Kinetics of the Epoxidation of Crotonic Acid by Aqueous Hydrogen Peroxide Catalysed by Sodium Orthovanadatae

Aims And Background

Since epoxides have found use as reagents, intermediate, end products, epoxidation of olefins and other organic substrates by inorganic oxidants catalysed by trasitional derivative has remained a primary goal in recent years. H₂O₂ becomes an efficient epoxidising agent of unsaturated compounds under the catalytic action of certain transition metal derivative. We have examined the catalytic activity of W (VI) and Mo (VI) compounds in some oxidation reaction with H₂O₂ as the oxidant^1-3. In the present paper we report the oxidation of unsaturated organic substrate crotonic acid by V (V)-H₂O₂ system. The kinetic and thermodynamic parameters associated with the reaction were evaluated and the probable mechanism and rate laws were proposed.

Experimental

Materials : The substrate, crotonic acid of Ridel Dehem quality was used and its solution was prepared by dissolving the requisite amount of sample in doubly distilled water. E. Merck and B.D.H. grade sodium orthovanadate and sodium perchlorate was used for the epoxidation reaction. A fresh solution of hydrogen peroxide was used for each experiment and its strength was checked titrimetrically using acidified iodide and sodium thiosulphate⁴.

Kinetic Measurements: The sodium orthovanadate sodium and reaction mixture of desired concentrations containing crotonic acid and H₂O₂ were thermostated at 40°C (+0.1°C) in a separate flask. The pH of the solution was maintained by triethanolamine. An Elico model L1 120 digital pH meter was used for pH measurements. The reaction was started by adding the sodium orthovanadate solution (5ml) with a standard rapid delivery pipette. The instant of half delivery was noted as zero time. The total volume of the solution used was always 50ml in order to keep the reaction conditions same. The progress of the reaction was monitored by iodometric determination of unreacted hydrogen peroxide in measured aliquots (5ml) of the reaction mixture at appropriate intervals of the time as reported earlier¹.

*For correspondence

 

By measuring the rate constants in the presence of added sodium perchlorate with all other condition constant, it was established that reaction rate were independent of the ionic strength of the medium. Subsequently the ionic strength of the reaction mixture was not controlled.

Stoichiometry and product analysis: The stoichiometry of the reaction mixture was ascertained by taking a known excess of H₂O₂ over crotonic acid. The presence of the epoxide in the reaction mixture was confirmed by periodate test for epoxide⁵. The use of free radical scavenger did not initiate polymerisation⁶.

Results

Kinetics of epoxidation of crotonic acid by aqueous hydrogen peroxide was investigated at several initial concentration of H₂O₂ crotonic acid, catalyst, NaClO₄ and at different pH. Under the conditions of [substrate] > [H₂O₂], plots of [H₂O₂] versus time were linear establishing a zero order dependence in hydrogen peroxide at all concentration of the reactants Table 1 give the kinetic data of the rate of hydrogen peroxide dependence.

Table 1: Variation of rate of epoxidation with [H₂O₂]; [crotonic acid] = 1.0 x 10^-1 mol dm^-3; [Na₃VO₄] = 2.0 × 10^-4 mol dm^-3; temp. = 40°C, pH = 3.26

[H2O2] × 103	1.00	2.00	3.00	4.00	5.00	6.00	7.00
108 k0 (mol dm-3 min-1)	0.068	0.073	0.072	0.071	0.076	0.078	0.068

The reaction is first order with respect to [Crotonic acid] as evident from Table 2. The observed zero order rate constant when divided by crotonic acid concentration gives a fairly constant value. A plot of lg k₀ versus 1g [crotonic acid] is a straight line with a unit slope indicating first order dependences of rate on the concentration of substrate.

Table 2: Variation of rate of epoxidation with [Crotonic acid]; [H₂O₂] = 4.0 × 10^-3 mol dm^-3; [Na₃VO₄] = 2.0 × 10^-4 mol dem^-3; temp. = 40°C, pH = 3.26.

[Crotonic acid] (mol dm-3)	1.50	2.00	2.50	3.00	4.00
108 k0 (mol dm-3 min-1)	0.056	0.070	0.088	0.097	0.140
106 k1 (min-1)	0.037	0.035	0.035	0.032	0.035

To study the dependence of rate on catalyst, kinetic experiments have been conducted at different concentration of Na₃VO₄ keeping the concentration of other reactants constant. The rate of reaction was found to be first order with respect to [catalyst] (Table 3). The unit slope of the plot of 1g k₀ against 1g [Na₃VO₄) is an indication of the first order dependence of the rate on the concentration of the sodium orthovanadate.

Table 3: Variation of rate of epoxidation with [Na₃VO₄]; [Crotonic acid] = 1.0 × 10^-1 mol dm^-3; [H₂O₂] = 4.0 × 10^-3 mol dm^-3; temp. = 40°C, pH = 3.26

[Na3VO4] × 104 (mol dm-3)	2.00	2.50	3.00	3.50	4.00
108 ko (mol dm-3 min-1)	0.07	0.09	0.10	0.14	0.19
104 k1 (min-1)	0.035	0.036	0.033	0.047

The pH dependence on the reaction was investigated at 40°C in the range 3.26-7.00 keeping the concentrations of all other reactants fixed. A perusal of Table 4 shows that the rate of reaction decrease gradually with an increase in pH in the range 3.26 to 5.50, reaches to a minimum and then slowly increases.

Table 4: Variation of rate of epoxidation on pH [Crotonic acid] = 1.0 × 10^-1 mol dm^-3; [H₂O₂] = 4.0 × 10^-3 mol dm^-3; [Na₃O₄] = 2.0 × 10^-4 mol dm^-3; temp. = 40°C

pH	3.26	3.50	4.00	4.50	5.50	6.50	7.00
k0 × 108 mol dm-3 min-1	0.070	0.060	0.50	0.40	0.35	0.050	0.060

The kinetics of epoxidation of crotonic acid by H₂O₂ catalyzed by sodium orthovanadate has been studied at different temperatures in the range 313 K to 328 K. A plot 1g k₀ of observed rate against the reciprocal of temperature was linear, and the values of activation parameters evaluated from the Arrhenius plot are presented in Table 5.

Table 5: Variation of rate of epoxidation of crotonic acid with temperature; [Crotonic acid] = 1.0 × 10^-1 mol
dm^-3; [H₂O₂] = 4.0 × 10^-3; mol dm^-3; pH = 3.26.

Temperature	k0 × 108 mol dm-3 min-1	Ea KJ mol-1	DH*	DG* KJ mol-1	-DS* JK-1 mol-1
313	0.070	–	45.02	131.22	275.40
318	0.110	–	44.98	132.60	275.63
323	0.124	48.00	44.94	133.98	275.64
238	0.190	47.25	44.90	135.36	275.60

Discussion

The preliminary experimental data suggest that H₂O₂ alone cannot epoxidise the substrate. The catalyst alone also fails to bring about epoxidation. This suggests the involvements of some oxygenated form of vanadium as oxygen carrier.

It has been shown⁷ that sodium orthovanadate changes into vanadium pentoxide in the pH range 2.2 to 6.5 Vanadium pentoxide at once reacts with hydrogen peroxide to produce a single peroxy complex as shown a study of Flood et al⁸ and inconclusively described as H₃VO₅.

The mode of dependence on the pH may be ascribed to the detaching of a proton from the peroxyvanadic acid.

The decrease of the rate with the pH corresponds to the quantitative formation of H₂VO₅^–. The concentration of H₃VO₅ decrease probably due to its conversion into H₂VO₅^– which is a slower oxidising agent than undissociated peroxyvanadic acid.

The mechanism of epoxidation of crotonic acid may be proposed by the following steps :

The above rate expression is fully consistent with the experimental observations.

It is well known that the nature of activated complex is broadly reflected in the values of S*. The entropy of activation is negative, indicating that the rate will be slower. A large negative entropies may be ascribed to a highly oriented transitions state. It has been attributed to bond formation of the substrate the oxygen from the peracids to the double bond.

Similar trend was reported by Ballisteri et al.⁹ that vanadium forms peroxospecies in the presence of hydrogen peroxide. This peroxspecies is electrophilic in nature which transfer oxygen to the unsaturated acids to form the epoxide.

References

1. G.L. Agarwal, A. Shrivastava; Proc. Ind. Natn. Sci. Acad., 59, 169 (1993).
2. G.L. Agarwal, A. Shrivastava; Revaue. Roum. de Chemie., 59, 751 (1994).
3. G.L. Agarwal; J. Ind. Council. Chem. XII, 4, (1996).
4. F.A. Miller, C.M. Wilkins; Anal. Chem., 24, 1853 (1952).
5. D.J. Pasto, C.R. Johnson; Organic structure Determination, Prentice-Hall NJ, 376 (1969).
6. P.J. Flory; Principles of polymer Chemistry Cornel. Univ. Ethaca. USA (1953).
7. H. Remy; Tretise in Inorganic Chemistry, Amsterdan Elsevier II, 100 (1956).
8. P. Flood, T.J. Lewis, D.H. Richard; J. Chem. Soc., 6, 5024, (1963).
9. F.P. Ballistreri, A. bazoo, G.A. Tomasell, R.M. Tosceno; J. Org. Chem., 57, 7074 (1992).

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