INTRODUCTION:

Chromium compounds have been widely used in aqueous and non- aqueous medium for the oxidation of a variety of organic compounds^[1-13]. Chromium compounds especially Cr(VI) reagents have been proved to be versatile reagents capable of oxidizing almost all the oxidizable organic functional groups. The development of newer Cr(VI) reagents for the oxidation of organic substrates continues to be of interest. Chemical kinetics is concerned with the rates of chemical reactions, that is, with the quantitative description of how fast chemical reactions occur and the factors affecting these rates. The chemist uses kinetics as a tool to understand fundamental aspects of reaction pathways, a subject that continues to evolve with ongoing research.

Lactic acid is oxidized to corresponding aldehyde (Acetaldehyde) by C-H Bond fission in slow step with transfer of Hydride ion. Selective and mild oxidation by Cr(VI) reagents were achieved by the use of new oxidizing agents^[1-13] for the study of kinetics and mechanism of organic compounds. Kinetic Study of TPAFC and TPACC employed is an efficient reagent for oxidation of alpha Hydroxy acids to carbonyl compounds. Oxidants employed are readily soluble in water, it is economic and easy to prepare in a good yield (97%).

Two different oxidant TPAFC and TPACC were used for their comparative study on oxidation of alpha Hydroxy acids in presence of Benzalkonium chloride. Perchloric acid is used as a medium. Some of the reported chromium(VI) reagents suffer from disadvantages like Instability, hygroscopic, low selectivity, Photosensitivity, long reaction time, low solubility and need of large excess reagents and difficulty of preparation. To overcome these disadvantages, two new mild, efficient and stable reagent TPAFC and TPACC were new promising reagent which improves the workup efficiency and durability for the oxidation of Lactic acid.

Rate of the reaction have been determined at room temperature. On Increasing the concentration of Chromium (VI) oxidant the rate is found to be constant, whereas rate is increased by increase in concentration of substrate, Perchloric acid and micelle. Decrease in Absorbance from 373nm for TPAFC and 370.5nm for TPACC is observed. Rate of the reaction also been determined at four different temperatures from 303K to 318K for lactic acid by keeping other reagents at constant temperature. Temperature is maintained using constant temperature bath. Activation parameters were calculated from the graph by plotting log(k/T) versus (1/T) is a linear.

MATERIALS AND METHODS:

All the chemicals used are of Analytical grade. Lactic acid, Perchloric acid were commercial products (E. Merck Ltd, Mumbai, India) and directly used. Tripropylamine and Benzalkonium chloride were Purchased from SD fine chemicals, India. Double Distilled water were used as solvent. Perchloric acid was standardized using standard sodium carbonate (Merck, India) solution with methyl orange as Indicator.

Benzalkonium chloride used as a micellar catalyst also known as Alkyl dimethyl Benzylammonium chloride is a cationic surfactant. They are the organic salt of Quaternary ammonium compound which is colourless and readily soluble in water. It is also used as Phase transfer catalyst and synthesis of organic drugs. It has molecular formula [C₆H₅CH₂N (CH₃)₂R ] Cl where R=n-C₁₂H₂₅ and molecular weight 360-375. The surfactants were purified by adopting earlier procedure.

Halochromates have been used as mild and selective oxidizing reagents for the oxidation of organic substrates continues to be of interest. Extensive kinetic and mechanistic studies on the oxidation of organic compounds by chromium reagents have revealed that such reaction ordinarily involve a three electron change, where Cr(VI) species are reduced to Cr(III). In recent years, the kinetics and mechanism of oxidation reactions involving Cr (VI) for a number of substrates have been fair well studied.

Kinetic methods

Elico UV-Visible (FL244) Spectrophotometer has been used to study the oxidation of Lactic acid by two different oxidant TPAFC and TPACC in presence of micellar catalyst. The kinetics studies were carried out by allowing reactions in glass stopper 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.

Rate Measurement:

The rate measurement were carried out on 30 ± 0.2° C in 100% 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. The reactions were carried out under pseudo-first order conditions, keeping the substrate concentration always in excess. The pseudo-first order rate constant of each kinetic run was evaluated from the slope of the linear plot of log (a-x) versus time, according to the first order rate equation by the method of least square.

k = (2.303/t)*log (a/ (a-x))

k₁ = 2.303 x slope expressed in sec^-1 where k₁ is the pseudo-first order rate constant, ‘t’ is the time in sec. and ‘a’ and (a-x) denote the initial concentration and concentration at time ‘t’ respectively of oxidant.

Thermodynamic methods

Time is a variable in kinetics but not in thermodynamics; rates dealt with in the latter are with respect to temperature, pressure, etc., but not with respect to time; equilibrium is a time independent state. Thermodynamic parameters such as Activation Energy, Frequency factor, Enthalpy of Activation, Entropy of Activation and free energy of Activation has been calculated at four different temperature from the equations given below. From Arrhenius Equation the speed of the chemical reaction increases exponentially with temperature.

k = A.exp[-E_a/RT] ------ (1)

log A = log k₁ + [E_a/2.303RT] ------ (2)

The equation is in accordance with empirical fact that for most of reactions plot of 3 + log k versus 10³ / T is a Arrhenius plot which gives a straight line and slope is -E_a /2.303R, E_a calculated in this way is called Arrhenius activation energy. Intercept gives the value of log A.

∆H = E_a – RT ----- (3)

∆S = 2.303 R (logA – log exp [k_BT/h]) ----- (4)

∆G = ∆H - T∆S ------ (5)

Thermodynamic parameters can also be calculated from Eyring equation, According to Transition state theory

k₁ = [k_BT/h] exp (-∆G / RT) ------ (6)

k₁ = [k_BT/h] exp (-∆H / RT) exp (∆S/ R) ----- (7)

Taking log on both sides

Log (k/T) = log (k_B/h) - (∆H /2.303RT) + (∆S/2.303R) --- (8)

also ∆H = E_a

Log (k/T) = (10.32) - (E_a / 19.148T) + (∆S/ 19.148) -- (9)

A plot of 3 + Log (k/T) versus 10³ / T is a Eyring plot which gives a straight line with slope = (E_a / 19.148) and intercept = (10.32) + (∆S/ 19.148) from slope and intercept E_a, ∆S is evaluated. Thus there is relatively little difference between Thermodynamic parameters evaluated from Arrhenius and Eyring plot.

Two oxidants TPAFC and TPACC were synthesized by the given procedure [5],[13]

Preparation of Tripropylammonium Fluorochromate TPAFC, (C₃H₇)₃NH [CrO₃F]

Chromium (VI) oxide (10g, 10 mmol) and 9.0 mL (20 mmol) 40 % hydrofluoric acid were added to 20 mL of water in a 100 mL polyethylene beaker with stirring at 0^oC. To the resultant orange solution, Triproylamine (14ml, 20mmol) was added drop wise and stirring was continued over a period of half an hour. The precipitated solid was isolated by filtration and washed with petroleum ether and dried under vacuum for 2 hours. Yield: 45.8(98%); mp 142^oC.IR spectral data shows peak at 904,647, 949 cm^-1.Electronic absorption occurs at 22321 cm^-1. UV/Visible and¹H-NMR and ¹⁹F-NMR was all consistent with the TRIPAFC structure. The pH of 0.01 M solution of TPAFC in water was 3.3. Purity of the oxidant was checked by Iodimetric procedure

Preparation of Tripropylammonium Chlorochromate TPACC, (C₃H₇)₃NH [CrO₃Cl]

Chromium (VI) oxide (10g, 10m mol) and 2.5 mL (15 mmol) 40 % hydrofluoric acid were added to 20 mL of water in a 100 mL polyethylene beaker with stirring at 0^oC. To the resultant orange solution, Triproylamine (15ml, 15mmol) was added drop wise and stirring was continued over a period of half an hour. The precipitated solid was isolated by filtration and washed with petroleum ether and dried under vacuum for 2 hours. Yield: 45.8(95%); mp 135^oC.IR spectral data shows peak at 901, 432, 949 cm^-1.Electronic absorption occurs at 22123 cm^-1. UV/Visible and¹H-NMR and ¹³C-NMR was all consistent with the TPAFC

Product Analysis

The carbonyl compound formed during the oxidation of lactic acid by TPAFC and TPACC was analyzed by the following general procedure. The reaction mixture, after 9 half lives was neutralized to pH = 6.0 by the addition of saturated KHCO₃ solution and the resultant solution are filtered off. The filtrate was extracted with diethyl ether several times and the ether extracts were made up to known volume. The amount of acetaldehyde formed was determined by measuring the absorbance at 250nm (Ԑ = 11400 dm³mol^-1cm^-1). Acetaldehyde formed was analyzed as 2, 4-dinitrophenyl-hydrazone derivative (m.pt-168°C).

Stoichiometry and polymerization test

The stiochiometric studies for the TPAFC and TPACC oxidation of Lactic acid in the presence of micelle were carried out at 30 ± 0.2°C. The stoichiometry was calculated from the ratio between reacted [oxidant] and [substrate].

Polymerisation test with acrylonitrile was carried out to check the formation of intermediate radicals during the oxidation of Lactic acid by TPAFC and TPACC in perchloric acid medium.

RESULT AND DISCUSSION:

The oxidation kinetics of lactic acid with two different oxidant TPAFC and TPACC were attempted.

Dependence of rate on varying TPAFC and TPACC concentration :-

Concentration of both TPAFC and TPACC varied at 1.0x10^-2, 2.0x10^-2, 3.0x10^-2, 6.0x10^-2, keeping other concentrations of

Lactic acid, perchloric acid and temperature constant. This variation seems to be second order. The Plot of Log k_obs versus Log[TPAFC] and Log[TPACC] for different initial concentration is linear(k = 2.303xslope) shows the first order dependence of rate of Cr(VI) reagents.

Rate= k₂ [lactic acid] [Cr (VI)] ----- (10)

Table -1

[L] = 4.0 x 10^-1 mol dm^-3 [HClO₄] = 4.0 x 10^-1 mol dm^-3

[TPACC] = 1.0 x 10^-2 mol dm^-3Temperature = 30 ±0.2°C

L = Lactic Acid

Time(Sec)

(a – x) mol dm^-3

10³k₁ s^-1

120

180

240

300

360

420

480

540

600

3.402

2.99

2.6219

2.2412

2.0542

1.7828

1.5524

1.361

1.1936

1.07

2.17

2.16

2.17

2.28

2.12

2.18

2.15

Theoretical

Graphical

2.18

2.16

Table -2

[L] = 4.0 x 10^-1 mol dm^-3 [HClO₄] = 4.0 x 10^-1 mol dm^-3

[TPAFC] = 1.0 x 10^-2 mol dm^-3Temperature = 30 ±0.2°C

L = Lactic Acid

Time(Sec)

(a – x) mol dm^-3

10³k₁ s^-1

120

180

240

300

360

420

480

540

600

660

0.5846

0.572

0.559

0.5465

0.5339

0.523

0.51

0.49858

0.4886

0.47676

0.4658

3.78

3.7

3.76

3.78

3.72

3.79

3.78

3.7

3.78

3.79

Theoretical

Graphical

3.76

3.839

Figure 1: Pseudo first order plot of 3+logk versus time rate dependence for TPAFC and TPACC

Table 3: comparison of rate of TPAFC and TPACC

[L] = 4.0 x 10^-1 mol dm^-3 [HClO₄] = 4.0 x 10^-1 mol dm^-3

Temperature = 30 ±0.2⁰C L = Lactic Acid

10² [TPAFC]

10³k₁ s^-1

10² [TPACC]

10³k₁ s^-1

1.0

2.0

3.0

6.0

3.76

3.82

3.765

3.821

1.0

2.0

3.0

6.0

2.16

2.05

2.13

2.18

From table 2 concentration variation of both TPAFC and TPACC shows nearness value shows the reaction follows Pseudo first order reaction.

Dependence of rate on varying Lactic acid concentration

On increasing the concentration of Lactic acid at 1.0x10^-2, 2.0x10^-2, 3.0x10^-2, and 6.0x10^-2 and by keeping other constituents constants, the rate increases proportionately. The plot of 4+logk versus 2+log[LA] is a linear with slope nearness to Unity shows the first order rate dependence of Lactic acid. The rate is compared for the both the Cr(VI) reagents.

Figure 2: rate dependence of Lactic acid plot of logk versus log[LA]

Table 4: effect of Lactic acid variation

[TPACC] = 1.0 x 10^-2 mol dm^-3 [HClO₄] = 4.0 x 10^-1 mol dm^-3

[TPAFC] = 1.0 x 10^-2 mol dm^-3Temperature = 30 ±0.2°C

10² [LA]

[TPAFC]10⁴k₁

[TPAFC]10⁴k₂

[TPACC]10⁴k₁

[TPACC]10⁴k₂

1.0

2.0

3.0

6.0

2.875

5.75

8.71

17.25

2.87

2.90

2.87

5.4

10.8

16.2

32.4

5.4

From the above table it is found that rate of lactic acid oxidation is faster in TPAFC than TPACC on varying perchloric acid concentration.

Figure 3: rate dependence of perchloric acid plot of logk vs log[HCLO₄]

Dependence of rate on varying Micellar concentration

On varying the concentration of micelle Benzalkonium chloride(BKC) at 0.0052,0.01,0.015 and 0.03 mol dm-3 shows a progressive increase in the rate.A plot of logk versus 3+[micelle] gives sigmoidal curve with CMC value (1.1±0.6)x10-3.Comparision of rate is given below

Table 6: Effect of micellar variation

[L] = 4.0 x 10^-1 mol dm^-3 [TPAFC] = 1.0 x 10^-2 mol dm^-3

[TPACC] = 1.0 x 10^-2 mol dm^-3Temperature = 30 ±0.2⁰C

L = Lactic Acid [HClO₄] = 4.0 x 10^-1 mol dm^-3

10² [micelle]

[TPAFC]10⁴k₁

[TPACC]10⁴k₁

0.0052

0.01

0.015

0.03

12.0

13.1

15.6

17.81

7.7

13.7

15.1

17.2

Figure 4 : rate dependence of micellar variation for TPAFC and TPACC

Thermodynamic parameters:

Oxidation of Lactic acid by TPAFC and TPACC was carried out at four different temperatures keeping all reactant concentrations constant. Value of rate constant k1 is given in Table 7

Table 7: Effect of variation of temperature

[L] = 4.0 x 10^-1 mol dm^-3 [HClO₄] = 4.0 x 10^-1 mol dm^-3

[TPAFC] = 1.0 x 10^-2 mol dm^-3 [TPACC] = 1.0 x 10^-2 mol dm^-3

L = Lactic Acid

Temperature(K)

[TPAFC]10³k₁

[TPACC]10³k₁

303

308

313

318

3.76

4.31

4.84

5.45

2.18

2.46

2.77

3.10

Arrhenius plot of log k Vs 1/T is given in figure and Eyring’s plot of log k/T Vs 1/T is given in figure . Thermodynamic parameters calculated is given in the table 8 and table 9

Figure 8: Arrhenius plot of log k Vs [1/T] to calculate thermodynamic parameters

Table 8. Arrhenius parameters at 313K for the oxidation of lactic acid by TPACC and TPAFC

Arrhenius parameters	[TPAFC]	[TPACC]
Ea KJmol^-1	19.654	19.148
∆H KJmol^-1	17.052	16.545
-∆S JK^-1mol^-1	177.8	192.5
∆G KJmol^-1	72.71	76.798
Log A	3.964	3.195

Figure 9 : Eyring’s plot of log(k/T) vs [1/T] to calculate thermodynamic parameters

Table 9. Eyrings plot parameters at 313K for the oxidation of lactic acid by TPACC and TPAFC

Eyring’s parameters	[TPAFC]	[TPACC]
Ea KJmol^-1	16.176	17.2677
∆H KJmol^-1	13.574	14.666
-∆S JK^-1mol^-1	178.46	173.86
∆G KJmol^-1	69.43	69.08
Log A	3.964	3.195

Polymerization test and stoichiometry

To a solution of 0.1 mol dm^-3 of Lactic acid in perchloric acid medium, a few drops of acrylonitrile was added and shaken well. To this 5 ml of 0.01 mol dm^-3 solution of oxidant in aqueous medium and stirred well and kept under nitrogen atmosphere in a thermostat for one hour. No polymer formation was observed which indicates the absence of radical formation as a intermediate during the course of the reaction. The estimation of unreacted oxidant TPAFC and TPACC indicated that one mole of the oxidant was consumed by 0.65 mole of the Lactic acid (1.00: 0.65) ratio.

CONCLUSION:

The lactic acid oxidation by two new reagents TPAFC and TPACC have been investigated in perchloric acid medium at room temperature. The oxidation of lactic acid is first order with respect to each TPAFC, TPACC, perchloric acid. This oxidation is catalysed by cationic micelle-Benzalkonium chloride. Effect of variation of TPAFC; TPACC on lactic acid has no progress of increase on rate. On varying the concentration of substrate, perchloric acid shows a progressive increase in the reaction rate with slope is near to unity. variation of micellar concentration shows a progress of increase in the rate and a plot gives sigmoidal curve.

Rate of oxidation of lactic acid for TPACC is faster than TPAFC and rate of lactic acid oxidation is faster in TPAFC than TPACC on varying perchloric acid concentration. Polymerisation test no intermediate is formed during the course of the reaction. Stoichiometry data shows that one mole of the oxidant was consumed by 0.65 mole of the Lactic acid. Thermodynamic parameters are calculated at four different temperatures, Arrhenius and Eyring plots were given.

ACKNOWLEDGEMENT:

The author wish to thank to Mr. P. Palanivel, PG Department of chemistry, Govt. Arts college, Krishnagiri, India to give their valuable support and Dr. P. Rajkumar, Department of chemistry, Priyadarshini Engineering college, Vaniambadi, Tamil Nadu, India for his valuable suggestions.

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Received on 10.02.2016 Modified on 07.03.2016

Asian J. Research Chem. 9(4): April, 2016; Page 170-176

DOI: 10.5958/0974-4150.2016.00027.4