Reactivity of Phenoxy Acetic Acid and Oxidation Kinetics Using Pyrazinium Chlorochromate
Ilavenil K. K1*, Anusha. R2, Senthil Kumar. V3 and Anbarasu. K4
1 Department of Chemistry, Nehru Memorial College, Affiliated to Bharathidasan University, Tiruchirappalli, Tamilnadu - 621 007
2 Department of Chemistry, MIT College of Arts and Science for Women, Affiliated to Bharathidasan University, Musiri, Tamilnadu - 621 211
3Department of Mechanical Engineering, SRM-TRP Engineering College, Affiliated to Anna University, Irungalur, Tiruchirappalli, Tamilnadu – 621 105
4Department of Chemistry, Arignar Anna Govt. Arts College, Affliliated to Bharathidasan University, Musiri, Tamilnadu - 621 211
*[email protected] ABSTRACT
The oxidation of phenoxy acetic acid by pyrazinium chlorochromate has been studied in aqueous acetic acid medium (60 %). The reaction showed fractional order for substrate and HClO4, first order for the oxidant and the low dielectric constant favors the reaction. Using the Eyring least square method the activation and thermodynamic parameters were evaluated. The enthalpy of change in activation (ΔH# = 6.56 KJ/mol), entropy of change in activation (ΔS# = -208.43 KJ/mol) and the activation energy, Ea = 9.08 KJ/mol were calculated from the temperature effects. The linear graph obtained proved good regression and slope values. The phenoxy acetic acid is oxidized into quinone through the formation of activated complex. The absence of free radical mechanism has been ruled out and the involvement of two electrons transferred by manganous ions facilitated the reaction and the plausible mechanism has been proposed.
Keywords:Pyrazinium chlorochromate, phenoxy acetic acid, rate constant, mechanism, Ionic strength, manganous sulphate.
Introduction
Numerous heterocyclic organic compounds has been synthesized and employed as an oxidizing agent. Hexavalent chromium is a powerful agent capable to oxidize various organic functional groups (Wiberg, 1965). The commonly used harmful compounds are degraded using a heterocyclic compound which reduces the toxicity of the compound.
The oxidation mechanism of toxic compounds into harmless products is the basic objective of the investigation.
Chemical kinetics is an important tool to analyse the mechanism of the reaction (Elango et al., 2005). A methodical study has been made to set up the reactivity of the compounds. The thermodynamic parameters and isokinetic relation helps to decide the mechanism of the reaction (Elango et al., 2007). The hexavalent and trivalent chromium differ in their physiochemical property. The oxidation and reactivity of few acids using PZCC in the order, mandelic acid > lactic acid > glycolic acid (Sekar & Prabakaran 2008 c) was reported. The order and reactivity of benzaldehyde (Sekar & Prabakaran 2008 a), oxidation of pentammine cobalt (III) complexes of alpha hydroxy acids in the micelles medium (Nazim Ahmed & Mansur Ahmed 2013) using the oxidant pyrazinium chlorochromate has been investigated. The oxidation of phenoxy acetic acid using chromium (VI) reagents is less reported. PAA is employed as plant regulators, dyes, insecticides and antimicrobial agent. Various derivatives of PAA has been synthesised and used as pharmaceutical agents. The oxidation of PAA using pyridinium hydrobromide perbromide showed first order kinetics and Michaelis-Menten equation for the [PAA] (Karunakaran & Elango 1996). Meta and para substituted phenoxy acetic acid exhibiting first-order kinetics for the concentration of substrate, oxidant and second order with respect to the acid (Sekar & Anbarasu 2011) and the oxidation using tetrakis (pyridine) cobalt (II) chromate showed first order kinetics for [oxidant] and [PAA] (Palaniappan et al., 2012). Phenoxy acetic acid is a widely used insecticides and the oxidation of it was done using pyrazinium chlorochromate compound.
Experimental
The pyrazinium chlorochromate was prepared by the standard method (Davis and Sheets 1983).
The phenoxy acetic acid was prepared using standard procedure (Koelsch 1931) and the purity was checked by melting point method (98.5 0C). The concentration of phenoxy acetic acid was taken in surplus over the concentration of the oxidant, C4N2H5CrO3Cl. This pseudo order condition was performed at fixed temperature and the reduction in the concentration of pyrazinium chlorochromate was observed using digital spectrometer at the visible range (470 nm) and the solutions were maintained at constant temperature in a thermostat to an accuracy of
± 0.5 oC. The response was observed by adding known volume of PZCC into the reaction flask at regular intervals of time. The linear regression (r = 0.991) was recorded with 85 % completion of the reaction with linear plots of log absorbance versus time. The kinetic runs were reproduced in
± 2%.
Figure 1. Preparation of Phenoxy acetic acid and Pyrazinium chlorochromate
Spectral elucidation of Phenoxy acetic acid (PAA)
Infra-red spectrum exhibited peak at 3426.51 cm-1, the stretching frequency of the group (–O-H) and it is weak and broad, bending frequency at 693.40 cm-1 is sharp and medium. The presence of –C-OH in the acid exhibited a sharp and strong peak at 1172.25 cm-1, the aliphatic –C=O at 1607.11cm-1 sharp and strong and due to the ether linkage in the compound the observed value is decreased. The aromatic –CH2 at 3250 cm-1 is weak and broad, the Ar-O (ether) bestows a sharp
and medium peak at 1043.36 cm-1 and this middle value is expected due to the presence of phenyl ring and acidic group on either side of the ether group. Ultraviolet -Visible spectrum showed absorbance peaks at 216.85 nm (sharp), 268.70 nm (weak), Ultraviolet -Visible transmittance peaks at 216.85 nm (sharp), 268.70 nm (sharp).
Figure 2. IR spectrum of Phenoxy acetic acid
Figure 3. UV Absorbance Spectrum of Phenoxy Acetic Acid
Figure 4.UV Transmittance Spectrum of Phenoxy Acetic Acid
Stoichiometry and Product analysis
The ratio of [PzCC]: [phenoxy acetic acid] and its stoichiometry was confirmed by taking excess of the oxidant over the substrate and was found to be 1:2. The mixture of phenoxy acetic acid in the solvent and pyrazinium chlorochromate dissolved in water was warmed and the product obtained was dried overs sodium sulphate. The product quinone was filtered, cooled and confirmed from the spectral analysis of IR and UV spectrum given in the above (Figure.2, 3, 4).
Kinetic Studies
Pseudo-first order condition was maintained for the oxidation of phenoxy acetic acid (PAA) using pyrazinium chlorochromate (PzCC) in aqueous acetic acid solvent.
The [acid] and [PAA] was fixed and the reaction rate was observed to be first order with respect to PzCC as evidenced by a good linearity in the plot of log absorbance versus time (Figure. 5).
The presence of acid chromate affected the reaction rate (Table 1) with increase in the [PzCC]
and it is evidenced from the plot of 1/k1versus [PzCC]-1 as shown in the figure 6. At constant temperature the reaction rate increased with the rise on [PAA] and from the graph the slope value was found to be B = 0.433, r = 0.998 and fractional order with respect to the substrate (Figure 7).
Table 1 Kinetic data for the oxidation of phenoxy acetic acid
102[C4N2H5CrO3Cl] mol dm-3
103[C8H8O3] (mol dm-
3) 101[H+] (mol dm-3) k1 104s-1
1.6 2.25 2.0 14.06
1.8 2.25 2.0 11.08
2.0 2.25 2.0 8.03
2.2 2.25 2.0 6.56
2.4 2.25 2.0 5.17
2.0 1.95 2.0 7.49
2.0 2.25 2.0 8.03
2.0 2.55 2.0 8.49
2.0 2.85 2.0 8.87
2.0 3.15 2.0 9.22
2.0 2.25 1.6 5.70
2.0 2.25 2.0 8.03
2.0 2.25 2.4 9.84
2.0 2.25 2.8 12.33
2.0 2.25 3.2 15.45
AcOH:H2O=60:40 (v/v) Temperature= 303K
Figure 5. Plot of log absorbance versus time
Figure 6. Plot of 1/k1 versus 1/[PzCC]
Figure 7. Plot of log k1versus log [S]
The different concentration of the catalyst (perchloric acid) increased the rate from 1.6 x 10-1 to 3.2 x 10-1moldm-3 shown in the table 2. The plot of log k1versus log [acid] gave a linear graph with B = 1.403, r = 0.998 as given in the figure 8. The protonated form of the oxidant resulted in fractional order kinetics (Ravishankar1998). Varying concentration of sodium perchlorate had no effect on the reaction rate due to the ionic species in the slow step. The solvent effect produced a linear graph (Figure. 9) with B = +44.06, r = 0.998. This linearity confirms the interaction between the neutral molecule and the ion in the rate determining step (Sekar&Saktivel 2013). By the addition of different [MnSO4] at 303K, the rate constant of the reaction decreased slowly since the Mn2+ are involved in the reaction by transferring two electrons (Graham 1958).
Figure 8. Plot of log k1versus log [H+]
Table 2. Rate data for oxidation of phenoxy acetic acid by PzCC, sodium perchlorate and manganous sulphate at 303K
AcOH:H2O (v/v) [NaClO4] 104 (mol dm-3) [MnSO4]103 (mol dm -3) k1104 (s-1)
60:40 - - 8.03
62:38 - - 9.17
64:36 - - 10.41
66:34 - - 12.08
68:32 - - 14.17
70:30 - - 16.88
60:40 4.0 - 8.12
60:40 8.0 - 8.06
60:40 12.0 - 7.94
60:40 16.0 - 8.06
60:40 - 4.0 6.88
60:40 - 8.0 6.71
60:40 - 12.0 6.55
60:40 - 16.0 6.41
60:40
[PzCC] = 2.0 x 10-3 mol dm-3; [PAA] = 2.25 x 10-2 mol dm-3; [H+] = 1.0 x 10-1mol dm-3
Figure 9. Plot of log k1versus 1/D
Acrylonitrile Addition
The substrate, acid, solvent and the oxidant were taken in a beaker and to that mixture
acrylonitrile was added and left undisturbed for a day. There was no precipitate formed and this helps to conclude the absence of free radical formation.
Temperature Effect
The oxidation reaction of phenoxy aceticacid was subjected to different temperatures like, 298K, 303K, 313K and 323K by having other parameters as constant. By the method of Eyring’s least square method the activation parameters and thermodynamic parameters was evaluated (Eyring1935) (Leffler 1963).The rate constant value increased with increase in the temperature (Table 3). The plot of log k2/T versus 1/T gave a linear plot (Figure 10).
ΔH#= 6.56 kJ mol-1 ΔS#= -208.43 JK-1mol-1 ΔG# = 69.71 kJ mol-1 at 303 K Ea = 9.08 kJ mol-1 at 303 K
Table 3. Kinetic data for the Variation with Temperature
[Phenoxy aceticacid] = 2.25 x 10-2 mol dm-3 AcOH: H2O=60:40 (v/v)
[PzCC] = 2.0 x 10-3mol dm-3 [H+] = 2.0 x 10-1 mol dm-3 Temperature K k1 104s-1
293 6.88
303 8.03
313 10.21
323 13.08
Figure 10. Plot of log k2/T versus 1/T Mechanism of Oxidation of PhenoxyAceticacid
The kinetic measurements prove that the oxidation reaction is unit order for the [O] and fractional order dependence with respect to the concentration of substrate and perchloric acid. The solvent CH3COOH + H2O prove to be vital in the reaction rate. The low dielectric constant and ionic strength of the medium facilitate the reaction. The absence of free radical due to the addition of polymer acrylonitrile does not hinder the rate constant of the reaction. The two electron transfer from the manganese sulphate retards the reaction rate and based on this evidence the rate law and the mechanism has been proposed in Scheme 1.
Scheme 1. Mechanism of Oxidation of Phenoxy aceticacid by Pyrazinium chlorochromate
Rate law
Conclusion
The reaction rate was determined by the standard oxidation kinetic measurements as discussed.
The stoichiometry was proved that one mole of PAA required two moles of PzCC and the ratio was 1:2. The rigidity of the activated complex was due to the decrease in entropy change in the reaction. The nature of the pyrazine ring (presence of two nitrogen atoms) influenced the reaction rate and the rate values decreased effectively for the [O].
Acknowledgement
I extend my sincere gratitude to my Research supervisor, the President of Nehru Memorial College for his support in Research and Development and my Family for their care.
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