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JASC: Journal of Applied Science and Computations ISSN NO: 1076-5131

STRUCTURE AND REACTIVITY OF OXIDATION OF CYCLIC BY QUINOXALINIUM DICHROMATE

K.Anbarasu*, S.Chandrasekar, M.Venkatapathy and S.Suriya Department of Chemistry, Arignar Anna Govt. Arts College, Musiri-621 211, Tamilnadu, India Email: [email protected]

Abstract--- The structural behavior of few cyclic ketones, oxidation mechanism and absorbance measurements by the oxidant, quinoxalinium dichromate was investigated. The order of the reaction with respect to [cyclic ketones] and [QxDC] obeys first order kinetics. The [H+] followed fractional order and the plausible mechanism has been discussed. The free radical mechanism was not observed for the cyclic ketones. The rate of the reaction remains constant for the addition of sodium chloride. Activation and thermodynamic parameters were calculated from Eyring’s equation using thermostat at 303K, 313K, 323K and 333K. The orders of reactivity was noted among the cyclic ketones viz., Cyclohexanone < Cycloheptanone < Cyclopentanone < Cyclooctanone.

Key Words: Cyclic ketones, structural behavior, oxidation, kinetics, thermodynamic parameters, mechanism. I. INTRODUCTION

Cr (VI) oxidation reactions have been performed in aqueous acidic conditions. The source of Cr (VI) compounds were and sodium or . Cr (VI) have been proved to be versatile reagents capable of oxidising almost all the oxidisable organic functional groups [1-3]. Both the chromate and dichromate anions are strong oxidizing reagents at low pH: 2- + - 3+ Cr2O7 + 14 H3O + 6 e → 2 Cr + 21 H2O However, there are moderately oxidizing at high pH: 2- - − CrO4 + 4 H2O + 3 e → Cr(OH)3 + 5 OH

Many synthetic reagents have been developed in recent years with some success [4]. Some of the important entries in the list of reagents are pyrolinium chlorochromate [5], pyridinium chlorochromate (PCC)[6], pyridinium fluorochromate (PFC)[7], pyridinium dichromate (PDC)[8], pyridinium bromochromate (PBC)[9], quinolinium fluorochromate (QFC)[10] and Quinolinium chlorochromate (QCC)[11]. A survey into the literature was studied on the kinetics of oxidation of cyclic ketones with various oxidants [12-14]. The oxidation of cyclic ketones by QxDC has not been studied and this paper has evidenced the oxidation of few cyclic ketones.

Preparation of quinoxalinium dichromate (QxDC) Quinoxalinium dichromate (QxDC)[15] was prepared by addition of quinoxaline to a solution of CrO3 in water in a molar ratio of 1:1. An yellow, non-hygroscopic and stable solid was obtained. The structure of the product was confirmed by elemental analysis and its IR spectral studies.

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H N 2- Cr2O7 N H

Figure.1 Structure of QxDC

II. MATERIALS AND METHODS Materials: Cyclohexanone, cyclopentanone, cycloheptanone and cyclooctanone of AnalaR grade were purchased from Sigma Aldrich and the purity was ascertained by checking the boiling point. The solvent glacial was distilled with CrO3 and the distilled acid between 116 and 118°C was collected and utilized throughout the experiment. Doubly distilled water was used for the kinetic measurements.

Kinetic Measurements: The kinetics of oxidation of cyclohexanone by quinoxalinium dichromate was carried out in Acetic acid (75%) and water medium (25%). The absorbance was studied using digital spectrometer (ELICO CL 23 MINI SPEC). The wavelength was adjusted at 470 nm. The temperature was controlled using thermostat (ROYAL) to an accuracy of ±0.5 oC. The oxidation process was started by adding QxDC at 303 K. The cyclic ketones were maintained in excess than the oxidant to satisfy pseudo- first order condition. 75% completion of reaction was carried and the rate constants were determined using linear regression method using Microcal origin software. The results were reproducible with in ±2% error.

Stoichiometry: QxDC was added in excess to the mixture of substrate (Cyclohexanone), acid (HCl), solvent (AcOH+H2O) and kept at a night without being disturbed. The unreacted oxidant was determined iodometrically. The stoichiometry between cyclic and quinoxalinium dichromate was found to be 1:1. Product Analysis: The reaction mixture containing cyclic ketone (0.1 M) in acetic acid and QxDC (0.1M) in water was added and the medium was maintained using . After 48 h, the reaction mixture was extracted with and dried over anhydrous sodium sulphate. The ethereal layer was washed with water several times and kept on a water bath for ether evaporation and cooled to get the product. The products of oxidation of cyclic ketones were found to be corresponding dibasic acids by their spot tests [16]. Finally, the products were identified by the corresponding 2-hydroxy ketones.

III. RESULTS AND DISCUSSION Oxidation of Cyclic ketones: The oxidation mechanism and kinetic measurements of four different cyclic ketones viz. Cyclohexanone, cyclopentanone, cycloheptanone and cyclooctanone were studied using the oxidant QxDC in acetic acid and water medium catalysed by hydrochloric acid. Effect of varying the [QxDC]: The oxidation of cyclohexanone by QxDC was investigated at several concentrations of oxidant [QxDC], the [substrate], [HCl], AcOH, water and temperature remains constant. The plot of log absorbance versus time was linear indicating first order dependence of the reaction on [QxDC]. The pseudo-first order rate constants k1 (Table 1) were calculated from the linear square fit (r = 0.980).

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Effect of varying the [Cyclohexanone]: The rate of reaction was increased steadily on increasing the concentration of the substrate as shown in Table 1. The linear plot of log k1 versus log [s] with a slope (1.12) of unity and r = 0.996, it clearly indicates that the reaction has unit order dependence on the concentration of the cyclohexanone.

Effect of varying the hydrogen ion concentration: The effect of acidity was studied by varying the concentration of hydrochloric acid and rate constants were found to increase with the increase in the + concentration of hydrochloric acid (Table 1). The plot of log k1 versus log [H ] (Figure 2) was found to be linear with fractional slope (0.53) indicating fractional order dependence with respect to hydrogen ion concentration.

Effect of solvent and ionic strength: The reaction was carried out at different initial concentrations of sodium chloride while the other variables were kept constant. Increase in ionic strength of the medium by adding sodium chloride has no effect on the reaction rate (Table 2) indicating the involvement of charged species in the rate-determining step. The reaction was carried out at six different percentages of acetic acid- water mixtures while all other factors were constant. The rate of reaction increased with the increase in the percentage of acetic acid (Table 2). The plot of log k1 versus 1/D (Figure 3) was found to be linear (r= 0.997). This might be probably due to ion-dipole interaction [17] in the rate determining step.

Effect of added acrylonitrile: The reaction does not induce of acrylonitrile. The added acrylonitrile has no effect on the reaction mixture indicating the absence of free radical mechanism. Effect of varying the [Manganous sulphate]: The reaction was carried out with the varying concentrations of Mn2+ ions keeping all the other factors constant. There was a noticeable negative catalytic effect on the reaction rate for the addition of manganous sulphate (Table 2). Thus, it is possible that the reaction involves a two electron process. Mechanism: The cyclohexanone was oxidized by quinoxalinium dichromate to give 2-hydroxy cyclohexanone in presence of hydrochloric acid. The reaction shows first order with respect to oxidant, substrate and fractional order with respect to hydrogen ion concentration. In this case, the concentration of chromium (VI) is very much lower [18]. From these observations, the following mechanism (Scheme 1) was proposed and suitable rate law was derived.

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O OH C C H C CH 2 2 K1 HC CH2

H2C CH2 H2C CH2 C C H2 H2

K2 + 2HCrO - + C H N (C8H8N2)Cr2O7 H2O 4 8 8 2

OH OH C C C CH k O H 2 HO O CH 3 2H+ + H C 2 HO + Cr Cr H C CH2 CH2 slow 2 O O H2C HO OH C C H2 H2

C1 O OH -H+ HO HO C Cr + C1 H C CH2 HO Hydrolysis OH H2C CH2 C H2 Scheme 1. Mechanism of Oxidation of Cyclohexanone by QxDC Rate law Rate = k3[Product] + -d[QxDC] k3 K2 K1[O][S][H ] = 1+k [H+] dt 3 The proposed mechanism and the suitable rate law support all the observations made including the effect of solvent polarity and the negative entropy of activation. Effect of varying the temperature: The reaction has been studied at four different temperatures for the cyclic ketones viz., cyclo hexanone, cyclo pentanone, cyclo heptanone, cyclo octanone. The rate of reaction increased with the increase in the temperature (Table 3). The thermodynamic and activation parameters have been computed [19] from the linear plot of log (k2/T) versus 1/T of Eyring’s equation [20].

The negative values of the entropy of activation (ΔS#) indicate extensive solvation of the transition state over the reactants. Free energy of activation (ΔG#) values is nearly constant which indicates that all the cyclic ketones are oxidized and follows the same mechanism. Applying the isokinetic relationship and using the equation. # # # ΔH = ΔH0 + βΔS …… (1) A plot of ΔH# versus ΔS# gave a straight line (Figure 4). The isokinetic temperature β obtained from the slope is 359.0 K. Since the β value is higher than the experimental temperature, it indicates that this oxidation reaction is an enthalpy controlled reaction [21] and it follows a common mechanism. The plot of log k323 K versus log k313 K (Figure 5) gave a straight line with an excellent correlation co efficient r = 0.991. Such a good correlation indicates that all the cyclic ketones follow a common mechanism.

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Structure and reactivity of cyclic ketones: The kinetic data of the oxidation of the above cyclic ketones were analysed. The order of reactivity increases with the increasing size of the ring as noticed in the oxidation of cyclic ketones by acid chromate [22]. Because the cyclohexanone ring with six tetrahedral carbon atoms is highly symmetrical and stable. The hydrogen-hydrogen repulsion is reduced to a minimum in the chair form as a result of fully staggered constellation permitted by this form. The increase in the angle will lead to an increase in internal strain (positive strain). Therefore in cyclohexane derivatives I-strain [23] will oppose the reactions involving a change in covalency of a ring atom from either four to five or four to three. In the present study, the orders of reactivity among the cyclic ketones are Cyclohexanone < Cycloheptanone < Cyclopentanone < Cyclooctanone.

REFERENCES

1. U.P. Boyer, M. Lardy and K. Myrback, ‘The Enzymes’, vol. 1, pp. 402, 1959 Academic Press, Newyork. 2. K.B. Wiberg, ‘Oxidation by and Chromyl Compounds – Oxidation in Organic Chemistry’, Newport, pp. 69, 1965. 3. D.G. Lee and R.L. Augestine, ‘Oxidation’, 2nd Ed., Marcel Dekker, Newyork 1969. 4. L.F. Fieser and M. Fieser, ‘Reagents for ’, John Wiley Edition, New York, Vol. 1, pp. 11, 1967. 5. M. Mamaghani, F. Shirini and F. Parsar, ‘Prolinium Chlorochromate as a New Mild and Efficient Oxidant for ’, Russian Journal of Organic Chemistry, vol. 38, no. 8, pp. 1113, 2002. 6. E.J. Corey and J.W. Suggs, ‘Pyridinium Chlorochromate; an Efficient for Oxidation of Primary and Secondary Alcohols to Carbonyl Compounds, Tetrahedron Letters, pp. 2647, 1975. 7. M.N. Bhattacharjee, M.K. Chaudhuri, H.S. Dasgupta and N. Roy, ‘Pyridinium Fluorochromate; A New and Efficient Oxidant for Organic Substrates’, Synthesis, pp. 588-590, 1982. 8. E.J. Corey and G. Schmidt, ‘Useful Procedures for the Oxidation of Alcohols Involving Pyridinium Dichromate in Aprotic Media’, Tetrahedron Letters, pp. 399-402, 1979. 9. N. Narayanan and T.R., Balasubramanian, ‘Pyridinium Bromochromate-A New Reagent for Bromination and Oxidation’, Indian Journal of Chemistry, vol. 25B, pp. 228-229, 1986. 10. V. Murugesan and A. Pandurangan, ‘Quinolinium Fluorochromate; A New Reagent for the Oxidation of Organic Compounds’, Indian Journal of Chemistry, vol. 31B, no. 7, pp. 377-378, 1992. 11. J. Singh, P.S. Kalsi, G.S. Jawanda and B.R. Chhabra, ‘Quinolinium Chlorochromate; A Selective ’, Chemistry and Industry, pp. 751, 1986. 12. K.G. Sekar and A.N. Palaniappan, ‘Structural Reactivity in the Oxidations of Cyclic Ketones by Quinolonium Fluorochromate, Oxidation Communications, vol. 31, no. 3, pp. 606-612, 2008. 13. B.K. Kumar, P. Saroja and S. Kandlikar, ‘Uncatalysed and HClO4 Catalysed Oxidation of Cyclic Ketones by Pyridinium Chlorochromate in Aquo-Acetic Acid Medium’, Oxidation Communications, vol. 26, no.2, pp. 223-234, 2003. 14. S. Das, E.R. Rani and M.K. Mahanti, ‘Kinetics and mechanism of the oxidative cleavage of cyclic ketones by quinolinium dichromate’, Kinetics and , vol. 48, no. 3, pp. 381-389, 2007. 15. N. Degirmenbasi and B. Ozgun, ‘Quinaldinium Fluorochromate and Quinaldinium Dichromate: Two New and Efficient Reagents for the Oxidation of Alcohols’, Monatshefte Fur Chemie, vol. 135, pp. 407–410, 2004. 16. F. Fiegel, ‘Spot Tests in Organic Analysis, 7th Edition, Elsevier Publishing Company, London, 1966.

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17. E. Quinlan and E.S. Amis, ‘The Alkaline Hydrolysis of Methyl Propionate in -Water Mixtures and Solutions of Different Ionic Strength’, Journal of American Chemical Society, vol. 77, no. 16, pp. 4187- 4440, 1955. 18. KB. Wiberg and T. Mill, ‘The Kinetics of the Chromic Acid Oxidation of Benzaldehyde’, Journal of American Chemical Society, vol. 80, no.12, pp. 3022-3029, 1958. 19. N. Nalwaya, A. Jain and B.L. Hiran, ‘Kinetics of Oxidation of Glycine by Pyridinium Bromochromate in Acetic Acid Medium’, Journal of Indian Chemical Society, vol. 79, no. 7, pp. 587-589, 2002. 20. J.E. Leftler, ‘Entropy Requirements of the Hammett Relationship’, Journal of Chemical Physics, vol. 23, no. 11, 2199-2200, 1955. 21. PR. Saradamani and V. Jegannathan, ‘Kinetics and Mechanism of the Oxidation of Aromatic by N- Bromoacetamide’, Indian Journal of Chemistry, vol. 29A, no. 1-4, pp. 700-702, 1990. 22. K. Vijayalakshmi and E.V. Sundaram, Journal of Indian Chemical Society, vol. 55, pp. 567, 1978. 23. R. Gurumurthy, B. Karthikeyan and M. Selvaraju, Oxidation Communications, vol. 22, no. 1, pp. 103, 1999.

Volume V, Issue XII, December/2018 Page No:1453 JASC:Table Journal 1: Rate ofdata Applied for the oxidation Science of andcyclo Computationshexanone by QxDC at 303 K. ISSN NO: 1076-5131 3 2 1 4 [QxDC] 10 [cyclo hexanone] 10 [HCl] 10 k1 10 mol dm-3 mol dm-3 mol dm-3 (s-1) 0.7 2.25 2.0 7.36 1.4 2.25 2.0 7.20 2.1 2.25 2.0 7.39 2.8 2.25 2.0 7.26 1.40 1.50 2.0 5.08 1.40 3.00 2.0 10.61 1.40 3.75 2.0 13.69 1.40 4.50 2.0 17.09 1.40 2.25 0.80 4.92 1.40 2.25 3.20 9.54 1.40 2.25 4.40 11.55 1.40 2.25 5.60 14.10

% AcOH:H2O = 75:25 (v∕v)

Table 2: Rate data for the oxidation of cyclo hexanone by QxDC at 303 K. 4 4 4 % AcOH:H2O [NaCl] 10 [MnSO4] 10 k110 (v∕v)q mold m-3 mold m-3 (s-1) 60:40 - - 5.71 65:35 - - 6.02 70:30 - - 6.48 75:25 - - 7.20 80:20 - - 8.15 85:15 - - 9.40 75:25 6.0 - 7.04 75:25 12.0 - 7.22 75:25 18.0 - 7.18 75:25 24.0 - 7.25 75:25 - 6.0 5.90 75:25 - 12.0 4.04 75:25 - 18.0 3.28 75:25 - 24.0 2.96 [QxDC] = 1.40 x 10-3 mol dm-3 [Cyclo hexanone] = 2.25 x 10-2 mol dm-3 [HCl] = 2.00 x 10-1 mol dm-3

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Table 3. Thermodynamic and activation parameters for the oxidation of Cyclic Ketones by QxDC [QxDC] = 1.40 x 10-3 mol dm-3 [Cyclic Ketones] = 2.25 x 10-2 mol dm-3 + -1 -3 [H ] = 2.00 x 10 mol dm % AcOH : H2O = 75:25 (v/v) 4 k1 x 10 (s-1) Ea Order -ΔS# ΔG# Cyclic ΔH# (kJmol- S.No. w.r.to (JK-1mol- (kJmol-1) r Ketones 303 K 313 K 323 K 333 K (kJmol-1) 1) at 303 substrate 1) at 303 K K Cyclo 1. 1.12 `7.20 12.63 20.73 35.54 18.12 170.82 69.88 20.64 0.999 hexanone Cyclo 2. 1.03 8.49 14.89 24.42 38.85 17.23 173.09 69.68 19.75 0.999 heptanone Cyclo 3. 0.97 10.67 16.04 26.12 40.44 15.19 179.15 69.47 17.71 0.998 pentanone Cyclo 4. 1.03 12.65 18.94 30.12 47.19 14.93 179.41 69.29 17.45 0.998 octanone

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-2.8 r = 0.992 -2.9 B = 0.53 -3 1 -3.1 log k -3.2

-3.3

-3.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 log[H+]

+ Figure 2. Plot of log k1 versus log [H ]

-3.0 r = 0.997 -3.1 B = +7.35

-3.1 1 -3.2 log k log -3.2

-3.3

-3.3 1.5 2.5 3.5 4.5 5.5 6.5

1 / D X 102

Figure 3. Plot of log k1 versus 1 / D

Volume V, Issue XII, December/2018 Page No:1456 JASC: Journal of Applied Science and Computations ISSN NO: 1076-5131 19 r = 0.999 cyclo hexanone B18 = 359.0

17 cyclo heptanone # H D cyclo16 pentanone

15 cyclo octanone

14 -180 -178 -176 -174 -172 -170 DS#

Figure 4. Isokinetic Plot of ΔH# versus ΔS#

-2.50

r = 0.991 -2.55

1 (323K) -2.60 log k log

-2.65

-2.70 -2.95 -2.90 -2.85 -2.80 -2.75 -2.70

log k1 (313K)

Figure 5. Exner plot of log k1 (323 K) versus log k1 (313 K)

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