3.454

5.759


3.656

3.656

3.656

 

4.572

4.558

4.412

2.360

2.302

2.303

 

1.523

4.561

7.619

 

2.201

6.618

10.996

 

1.150

3.391

5.683

 

3.041

3.011

3.083

 

4.410

4.439

4.461

 

2.280

2.291

2.203

[PMS]= 0.08 mol dm-3, [H2SO4]= 0.25 mol dm-3, [NaLS] = [CTAB] = [Triton] = 1.00 x10-3 mol dm-3, Temperature = 60 ± 0.2°C

 

Table 2: First order rate constants for PMS oxidation of Co(III) complexes α-Hydroxy acids  in the presence of NaLS, CTAB and Triton 

102[(NH3)5Co (III) - L] mol dm-3

104k1 (s-1)

NaLS

102k2

dm3mol-1s-1 NaLS

104 k1 (s-1)

CTAB

102k2 dm3mol-1s-1

 CTAB

104k1 (s-1)

Triton

102k2 dm3mol-1s-1

Triton

 Mandelato

0.5

1.5

2.5

Lactato

0.5

1.5

2.5

Glycolato

0.5

1.5

2.5


1.281

3.880

6.409

2.886

8.644

14.301

1.383

4.160

6.841


2.568

2.563

2.561

5.760

5.762

5.728

2.770

2.776

2.734


2.228

6.654

11.210

3.216

9.658

16.051

1.610

4.824

8.059


4.446

4.436

4.486

 

6.432

6.438

6.422

3.220

3.212

3.223

 

1.738

5.214

8.505

 

3.059

9.136

15.084

 

1.502

4.508

7.516

 

3.460

3.491

3.487

 

6.091

6.082

6.024

 

3.023

3.016

3.028

[PMS]= 0.08 mol dm-3, [H2SO4]= 0.25 mol dm-3,  [NaLS] = [CTAB]=  [Triton] = 1.00 x10-3 mol dm-3, Temperature = 60  ±  0.2°C

Table 3: First order rate constants for PMS oxidation of α-Amino acids in the presence of NaLS, CTAB and Triton 

102[α-amino acids]

mol dm-3

104k1 (s-1)

NaLS

102k2

dm3mol-1s-1 NaLS

104 k1 (s-1)

CTAB

102k2 dm3mol-1s-1

CTAB

104k1 (s-1)

Triton

102k2 dm3mol-1s-1

Triton

Glycine

0.5

1.5

2.5

Alanine

0.5

1.5

2.5

Isoleucine

0.5

1.5

2.5

N – acetylglycine

0.5

1.5

2.5

N – benzoylglycine

0.5

1.5

2.5


0.974

2.881

4.929

0.896

2.647

4.401

1.233

3.680

6.241

 

2.101

6.094

10.192

 

2.665

7.778

12.604


0.998

2.923

4.821

0.923

2.726

4.498

1.380

3.726

6.344

 

2.304

6.294

10.345

 

2.998

7.993

12.904


1.088

3.054

5.001

0.956

2.848

4.751

1.400

4.254

7.005

 

2.348

6.965

11.598

 

2.984

8.760

14.585


1.236

3.396

5.226

 

0.942

2.938

4.882

1.500

4.402

7.193

 

2.666

6.985

11.987

 

3.045

8.999

14.895

 

0.991

2.973

4.955

 

0.920

2.761

4.638

 

1.310

3.931

6.558

 

2.231

6.693

11.150

 

2.831

8.403

13.913

 

0.986

2.991

5.084

 

0.891

2.632

4.592

 

1.402

3.894

6.636

 

2.318

6.593

11.238

 

2.789

8.038

14.036

[PMS]= 0.08 mol dm-3, [H2SO4] = 0.25 mol dm-3, [NaLS] = [CTAB]=  [Triton] = 1.00 x10-3 mol dm-3, Temperature = 27 ±  0.2°C

 

Table 4: First order rate constants for PMS oxidation of Co(III) complexes of  α-amino acids in the presence of NaLS, CTAB and Triton 

102[(NH3)5Co (III) - L] mol  dm-3

104k1 (s-1)

NaLS

102k2 dm3mol-1s-1

NaLS

104 k1 (s-1)

 CTAB

102k2 dm3mol-1s-1

 CTAB

104k1 (s-1)

Triton

102k2 dm3mol-1s-1

Triton

Glycinato

0.5

1.5

2.5

Alaninato

0.5

1.5

2.5

Isoleucinato

0.5

1.5

2.5

N – acetylglycinato

0.5

1.5

2.5

N – benzoylglycinato

0.5

1.5

2.5


1.204

3.631

5.990

1.126

3.157

5.641

1.783

4.890

7.891

 

2.551

7.394

12.122

 

3.395

8.458

15.674


1.368

3.563

5.891

1.283

3.206

5.318

1.670

4.906

7.704

 

2.684

7.494

12.275

 

3.398

8.563

15.784


1.567

4.567

7.498

1.367

3.814

5.908

1.906

5.294

8.021

 

2.812

7.907

13.097

 

3.504

10.000

16.594


1.678

4.609

7.590

 

1.490

3.908

6.000

1.897

5.367

8.209

 

2.976

7.821

13.245

 

3.670

10.176

16.601

 

 

1.396

4.146

6.903

 

1.228

3.686

6.165

 

1.852

5.294

8.001

 

2.671

7.864

12.932

 

3.401

9.789

16.203

 

1.409

4.087

6.876

 

1.294

3.578

6.292

 

1.936

5.356

8.109

 

2.756

7.890

12.424

 

3.513

9.876

16.146

[PMS]= 0.08 mol dm-3, [H2SO4]= 0.25 mol dm-3, [NaLS] = [CTAB] =  [Triton] = 1.00 x10-3 mol dm-3, Temperature = 27  ±  0.2°C

 

The rate of the reaction for PMS Oxidation of  Co(III)  complexes of α-amino acids are arranged in the following increasing order.

Alaninato < Glycinato < Isoleucinato < N-acetylglycinato < N-benzoylglycinato

 

The similar trends are followed in the unbound  ligand also. Based on the oxidation of PMS with α-amino acids the following rate law has been deduced.

 

Rate= k2 [α-amino acids] [PMS]

 

From a comparison, the specific rates for PMS oxidation of the respective Co (III) complexes of α-hydroxy acids and α-amino acids, one can infer that the oxidation rates of α-hydroxy acids and α-amino acids are not significantly affected by complex formation.  This may be due to the point of attack lies away from the Co (III) centre so that its electrostatic influence is less felt on the position of attack.  There is, however a considerable change in the specific rate of PMS oxidation of the Co (III) α-hydroxy acid and α-amino acid complex as the two Co(III) centre's can exert greater electrostatic influence over the reacting centre.  This suggests that PMS attacks the O-H centre in the slow step of the reaction leading to ligand oxidation takes place for α-hydroxy acid complexes and α-amino acid complexes PMS attacks the -NH2 or -NH center in the slow step of the reaction leading to the formation of a radical -NH or - N-.

 

In α- hydroxy acids,  the specific rate of the lactato complex is more when compared to both the rate of unbound ligand and mandelato complex is due to the ligation of lactic acid to cobalt(III) centre has probably increased its reactivity towards PMS and this effect seems to be more specific for ligands only.  In NMR spectrum of lactato complex the alpha methine proton has undergone considerable downfield shift compared to the alpha C-H proton of the unbound ligand [∂ C-H= 1.73 ppm in lactic acid and ∂  C-H =2.30 ppm in lactato complex whereas ∂  C-H = 4.75 ppm in mandelic acid ∂ C-H = 3.85 ppm in the respective complex].  Suggesting an increase in acidic nature of methine proton of lactic acid is due to ligation to metal centre.  If the reaction proceeds through a performed chromate ester, then the rate of alpha C-H will be enhanced, resulting in an increased rate of oxidation of lactato complex such a precursor complex may be sterically hindered in the case of mandelato and glycolato complexes.

 

 

Table 5: Stoichiometric data for PMS Oxidation of α-hydroxy acids and α-amino acids in the presence of  NaLS , CTAB and Triton

103[Compound]

mol dm-3

102[PMS]initial

mol dm-3

102[PMS]final

mol dm-3

∆103[PMS]

mol dm-3

[Compound]:

∆[PMS]

Mandelic acid

1.0

4.0

Lactic acid

1.0

4.0

Glycolic acid

1.0

4.0

Glycine

1.0

2.0

Alanine

1.0

2.0

Isoleucine

1.0

2.0

N-acetylglycine

1.0

2.0

N-benzoylglycine

1.0

2.0

 

1.0

2.0

 

1.0

2.0

 

1.0

2.0

 

1.0

2.0

 

1.0

2.0

 

1.0

2.0

 

1.0

2.0

 

1.0

2.0

 

0.89

1.60

 

0.90

1.60

 

0.88

1.57

 

0.89

1.80

 

0.91

1.81

 

0.88

1.78

 

0.90

1.82

 

0.89

1.79

 

1.10

4.00

 

1.00

4.00

 

1.20

4.30

 

1.10

2.00

 

1.00

1.90

 

1.20

2.20

 

1.00

1.80

 

1.10

2.10

 

1.00 : 1.10

1.00 : 1.00

 

1.00 : 1.00

1.00 : 1.00

 

1.00 : 1.20

1.00 : 1.07

 

1.00 : 1.10

1.00 : 1.00

 

1.00 : 1.00

1.00 : 0.95

 

1.00 : 1.20

1.00 : 1.07

 

1.00 : 1.00

1.00 : 0.97

 

1.00 : 1.10

1.00 : 1.05

[H2SO4] = 0.25 mol dm-3, [NaLS] = [CTAB]= [Triton] = 1.00 x10-3 mol dm-3

 

Table 6: Stoichiometric data for PMS Oxidation of Co(III) bound and unbound α-hydroxy acids and α-amino acids  in the presence of NaLS,  CTAB  and Triton

102[(NH3)5Co (III) - L]

mol  dm-3

102[PMS]initial

mol dm-3

102[PMS]final

mol dm-3

∆103[PMS]

mol dm-3

[Compound]:

∆[PMS]

 Mandelato

1.0

4.0

 Lactato

1.0

4.0

 Glycolato

1.0

4.0

 Glycinato

1.0

2.0

 Alaninato

1.0

2.0

 Isoleucinato

1.0

2.0

 N-acetylglycinato

1.0

2.0

 N-benzoylglycinato

1.0

2.0

 

1.0

2.0

 

1.0

2.0

 

1.0

2.0

 

1.0

2.0

 

1.0

2.0

 

1.0

2.0

 

1.0

2.0

 

1.0

2.0

 

0.95

1.99

 

0.96

2.00

 

0.95

1.98

 

0.95

1.90

 

0.94

1.90

 

0.94

1.88

 

0.95

1.90

 

0.95

1.89

 

0.50

2.01

 

0.49

2.00

 

0.50

2.02

 

0.50

1.00

 

0.60

1.00

 

0.60

1.20

 

0.50

1.00

 

0.50

1.10

 

2.00 : 1.00

2.00 : 1.00

 

2.00 : 0.98

2.00 : 1.00

 

2.00 : 1.00

2.00 : 1.01

 

2.00 : 1.00

2.00 : 1.00

 

2.00 : 1.00

2.00 : 1.00

 

2.00 : 1.20

2.00 : 1.20

 

2.00 : 1.00

2.00 : 1.00

 

2.00 : 1.00

2.00 : 1.10

[H2SO4] = 0.25 mol dm-3, [NaLS] = [CTAB] =[Triton] = 1.00 x10-3 mol dm-3

 

The stoichiometric results, coupled with kinetic data and product analysis, can be accounted by the following reaction schemes.

 

Scheme I

 

 

In  α-amino acids, the reduction in the specific rate of PMS oxidation of Co(III) complexes of Glycine, Alanine, Isoleucine, N-acetylglycine and N- benzoylglycine, compared to that for the unbound ligands, points to significant electronic influence of the acyl group and also the electrostatic influence of the Co(III) center at the seat of attack, viz. -NH group. Such an electrostatic influence due to ligation of α-amino acids to the Co(III) center (in the monomer only) seems to be absent as the amino nitrogen is protonated both in the complex and in the unbound ligand, exerting possibly the same electrostatic influence at the set of attack.

 

The stoichiometric results indicate that for one mole of Cobalt (III) complex, about 0.5 mole of PMS is consumed, whereas with the unbound ligands for 1 mole of α-hydroxy acids and α-amino acids  about 1.0 mole of PMS is consumed (Table 5 and Table 6).

 

Scheme-I proposes that Oxidation of Pentaamminecobalt(III) complexes of both bound and unbound ligands in micellar medium.  Thus, the kinetics of one electron transfer route seems to be unavailable for PMS with Cobalt (III) bound and unbound complexs of α-hydroxy acids in micellar medium, PMS oxidizes Cobalt(III) bound and unbound α-hydroxy acids.  It rules out the synchronous C-C bond fission and electron transfer to Cobalt (III) centre. Oxidation of above complexes increases with increase of concentration and temperature.  With increase in micellar concentration an increase in the rate is observed.  The added CTAB enhances the rate of Oxidation of a reaction much more than NaLS.  Similar trends have been observed in lactato and glycolato Co(III) complexes.

 

 

Scheme II

 

Scheme II proposes that PMS oxidizes OH centre of the α-hydroxy acids at a rate of comparable to that of the free ligand and there is 100% reduction at the Proton centre, forms a Permonosulphuric acid ester which can decompose in a slow step, proceeds through   C-C bond fission leading to the formation of carbonyl compounds with the evolution of carbon dioxide and H2 gas.  As 1 mole of α-hydroxy acids consumes 1 mole of PMS yielding nearly equal amount of carbonyl compounds.

 

 

Scheme III

 

Scheme IV

 

Scheme III explains the reaction between PMS oxidation of Co(III) complexes of α-amino acids exhibits total second order kinetics first order with respect to each reactant.  The rate of reaction carries with first power of AcOH concentration.  Decrease in absorbance at 502nm corresponding to the reduction of Co(III) bound complex.  The ligation of Carboxylic acid of α-amino acids by Co(III) changes the order with respect to Co(III) complex to unity.  No possibility of binuclear complex formation between PMS and Co(III) complexes.  One electron transfer to PMS acetate may occur by an outer-sphere path in the slow step as shown by Scheme III.

 

Scheme IV proposes that electron transfer possibly occurs within the intermediate complex.  The formation of binuclear complex with high association constant K, between PMS acetate and α-amino acids possibly due to the ligation of PMS acetate to free carbonyl end. (Absent in a Co(III) complex).

 

CONCLUSION:

The kinetic analysis of one electron transfer route seems to be unavailable for PMS with Cobalt(III) bound and unbound complexes of α-hydroxy acids and α-amino acids in micellar medium, PMS oxidizes Cobalt(III) bound and unbound α-hydroxy acids and  α-amino acids through free radical. It explains the synchronous C-C bond fission and electron transfer to Cobalt(III) centre.  On comparing these two substrates with PMS oxidant α-hydroxy acids react faster than α-amino acids. The Oxidation of above complexes increases with increase of concentration and Temperature, with increase in micellar concentration an increase in the rate is observed.  The added CTAB enhances the rate of Oxidation of a reaction much more than NaLS and Triton. Among three different micelles Triton is react faster than NaLS but lesser than CTAB. But in the case of NaLS, it is reacting slowly than cationic and neutral micelles.  The 1 mole of Co(III) complexes of α-hydroxy acids and α-amino acids Consumes 0.5 mole of PMS, whereas 1 mole of unbound α-hydroxy acids and α-amino acids consumes 1.0 mole of PMS.  The reaction goes by free radical mechanism was proved by acrylonitrile polymerization.

 

ACKNOWLEDGEMENT:

I express my sincere thanks to PG and Research Department of Chemistry, Islamiah College, Vaniyambadi and the Management of Priyadarshini Engineering College, Vaniyambadi for providing the necessary facilities for the successful completion of this research work.  Also I express my  sincere gratitude to my  Thiruvalluvar University, Vellore  for providing me  an opportunity for completing research work successfully.

 

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Received on 16.03.2013       Modified on 27.03.2013

Accepted on 29.03.2013      © AJRC All right reserved

Asian J. Research Chem. 6(4):  April 2013; Page   364-371