Kinetics of oxidation of 3- and 5-formylsalicylic acids with potassium permanganate in alkaline medium.
Potassium permanganate (KMn[O.sub.4]) has long been used as an oxidant for a variety of organic functional groups, either in aqueous or nonaqueous media [1,2]. However, little attention has been paid to the kinetic of oxidation of aldehyds especially aromatic ones. This can be attributed to the power of permanganate as an oxidant which often leads to a complete degradation of the oxidizable species . That is why catalysts or rabid distillation of the oxidation products are often applied during oxidation with permanganate . In this work two aromatic aldehyds, 3-fsa and 5-fsa, are used as a representation of aromatic aldehydes. The choice of the first is due to its importance as a ligand and ligand-forming compound while the other ,5-fsa, is selected to investigate the effect of the electronic environment on the oxidation process. The oxidation has been found to involve the formyl group and thus in this work the kinetics of oxidation of 3-fsa and 5-fsa to 2-hydroxy-1,3-benzenedicarboxilic acid and 2-hydroxy-1,5-benzenedicarboxilic acid respectively have been studied. This paper provides a method for preparation of the dicarboxylic acids from the corresponding aldehydocarboxilic ones with a good yield and high purity.
A little attention has also been focused to the spectrophotometric detection of hypomanganate(V) and manganate(VI) short-lived transient species during oxidation with permanganate ion. This is because of their extremely short lifetime owing to fast disproportionation [5-9]. However, Shaker [10,11] and El-Khatib  pointed out the rapid formation of Mn(V) as a short-lived intermediate complex during kinetic studies on the oxidation of some polysaccharides. It was reported that the 606 nm absorption band corresponds to manganate (VI) [13,14] and it was reported [5,14-16] that hypomanganate(V) has a band at a wavelength around 700 nm. In the present work it was found that these short life-timed intermediates are detectable spectrophotometrically. A comparison of activation energies and order of reaction with respect to the two acids showed that, oxidation of 5-fsa is easier than that of 3-fsa. This result was found to agree with the theoretical quantum chemical calculations based on the B3LYP/6-31G(d) level of theory [17,18] which has been performed using Gaussian 98 .
Chemicals and reagents
3-fsa and 5-fsa have been prepared according to literature . Potassium permanganate solutions of the required concentrations were prepared by dissolving KMn[O.sub.4] (reagent grade, Fisher Scientific, Pittsburgh, PA) in deionized (DI) water. All other chemicals are reagent grade. Sodium perchlorate solution was used to maintain constant ionic strength of [micro] = 0.1 M. The carbonate-free NaOH solution was prepared and standardized by using potassium acid phthalate (KHP). It is worthy to note that due to the insolubility of the acids in water, it was converted to the mono sodium salt by adding sodium hydroxide.
Instruments and spectrophotometric measurements
The spectral changes during the oxidation of the aldehydic acids by potassium permanganate in alkaline medium (pH [approximately equal to] 12) was monitored in a thermostated cell compartment within [+ or -] 0.05[degrees]C using Shimadzu UV-2101/3101 PC. All the pH measurements were made using a pH meter (CONCORT-C 925).
Stoichiometry and spectroscopic studies
Different ratios of the reductant (in excess) to oxidant were mixed at pH [approximately equal to] 12 and a constant ionic strength of 0.1 M, and then equilibrated for 48 h at room temperature. Estimation of unreacted Mn[O.sub.4.sup.-] was made by filtration of the precipitated Mn[O.sub.2] and then the absorbance of the filtrate at 525 nm was measured. All the UV-vis measurements have been carried out by mixing the reactants in presence of excess of the reductant, at least five folds, and in presence of sodium perchlorate to maintain constant ionic strength, 0.1 M.
Results and discussion
Structural studies of 3-fsa and 5-fsa
The B3LYP/6-31G(d) level of theory[17,18] has been applied to calculate the ionization potential and the atomic charge densities of the optimum structures of the uni- and bi-charged species of 3-fsa and 5-fsa, Table1. The values of the ionization potential for the uni-charged 3-fsa and 5-fsa, [A.sub.1] and [B.sub.1] respectively, were found to be more than those for the bi-charged species, [A.sub.2] and [B.sub.2] respectively. This indicates that the oxidation must be easier when the two acids are bi-charged (higher pH values).
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The theoretical calculations also showed that the ionization potential of the unicharged 5-fsa (+0.06698 au) is lower than that of the uni-charged 3-fsa (+0.06986 au) and accordingly electron loss (oxidation) will be easier for the first. Thus according to the theoretical comparison, the reaction between 5-fsa and KMn[O.sub.4] is expected to be of lower activation energy than that for 3-fsa.
Stoichiometry and identification of the oxidation products
The Stoichiometry [Mn[O.sub.4.sup.-]] consumed/[[3-fsa or 5-fsa].sub.0], was found to be 8.0 [+ or -] 0.1, indicating a probable complete degradation of the acids. Thus to avoid the multi-step reactions, the reaction was studied under pseudo order condition by adding large excess of acids, at least five folds that of KMn[O.sub.4]. Under this condition, oxidation is expected to involve the formyl group. That was confirmed through mixing of the reactants in a 2:3 molar ratio, as suggested by electron-half-ionic equation, the mixture was then equilibrated for 48h at pH = 12 and then the precipitated Mn[O.sub.2] was filtered out. The filtrate was then treated with few drops of concentrated HCl and was then refluxed for 20 min. On cooling, a crystalline precipitate formed which was filtered out, washed with hot water, dried and identified by melting point comparison, IR and mass spectra (fig. 1).
[FIGURE 1 OMITTED]
A pronounced change in the IR spectrum was observed with evidences of the conversion of the formyl group to the carboxylic group. The mass spectrum showed the molecular weight of the corresponding bi-acidic compound. The fragmentation pattern corresponding to the mass spectrum of 3-fsa is given in scheme 3.
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the sharp melting point of the oxidation products and the high resolution of the IR and mass spectra indicates a high purity of the two produced acids and the yield in both cases was 92 [+ or -] 1% thus preparation of the two acids can be performed through this easy simple method. The fact that mixing the reactants in a 2 : 3 molar ratio leads to oxidation of the formyl group confirms that oxidation will involve the formyl group in presence of large excess of the two acids as implemented in this study.
Identification of Mn (V) and Mn (VI)
The oxidation of 3-fsa and 5-fsa with KMn[O.sub.4] in alkaline medium has been confirmed through monitoring the reaction by a spectrophotometer in a repeated scan conditions fig. (2,3). It was found that the bands at 545 nm and 525 nm, suffer a decrease in the absorbance with time indicating a consumption of KMn[O.sub.4]. On the other hand a band with a maximum absorbance at 606 nm begins to appear with increasing of its absorbance by time in higher pH values ([approximately equal to]12). This band has been reported to correspond to the Mn(VI) [13,14], and since KMn[O.sub.4] has a lower absorbance at this wavelength, it is not difficult to identify. Another band of a very low intensity and a very short life time, at the wave length range (700-750) was also pronounced but at lower pH values ([less than or equal to] 11). This band has been reported to correspond the hypomanganate ion Mn (V) [5,14-16]. The stability of the Mn (VI) at higher pH values may be attributed to its complexation with the phenol group in its ionic form.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Reaction order and rate constants
Generally, when reaction mechanisms are unknown, the rate law describing a particular chemical reaction can be deduced from experimentally measured time-concentration data for one or all of the reactants [21,22]. Kinetic rate methods including the differential method, the integral method and the initial rate method are commonly used to abstract kinetic parameters for developing rate laws [21,23]. The integral method that was commonly selected for the reactions of known reaction order with respect to reactants was used to determine the rate parameters. The rate law for the degradation of KMn[O.sub.4] by the acids was assumed to be the form of Eq. (1).
-[r.sub.oxidant] = [k.sub.([alpha] +[beta])] [[oxidant].sup.[alpha]] [[reductant].sup.[beta]] (1)
Where, -[r.sub.oxidant] = -d[KMn[O.sub.4]]/dt is the rate expression for KMn[O.sub.4], [k.sub.([alpha] +[beta])] the overall rate constant, '[alpha]' and '[beta]' the reaction order in KMn[O.sub.4] and 5-fsa or 3-fsa respectively. Under the condition of excess 3-fsa or 5-fsa over KMn[O.sub.4], Eqs. (2) and (3) can be derived from Eq. (1).
-[r.sub.oxidant] = [k.sub.obs] [[oxidant].sup.[alpha]] (2)
[K.sub.obs] = [k.sub.([alpha] +[beta])] [[reductant].sub.0.sup.[beta]] (3)
Where [k.sub.obs] is the pseudo-first order rate constant ([s.sup.-1]) for KMn[O.sub.4], K the overall order rate constant ([M.sup.1-n] [s.sup.-1]), [[reductant].sub.0.sup.[beta]] the initial 5-fsa or 3-fsa concentration to the power [beta] (M). This hypothesis is verified for KMn[O.sub.4], while the reaction was found to be fractional first order in 5-fsa and 3-fsa through the experimental data. The experiments were conducted at the initial ratio of [reductant]0/[oxidant]0 of at least five folds under isothermal conditions. The data of the degradation of KMn[O.sub.4] by the acids over the course of the reactions were fit with a first order decay model. A typical graph showing the degradation of KMn[O.sub.4] by the two acids is presented in Figs. 4 and 5. The results in Fig. 4 , indicate that the reaction is first order with respect to KMn[O.sub.4] when reacting with the acids, as evidenced by the linear correlation relationship (e.g. [R.sup.2] [approximately equal to] 1) among the data points for all runs. The observed pseudo-first order rate constants are shown in Table 2. Based on Eq. (3) it is possible to investigate the reaction order in 3-fsa and 5-fsa. As implied by Eq. (3), under constant temperature and pH the pseudo-first order rate constant, [k.sub.obs], is proportional to [[reductant].sub.0.sup.[beta]] when a relatively constant reductant concentration is maintained during the reaction. The slope of the line in the plot of ln [k.sub.obs] versus ln [[reductant].sub.0] will be close to one if the reaction is first order in reductant. The lines in Fig. 5 shows a slope of 0.7 and 0.45 indicating that it is fractional first order with respect to 3-fsa and 5-fsa respectively when reacting with KMn[O.sub.4]. The rate law for the oxidation of 3-fsa and 5-fsa by KMn[O.sub.4] can thus be written as
[-r.sub.KMnO4] = k [KMn[O.sub.4]][[5-fsa].sup.0.45] (4)
[-r.sub.KMnO4] = k [KMn[O.sub.4]][[3-fsa].sup.0.7] (5)
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
k is calculated by k = [k.sub.obs] / [[5-fsa].sub.0.sup.0.45] and k = [k.sub.obs] / [[3-fsa].sub.0.sup.0.7]
In order to determine the activation parameters of the reaction between KMn[O.sub.4] and 5-fsa or 3-fsa the rate constants at different temperatures (10, 15, 20, 25, 30 and 35 [degrees]C) were determined. It is evident that the reaction rate increases with the increase in temperature (Table 3). The overall rate constants at these temperatures (Table 3) were used to calculate the activation energy using the linearized Arrhenius equation.
In k = lnA - Ea/RT (6)
Where; k is the overall rate constant, A the frequency factor, Ea the activation energy (kcal [mol.sup.-1]), R the universal gas constant, and T the absolute temperature (K). When ln k is plotted versus 1/T as shown in Fig. 6, the slope of the line will give "-Ea/R". The activation energy for the reaction between KMn[O.sub.4] and 5-fsa is 8.2324 [+ or -] 0.15 kcal [mol.sub.-1], which is lower than the activation energy for the reaction between KMn[O.sub.4] and 3-fsa is (9.713 [+ or -] 0.12 kcal [mol.sup.-1]) indicating that the theoretical expectation is in accordance with the experimental results. Furthermore, based on the transition state theory the enthalpy ([DELTA][H.sup.*]), entropy ([DELTA][S.sup.*]) and Gibbs free energy ([DELTA][G.sup.*]) of activation were determined using Eqs. (7)-(9), respectively, assuming that [DELTA][H.sup.*] and [DELTA][S.sup.*] are not functions of temperature in the studied range [24-26].
[DELTA][H.sup.*] = Ea -RT (7)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (8)
[DELTA][G.sup.*] = [DELTA][H.sup.*] = T[DELTA][S.sup.*] (9)
Where, [k.sub.B] is the Boltzmann's constant (1.38 x [10.sup.-23] [JK.sup.-1]), h the Planck constant (6.63 x [10.sup.-34] J s), and T the temperature (K).
[FIGURE 6 OMITTED]
Effect of pH and the reaction mechanism
The reaction was found to be unaffected by the pH in the range 7-11, but as the pH increase more higher the rate was found to increase dramatically that can be due to the deprotonation of the phenolic hydroxyl group at such higher pH values and thus giving the bi-charged speeches which is more reactive as descried in the theoretical study. All the measurements were carried out at pH = 12, lower values were avoided to avoid the possible disproportionation reaction of Mn(V) and Mn(VI) [5,9] and higher pH was avoided to slowdown the reaction rate. The reaction mechanism(scheme 4) is suggested to be initialized by a nucleophilic attack of Mn[O.sub.4.sup.-] to the formyl carbon atom. Though the calculated atomic charge densities of the two acids at the B3LYP/6-31G(d) level showed that the formyl carbon is a little more positively charged in 3-fsa (+0.350413) than in 5-fsa (+0.216991) and thus the attack is expected to be fast in 3-fsa but the steric factor may be the cause of why 5-fsa is still easier to be attacked and rabidly oxidized.
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Esam A. Orabi
Department of Chemistry, Faculty of Science, Assiut University, Assiut 71516, Egypt
Table 1: Calculated parameters of 3/fsa and 5/fsa molecule structures at B3LYP/6-31G(d) level of theory. Calculated parameters [A.sub.1] [A.sub.2] structure structure Energy (au.) -608.87241 -608.17952 Dipole moment (D) +7.0087 +7.1474 Ionization potential (au.) +0.06986 -0.09079 Electron affinity (au.) -0.03579 -0.18103 Calculated parameters [B.sub.1] [B.sub.2] structure structure Energy (au.) -608.86822 -608.18952 Dipole moment (D) +7.4066 +7.3413 Ionization potential (au.) +0.06698 -0.08519 Electron affinity (au.) -0.03942 -0.18641 Table 2: Rate constants for the oxidation of 3-fsa and 5-fsa with KMn[O.sub.4] (0.4 mM); pH [approximately equal to] 12; 20[degrees]C. Reductant [[Reductant].sub.0] [k.sub.obs] k ([M.sup.-n] (x [10.sup.-3] M) (x [10.sup.-3] [S.sup.-1]) [S.sup.-1]) 3-fsa 2.0 1.119 0.0867 2.8 1.408 0.0862 3.6 1.694 0.0870 4.4 1.926 0.0860 6.0 2.471 0.0888 5-fsa 2.8 0.599 0.0084 4.0 0.701 0.0084 5.2 0.795 0.0085 6.4 0.862 0.0084 7.6 0.932 0.0084 8.8 0.988 0.0083 10 1.080 0.0086 k is calculated by k = [k.sub.obs] / [[5-fsa].sub.0.sup.0.45] and k = [k.sub.obs]/[[3-fsa].sub.0.sup.0.7] Table 3: Rate constants for the oxidation of 3-fsa and 5-fsa (4.0mM) with KMn[O.sub.4] (0.4 mM); pH [approximately equal to] 12; at various temperatures. Reductant Temperature [k.sub.obs] k ([M.sup.-n] ([degrees]C) (x [10.sup.-3] [S.sup.-1]) [S.sup.-1]) 3-fsa 10 0.975 0.046500 15 1.401 0.066900 20 1.909 0.091100 25 2.372 0.113200 30 3.316 0.158200 35 3.983 0.190000 5-fsa 10 0.429 0.005144 15 0.545 0.006539 20 0.704 0.008441 25 0.863 0.010358 30 1.114 0.013369 Table 4: The activation parameters of the reaction between 3-fsa and 5-fsa with KMn[O.sub.4] at 20[degrees]C. Ea [DELTA][H.sup.*] Reductant [kcal [mol.sup.-1]) [kcal [mol.sup.-1]) 3-fsa 9.713 9.133 5-fsa 8.2324 7.652 [DELTA][S.sup.*] [DELTA][G.sup.*] Reductant [kcal [mol.sup.-1]) [kcal [mol.sup.-1]) 3-fsa -9.127 11.807 5-fsa -10.618 10.763
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|Author:||Orabi, Esam A.|
|Publication:||International Journal of Applied Chemistry|
|Date:||Jan 1, 2010|
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