Kinetic and Thermodynamic Study for the Oxidation of 4-Oxo-4-phenyl Butanoic Acid by Tripropylammonium fluorochromate in Aqueous Acetic Acid Medium

Tripropylammonium fluorochromate (TriPAFC) in acetic acid-water medium oxidizes 4oxo-4-phenyl butanoic acid (4-Oxo acid) in the presence of perchloric acid. The reaction is first order each in [TriPAFC], [4-Oxo acid] and [H+]. From the observed kinetic results a suitable mechanism has been proposed.


INTRODUCTION
The oxidation of organic substrates is an important aspect in modern organic synthesis, therefore, the search for new oxidizing agents is of interest to synthetic organic chemists.In recent years, there is continued interest in the development of new Cr(VI) reagents for the effective and selective oxidation of organic substrates, under mild conditions, because Cr(VI) is established as a versatile oxidant for the oxidation of a variety of organic compounds 1 .Many Cr(VI) reagents like benzimidazolium fluorochromate 2 , tetrahexylammonium fluorochromate 3 , tributylammonium chlorochromate 4 , quinolinium chlorochromate 5 , quinolinium fluorochromate 6 and tetraethylammonium chlorochromate 7 have been studied in recent years for the oxidation of various organic substrates.
Tripropylammonium fluorochromate is also one such oxidant developed recently [8][9][10][11] .It is a more efficient and stronger oxidizing agent.This new compound is more efficient for quantitative oxidation of several organic substrates and has certain advantages over similar oxidizing agents in terms of the amount of oxidant and solvent required, short reaction times and high yields.
Extensive studies on the mechanism of oxidation of 4-oxo acids by different oxidants have been the subject of study by various workers [12][13][14][15][16][17] .However, no detailed kinetic study of oxidation of 4oxo acids by TriPAFC have so far been attempted.We have been interested in the kinetic and mechanistic aspects of the oxidation by complexed Cr(VI) species, and several studies on halochromates have already been reported [18][19][20][21] .In continuation of our earlier work, we now report the kinetics and mechanism of oxidation of 4-oxo-4phenyl butanoic acid by TriPAFC in aqueous acetic acid medium.

Materials
The 4-oxo-4-phenyl butanic acid is synthesized by the Friedel-Craft's reaction between succinic anhydride and benzene in the presence of anhydrous aluminium chloride 14 .Tripropylammonium bromide and chromium trioxide were obtained from Fluka (Buchs, Switzerland).Acetic acid was purified by standard method and the fraction distilling at 118 o C was collected.Perchloric acid is used as the source of hydrogen ion.

Kinetic measurements
The pseudo -first-order conditions were attained by maintaining a large excess ( x 15 or more) of 4-Oxo acid over TriPAFC.The solvent was 50% acetic acid -50% water (v/v), unless specified otherwise.The reactions were followed, at constant temperatures (± 0.01 K), by monitoring the decrease in [TriPAFC] spectrophotometrically at 364 nm using UV-Vis spectrophotometer, Shimadzu UV-1800 model.The pseudo-first-order rate constant k obs , was evaluated from the linear (r = 0.990 to 0.999) plots of log [TriPAFC] against time for up to 80% reaction.The second order rate constant k 2 , was obtained from the relation k 2 = k obs / [4-Oxo acid].

Product analysis
Product study was made under mineral acid catalysed condition in 4-oxo-4-phenyl butanic acid.Keeping the concentration of TriPAFC in excess over 4-oxo-4-phenyl butanic acid, the two solutions were mixed together in 50% acetic acid -50% water (v/v).The reaction mixture was allowed to go to completion by keeping it in a thermostat at 303 K for 24 h to ensure the completion of the reaction.The product was obtained which had melting point of 121 o C. It was dissolved in benzene and a careful TLC analysis was done with benzoic acid and 4oxo-4-phenyl butanoic acid as references.Only one spot corresponding to benzoic acid was obtained.

Stoichiometric studies
The stoichiometric studies for the oxidation of 4-oxo acid by TriPAFC were carried out with oxidant in excess.The solvent composition 50% acetic acid -50% water (v/v) and [H + ] were maintained as in the corresponding rate measurements.The temperature was maintained at 303 K.The 4-oxo acid and TriPAFC were mixed in the ratio 1:4, 1:5, 1:6 and were allowed to react for 24 h at 303 K.The concentration of unreacted TriPAFC was determined.ˆ[TriPAFC] was calculated.The stoichiometry was calculated from the ratio between [4-Oxo acid] and [TriPAFC].The estimation of unreacted TriPAFC showed that 1 mol of TriPAFC reacts with 1 mol of 4-oxo-4-phenyl butanic acid.

RESULTS AND DISCUSSION
Tripropylammonium fluorochromate is easily prepared as reported in the literature 9 .The oxidation 4-oxo-4-phenyl butanic acid by TriPAFC has been conducted in 50% acetic acid and 50% water medium at 303 K, under pseudo first order conditions and the result obtained were discussed in the following paragraphs.

Order of the reaction
The reactions are first order with respect to TriPAFC as evidenced by a linear plot of log [TriPAFC] against time.The pseudo first-order rate constants do not depend on the initial concentration of TriPAFC (Table 1).This further confirms first-order dependence of rate on oxidant concentration.The concentration of 4-oxo acid is varied in the range of 1.2 x 10 -2 to 2.8 x 10 -2 mol dm -3 at 303 K and keeping all other reactant concentrations as constant and the rates were measured (Table 1).The data establish that the rate increases with increase in [4-Oxo acid] in a first-order fashion.Further, the plot of k 1 versus [4-Oxo acid] is excellently linear passing through origin (Figure 1).Also, the k 2 values remain  constant when [4-Oxo acid] is varied (Table 1).This result, coupled with the slope value of 0.98 for the double logarithmic plot between k obs and [4-Oxo acid] (Figure 2) confirms the first-order nature of the reaction with respect to [4-oxo acid].The acid catalysed nature of this oxidation is confirmed by an increase in the rate on the addition of H + .The plot of log k 1 versus log [H + ] is a straight line with the slope of 0.996 (Fig. 3).Therefore, order with respect to [H + ] is one.TriPAFC may become

Effect of Acidity
The reaction is catalyzed by hydrogen ions (Table -1).The acid-catalysis may well be attributed to a protonation of TriPAFC to give a stronger oxidant and electrophile.
The formation of a protonated Cr(VI) species has earlier been postulated in the reactions of structurally similar PFC 22 .

Effect of acrylonitrile and MnSO 4
The reaction did not promote polymerization of acrylonitrile indicating the absence of free radicals (Table -1).However, the addition of Mn(II) (0.003 mol dm -3), in the form of MnSO 4 retards the rate of oxidation.This indicates the involvement of Cr(IV) intermediate in the oxidation of 4-oxo acid by Cr(VI) reagent and confirms the two electron transfer process in the reaction.Mn(II) ion reduces Cr(IV) formed to Cr(III).In the absence of Mn(II) ion, formed Cr(IV) reduces Cr(VI) to Cr(V) and the oxidation of 4-oxo acid by Cr(V) is fast 23 .The decrease in the rate of Cr(VI) reduction on the addition of Mn(II) has been attributed to the removal of Cr(IV) by reaction 24 with Mn(II).

Effect of solvent polarity on reaction rate
The oxidation of 4-oxo-4-phenyl butanoic acid has been studied in the binary mixture of acetic acid and water as the solvent medium.For the oxidation of 4-oxo-4-phenyl butanoic acid, the reaction rate increased remarkably with the increase in the proportion of acetic acid in the solvent medium.These results are presented in Table 2.The plots of log k 1 against the inverse of the dielectric constant are linear with positive slopes, indicating an interaction between a positive ion and dipolar molecule (Fig 4).Amis (1967) holds the view that in an ion-dipole reaction involving a positive ionic reactant, the rate would decrease with increasing dielectric constant of the medium and if the reactant were to be a negatively charged ion, the rate would increase with the increasing dielectric constant 25 .
In the present study, the significant rate enhancement with the lowering of dielectric constant of the solvent medium may be attributed to the enolisation of the ketoacid group in the 4oxo-4-phenyl butanoic acid.The enolisation of the keto group of the oxoacid is facilitated by the increase in percentage of acetic acid in the solvent medium and this may also favors the rate enhancement.

Rate of enolisation by bromination method
It has been reported earlier in the case of oxidation of keto compounds that the oxidation proceeds via enolisation of the keto compounds.The rate of enolisation of keto compound is faster than the rate of oxidation.The reactive species of the substrate may be determined by enolisation, which is an acid as well as base catalysed reaction and proceeds by a concerted or push-pull mechanism.The rate of enolisation was determined by bromination method for the system under investigation.
The order of bromination reaction with respect to the 4-oxo-4-phenyl butanoic acid, bromine and H + has been determined.These data indicate that the bromination of the 4-oxo-4-phenyl butanoic acid is first order each with respect to the substrate and H + ion but zero order with respect to bromine.

Thermodynamic parameters
The kinetics of oxidation of 4-oxo acid is studied at four different temperatures viz., 298, 303, 308 and 313 K at various percentage of acetic acidwater medium.The second order rate constants were calculated (Table 3).The Arrhenius plot of log k 2 versus 1/T is found to be linear.The enthalpy of activation, entropy of activation and free energy of activation were calculated from k 2 at 298, 303, 308 and 313 K using the Eyring relationship by the method of least square and presented in Table 3.The least square method gives the values and standard errors of enthalpy and entropy of activation respectively.Statistical analysis of the Eyring equation clearly confirms that the standard errors of H # and S # correlate 26 .The entropy of activation is negative for 4-oxo acid.The negative entropy of activation in conjunction with other experimental data supports the mechanism of oxidation.

Mechanism of oxidation
A probable mechanism for the oxidation

CONCLUSIONS
The kinetics of oxidation of 4-Oxo acid by TriPAFC was studied in 50% acetic acid -50% water (v/v) at 303 K in the presence of perchloric acid.The stoichiometry study showed that 1 mol of TriPAFC reacts with 1 mol of 4-oxo-4-phenyl butanic acid.The oxidation of 4-oxo acid, in an atmosphere of nitrogen, failed to induce the polymerization of acrylonitrile.The reaction is first order each in [TriPAFC], [4-oxo acid] and [H + ].The rate of enolisation was determined by bromination method for the system under investigation.These data indicate that the bromination of the 4-oxo-4phenyl butanoic acid is first order each with respect to the substrate and H + ion but zero order with respect to bromine.Considering these facts and findings a suitable mechanism has been proposed for the oxidation.

Fig. 4 :Fig. 3 :
Fig. 4: Plot of 1 / D against log k 1 showing effect of solvent polarity at various temperatures