Kinetics and Thermodynamics of Oxidation of Benzhydrol by Tetrabutyl Ammonium Bromochromatein the Presence of Oxalic Acid

Introduction

Oxidation is a key reaction for different organic synthesis. Chromium(VI) compounds have proved to be versatile reagents capable of oxidizing almost every oxidiasable functional group.A number of new chromium(VI) containing compounds, with heterocyclic bases, likeisoquinolinium bromochromate¹, triphenylmethylposphonium chlorochromate², prolinium chlorochromate³, tripropylammonium fluorochromate⁴, benzimidazolium fluorochromate⁵, quinolinium chlorochromate⁶, tetrahexylammonium bromochromate⁷, 4-benzylpyridinium fluorochromate⁸, tetraethyl ammonium bromochromate⁹and tetrabutylammonium bromochromate¹⁰have been developed in recent years to improve the selectivity of oxidation of organic compounds.

The kinetics of oxidation of benzhydrols has been studied by many reagents such as 2,2’-bipyridyl-Cu(II) permanganate¹¹, pyridinium chlorochromate¹², chloramine-T¹³, N-bromo succinimide^{14, 15}, Tl(III)¹⁶ and  N-bromopthalimide¹⁷.  However, the kinetics of oxidation of benzhydrol by TBABC, a Cr(VI) reagent has not yetbeen studied. In continuation of our study for the oxidation of benzhydrol^18,19, we studied the kinetics of oxidation of benzhydrol by TBABC in the presence of oxalic acid (OA). A probable mechanism for the oxidation is also studied. 

Experimental

Materials

Tetrabutylammonium bromide and chromium trioxide were obtained from Fluka (Buchs, Switzerland). Benzhydrol (SRL,AR) andoxalic acid (Aldrich) were usedafter repeated crystallization from methanol.Acetic acid was purified by standard method and the fraction distilling at 118 ^oC was collected.

Preparation of Tetrabutylammonium Bromochromate, [N(C₄H₉)₄]Cro₃br

Tetrabutylammonium bromochromate (VI), [N(C₄H₉)₄]CrO₃Brwas easily prepared¹⁰ as follows: Chromium (VI) oxide (1 g, 10 mmol) was dissolved in MeCN and this solution was added to a solution of tetrabutylammonium bromide (3.22 g, 10 mmol) in MeCN under stirring at room temperature until an orange precipitate was formed. After 2 h stirring, the mixture was filtered. The solvent was evaporated at reduced pressure and the remaining solid was separated.

Scheme Click here to View Scheme

 

Kinetic Procedure

Reactions were carried out under pseudo first-order conditionswith a known excess of [benzhydrol]o over [TBABC]o at constanttemperature using 50% acetic acid – 50% water (v/v). A decrease in [TBABC] has been followed by spectrophotometric method using UV–Vis spectrophotometer, Shimadzu UV-1800 model.

Product Analysis

Product analysis was carried under kinetic conditions. In a typical experiment, benzyhrol (1.8 g, 0.01 mol) and TBABC (8.4 g, 0.02 mol) were made up to 100 ml of the solvent (50% acetic acid – 50 % water) and kept in the dark for 24 h  to ensure completion of the reaction. The solution was then treated with an excess (200 cm³) of a saturated solution of 2, 4 – dinitro phenyl hydrazine in 2 mol dm^-3 HCl and kept overnight in a refrigerator. The solvent was removed and the precipitated  2, 4 – dinitro phenyl hydrazone (DNP) was filtered and recrystallised from ethanol. The identityof product was established by comparing the m.p. ofthe DNP derivative with the literature value.The m.pt of DNP was 235 – 236 ^oC. This value is very close to m.pt of DNP of benzophenone (lit 237 ^oC).

Stoichiometric Studies

The stoichiomety of the reaction was determined by carrying out several sets of experiments with varying amounts of TBABC largely in excess over benzhydrol. The estimation of unreacted TBABC showed that the following reaction

3(C₆H₅)₂CHOH   +   2Cr(VI)        →         3(C₆H₅)₂CO + 6H⁺ + 2Cr(III)               (2) 

Results and Discussion

The oxidation of benzhydrolby TBABC has been conducted in 50% acetic acid and 50% water medium at 303K, under pseudo first order conditions and the result obtained were discussed in the following paragraphs.

Table 1: Rate constants for the oxidation of benzhydrolin the presence of oxalic acid by TBABC  in aqueous acetic acid medium at  303 Ka Click here to View table

 

Effect of varying OA concentration 

The concentration of oxalic acid is varied in the range of 0.0 x 10^-3 to 10.0 x 10^-3 mol dm^-1 at constant [TBABC], [BH] and [H⁺] at 303 K and the rates were measured (Table – 1).  When increasing the concentration of oxalic acid, the rate linearly increases.

Effect of varying TBABC concentration in the presence of OA

The values of k₁ were calculated in the presence of 6.0 x 10^-3mol dm^-3of oxalic acid.  The concentration of TBABC was varied in the range of 0.5 x 10^-3 to 2.5 x 10^-3 mol dm^-1 at constant [BH], [H⁺], [OA] at 303 K and the rates were measured (Table – 1). At various concentrations of TBABC, the k₁ values remains constant. This confirms the first order dependence on TBABC.

Effect of varying benzhydrol concentration in the presence of OA

The concentration of  BH is varied in the range of 1.2 x 10^-2 to 3.2 x 10^-2 mol dm^-1 at 303 K and keeping all other reactant concentrations as constant and the rates were measured (Table – 1). The k₁value linearly increases on increasing the concentration of benzhydrol. The plot of log k₁ versus log [BH] gave unit slope for BH (Fig. 1).  Under pseudo-first-order conditions, the plot of k₁versus [BH] is linear passing through origin. These resultsconfirm the first-order nature of the reaction with respect to [BH] in the presence of OA.

Effect of varying  perchloric acid  concentration in the presence of OA

Perchloric acid  has been used as a source of hydrogen ion in reaction medium. The concentration of H⁺ was varied in the range 0.12 to 0.44 mol dm^-1 keeping all other reactant concentration as constant at 303 K and the rates were measured (Table – 1). The k₁value linearly increases on increasing the concentration of H⁺. The plot of log k₁versuslog [H⁺] is a straight line with unit slope (Fig. 2).  Therefore, order with respect to H⁺ is one for BH in the presence of OA. TBABC may become protonated in the presence of acid and the protonated TBABC may function as an effective oxidant.

Figure 1: Showing order plot of  benzhydrol for the oxidation of benzhydrol by TBABC in the presence of oxalic acid  Click here to View figure

Figure 2: Showing order plot of perchloric acid for the oxidation of benzhydrol by TBABC in the presence of oxalic acid Click here to View figure

Effect of acrylonitrile and MnSO₄

Oxidation of benzhydrol, under nitrogen atmosphere,failed to induce polymerization of acrylonitrile.Further, addition of acrylonitrile had no effecton the rate(Table – 1). However, the addition of Mn(II) (0.003 mol dm^-3),in the form of MnSO₄ retards the rate of oxidation. Hence, the presence of Cr(IV) intermediate in the oxidationof benzhydrol by Cr(VI) is confirmed²⁰

Effect of Acidity

The reaction is catalyzed by hydrogen ions (Table 1). The acid-catalysis may well be attributed to a protonation of TBABC to give a stronger oxidant and electrophile.

O₂CrBrO^–N⁺(C₄H₉)₄+H⁺ ⇌ (OH)CrBrO^–N⁺(C₄H₉)₄(3)

The formation of a protonated Cr (VI) species has earlier been postulated in the reactions of structurally similar PCC²¹ and PFC²².

Effect of solvent polarity on reaction rate in the presence of OA

The oxidation of BH has been studied in the binary mixture of acetic acid and water as the solvent medium in the presence of OA. The concentration of acetic acid was varied from 30% to 70% and the rate were measured. The reaction rate increased remarkably with the increase in the proportion of acetic acid in the solvent medium(Table – 2).  Positive slope of log k₁ versus 1/D plot indicates that the reaction involves a cation–dipole type of interaction in the rate determining step²³ (Fig. 3).

Determination of activation parametersin the presence of OA

Rates of oxidation of benzhydrol were determinedat different temperatures between 298 and 313 K at various percentage of acetic acid-water medium in the presence of oxalic acid.  Various activation parameters were calculated and the values were presented in Table -3. The entropy of activation is negative for benzhydrol in the presence of oxalic acid.

Table 2: Pseudo-firstorder rate constants for the oxidation of benzhydrol by TBABCat various percentage of acetic acid-water medium in the presence of Oxalic acid at various temperatures Click here to View table

Table 3: Second order rate constants and activation parameters for the oxidation of benzhydrol by TBABC at various percentage of acetic acid-water medium in the presence of Oxalic acid Click here to View table

Figure 3: Plot of 1/D against log k1 showing effect of solvent polarity at various temperatures in the presence of oxalic acid  Click here to View figure

 

Mechanism of oxidationin the presence of OA

The findings with oxalic acid can be explained by considering the reaction mechanismoutlined in Scheme – 1. Oxalic acid readily form complexes(C₁) with Cr(VI) which are active oxidants²⁴.  The (C₁) complex then reacts with benzhydrol to form (C₂). The complex (C₂) is ternary complex and it undergoes redox decomposition by two electron transfer. The rate determining step involves a cyclic transition state.  In the rate determining step there is a simultaneous rupture ofC–C and C–H bonds to give a benzophenone and the Cr(IV)-OA complex.

Scheme 1: Mechanism of oxidation of benzhydrol by TBABC in the presence of oxalic acid Click here to View Scheme

 

Conclusions

The kinetics of oxidation of benzhydrol has been investigated in the presence of oxalic acid by spectrophotometric method. The values of k₁ were calculated in the presence of 6.0 x 10^-3mol dm^-3of oxalic acid.  The oxidation of benzhydrol by TBABC is first order each with respect to the benzhydrol, TBABC and hydrogen ion. The lowering of dielectric constant of reaction medium increases the reaction rate significantly. The reaction does not show the polymerization, which indicates the absence of free radical intermediate in the oxidation.

References

1. Vibhute,   A. Y.;  Patwari,  S. B.;   Khansole, S. V.;   Vibhute, Y. B.Chin.  Chem.  Lett.2009, 20, 256 – 260.
   CrossRef
2. Hajipour, A. R.; Khazdooz, L.; Ruoho, A. E.J. Iranian Chem. Soc. 2005, 2, 315-318.
   CrossRef
3. Mamaghani, M.;   Shirini, F.;  Parsa, F.Russ. J. Org. Chem.2002, 38,1113-1115.
   CrossRef
4. Yogananth, A.;  Mansoor, S.S. Oriental  J.  Chem. 2015, 31, 17-23.
5. Malik, V. S.;  Vannamuthu, I.; Shafi, S. S.;  Mansoor, S. S. Oriental J. Chem.2015, 31, 77-83.
   CrossRef
6. Elango, K. P.; Jayanthi, G.; Vijayakumar, G.J. Serb. Chem. Soc.2002, 67, 803 -808.
   CrossRef
7. Mansoor, S.S.; Asghar, B.H. J. Indian. Chem. Soc. 2013, 90, 1395 – 1401.
8. Ozgün,  B.; Yaylaoglu, A.; Sendil, K. Monatsh. Chem. 2007, 138, 161 – 163.
   CrossRef
9. Mansoor, S.S.; Shafi, S.S.Z. Phys. Chem., 2011, 225, 249 – 265.
   CrossRef
10. Ghammamy, G.; Mehrani K.; Afrand, H.; Hajighahramani, M. Afr.  J.  Pure Appl. Chem.2007, 1,8-10.
11. Grover, A.; Varshney, S.; Banerji, K.K.  Indian J. Chem.  1996, 35B, 171 – 178.
12. Venkataraman, K. S.; Sundaram, S.;  Venkatasubramanian, N. Indian  J. Chem. 1978, 16B, 84 – 88.
13. Rangappa, K.S.; Ramachandran,  H.;  Mahadevappa, D.S. J. Phys.  Org.  Chem.  1997, 10, 159 – 166.
    CrossRef
14. Venkatasubramanian, N.; Thiagarajan, V. Can.  J.  Chem.1969, 47, 694 – 697.
    CrossRef
15. Hiran, B.L.; Malkani, R. K.;  Rathore, N.  Kinet. Catal. 2005, 46, 360 – 365.
    CrossRef
16. Hiran, B. L.; Malkani, R. K.; Nalwaya, N.; Chand, K.; Rathore, N. Oxid. Commun.2004, 27, 98 – 102.
17. Jagdeesh, B.; Archana, C.; Balaji, M.; Fulchand, C.; Mazahar, F.; Milind, U. J. Indian Chem.  Soc. 2009, 86, 481 – 484.
18. Mansoor, S. S.; Shafi, S. S.; Ahmed, S. Z.  Arab. J. Chem.2013, http://dx.doi.org/10.1016/j.arabjc.2013.02.005.
    CrossRef
19. Mansoor, S. S.; Shafi, S. S. Reac. Kinet. Mech. Cat. 2010, 100, 21 – 31.
20. Karunakaran, C.;  Suresh, S. J. Phys. Org. Chem.2004, 17, 88-93.
    CrossRef
21. Sharma, V.;  Sharma, P. K.; Banerji, K. K. J. Indian Chem. Soc.1997, 74, 607 – 611.
22. Sharma, V.; Sharma, P.K.; Banerji, K. K.J. Chem. Research (S).1996, 290 – 291.
23. Amis, E. S. Solvent Effects on Reaction Rates and Mechanisms. Academic Press, New York,1967, 42.
24. Vanangamudi,  G.;  Srinivasan, S.  E – J. Chem., 2009, 6(3),920 – 927.

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