2.1 Materials and methods

Analytical reagent grade chemicals of high purity were used. Doubly distilled AnalaR grade DMF (Thomas Baker Ltd.) was used as a source of solvent.

2.2 Preparation of the oxidant (pyridinium fluorochromate)

PFC was prepared in the lab by the method described by Bhattacharjee et al. (1982), and its purity was checked by iodometric method, m.p. determination and IR spectral analysis [υ_max(KBr) = 3033, 2366, 1616, 1466, 1299, 949, 733 cm⁻¹]. The m.p. of PFC was obtained at 413 K which was confirmed with the literature value (413–415 K). The stock solution of PFC was prepared in DMF and it was stable. λ_max was observed at 354 nm.

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3,4,5-Trimethoxybenzaldehyde (white crystalline powder with light yellow color, A.R. grade, S.d. Fine-chem. Ltd.) was a commercial product used as such. It is slightly soluble in water, soluble in DMF, ethanol, acetic acid, ethyl ether chloroform, etc. Its purity was checked by m.p. and by TLC (4:1 ether–hexane) which revealed only one spot and R_f value comes out to be 0.57 (Slotta and Heller, 1930). Stock solution of TMBA was prepared by direct dissolving of a known volume in DMF medium. It was stored in refrigerator and used up to 1 week.

2.3 Standardization of 3,4,5-trimethoxybenzaldehyde

The solution of TMBA was standardized by the following method. In a 250 mL conical flask, 2 mg of 3,4,5-trimethoxybenzaldehyde was taken and dissolved in DMF medium. To this, 50 mL of 0.1 N sodium bisulfate solution prepared in DMF was added and was shaken vigorously. It was allowed to stand for half an hour. The excess bisulfate was titrated against 0.1 N iodine solution using starch as an indicator. A blank titration taking 50 mL of sodium bisulfate solution was also performed in a similar method and the quantity of TMBA was estimated by the difference in the volumes of iodine (I₂) used (U.S. Department of Health Education and Welfare, 1973): $$50{\text{mL of NaHSO}}_3=V{\text{mL of I}}_2$$ $${\text{Excess of NaHSO}}_3=V_1{\text{mL of I}}_2$$ $$V-V_1{\text{mL of I}}_2=[\text{aldehyde}]$$ $$\begin{array}{cc}\hfill & >\text{C⚌O}+{\text{NaHSO}}_3\to >\text{C}(\text{OH}){\text{SO}}_3\text{Na}\hfill \\ \hfill & \text{Sodium salt of alpha-hydroxy sulfonic acid}\hfill \end{array}$$ Acetic acid (S.d. Fine-chem. Ltd., AnalaR grade) used for the solvent variation was purified by refluxing with chromium trioxide followed by distillation over KMnO₄. Middle fraction boiling at 390–391 K (literature value 391.5 K) was used (Dippy et al., 1959). Due to the non-aqueous nature of the solvent DMF, TsOH (S.d. Fine-chem. Ltd., AnalaR grade) was used as a source of hydrogen ion. Its purity was checked by m.p. (379 K) (Merck Index) and the stock solution of TsOH was made by direct weighing of a known amount and dissolving it in DMF. Sodium perchlorate (B.D.H., AnalaR grade) was used for determining the ionic strength. Its purity was checked by the m.p. determination. Solution of PFC in DMF obeys Beer–Lambert’s Law in the concentration range of kinetic studies.

2.4 Kinetic measurements

The progress of the reaction was followed by measuring the absorbance of PFC at λ_max 354 nm in one cm thick cell placed in the thermostated compartment of Jasco model 7800 UV/VIS spectrophotometer (Shimadzu). No other reactant or product has any significant absorption at this wavelength. Pseudo-first order conditions were attained by keeping a large excess (×10 or greater) of the substrate over the oxidant. The reactions were carried out at a constant temperature (±0.1 K). The solvent was DMF and the reactions were followed by monitoring the decrease in the concentration of PFC up to 80% completion of the reaction. The pseudo-first-order rate constant, (k_obs) was evaluated from the linear (R² > 0.994) plot of log[PFC] against time. Duplicate kinetic runs showed that the rate constants were reproducible to within ±3%. The specific rate constant, k_s, was evaluated from the relation: k_s = k₁/[TMBA][H⁺]. The reactions were also carried out in DMF–acetic acid mixtures to study the effect of dielectric constant on the rate of the reactions. All reactions were performed under N₂ atmosphere.

2.5 Stoichiometry

In this oxidation PFC undergoes a two electron change. Oxidation state of chromium was proved by magnetic susceptibility, ESR and IR studies (Bhattacharjee et al., 1997). To determine the stoichiometry, PFC (0·001 mol) and TMBA (0·0001 mol) were made up to 100 mL DMF in the presence of 0.6 mol dm⁻³ TsOH at 303 K, in different experiments. The reaction mixture was allowed to stand for 24 h to ensure the completion of the reaction. The residual PFC was determined spectrophotometrically. Several determinations with different concentrations of PFC and TMBA showed that the reaction exhibited 1:1 stoichiometry, i.e., 1 mol of PFC was consumed with 1 mol of TMBA.

2.6 Product analysis

The oxidation of 3,4,5-trimethoxybenzaldehyde leads to the formation of 3,4,5-trimethoxybenzoic acid. The qualitative and quantitative product analyses were carried out under kinetic conditions, i.e., with an excess of the reductant. In a typical experiment, TMBA (0.1 mol) and PFC (0.01 mol) were made up to 50 mL with DMF and the mixture was kept in the dark for 24 h to ensure completion of the reaction in the presence of 0.6 mol dm⁻³ TsOH. After the completion of the reaction under kinetic conditions, the reaction mixture was treated with solid NaHCO₃ and then filtered. After complete neutralization, the reaction mixture was extracted with ether to remove unreacted TMBA. Non-aqueous layer was treated with conc. HCl drop by drop till bicarbonate was neutralized. Again add ether and shake. The product in ether was washed with cold water, dried and weighed (yield 80–85%) and identified as 3,4,5-trimethoxybenzoic acid by melting point analysis (442 K and this agreed with the literature value 441–445 K), TLC (4:1 benzene–absolute alcohol) which revealed only one spot. It was further confirmed by chemical test and FT-IR spectral analysis using KBr disc (>C⚌O stretching, 1686 cm⁻¹; O–H stretching, 3400 cm⁻¹; Aromatic C–H stretching, 3022.5 cm⁻¹; Aromatic C⚌C stretching, 1620.2, 1519.1 and 1320.0 cm⁻¹; asymmetric and symmetric C–H vibrations of CH₃ group are 2950 and 2840 cm⁻¹) (Fig. 1).

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Figure 1

Spectra of 3,4,5-trimethoxybenzoic acid taken by Perkin–Elmer FTIR 1725X spectrometer.

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3.1 Stability of the oxidant (pyridinium fluorochromate)

The solution of PFC in DMF solvent obeys Beer–Lambert’s Law at λ 354 nm. There was no change in optical density and spectra of PFC solution, without substrate in DMF on long standing or heating up to 333–343 K.

3.2 Effect of varying oxidant concentration

The concentration of PFC was varied in the range 0.25 × 10⁻³–2.50 × 10⁻³ mol dm⁻³ at constant [TMBA], [TsOH] at 303 K and the rates were measured. (Table 1) The near constancy in the value of k_obs (pseudo-first-order rate constant) irrespective of the concentration confirms the first order dependence on PFC.

3.3 Effect of varying substrate concentration

The concentration of the substrate TMBA was varied in the range 0.50 × 10⁻²–3.00 × 10⁻² mol dm⁻³ at 303 K and keeping all other reactant concentrations as constant and the rates were measured (Table 1). The rate of oxidation increased progressively on increasing the concentration of TMBA. The plot of log k₁ versus log[TMBA] gave the slope of 1.112 (R² = 0.99) indicating first order dependence on TMBA. Under pseudo-first-order conditions, the plot of 1/k₁ versus 1/[S] (where S is the substrate concentration) was linear with a negligible intercept indicating that the intermediate formed in a slow step got consumed in a subsequent fast step.

3.4 Effect of toluene para-sulfonic acid

TsOH has been used as a source of H⁺ in reaction medium. The concentration of TsOH was varied in the range 0.20–1.60 mol dm⁻³ keeping all other reactant concentration as constant at 303 K and the rates were measured (Table 1). The acid catalyzed nature of this oxidation is confirmed by an increase in the rate on the addition of TsOH. The plot of k₁ versus log[H⁺] is a straight line (R² = 0.993) with a slope of 1.066. Therefore, order with respect to H⁺ is one. PFC may become protonated in the presence of acid. The protonated PFC may function as an effective oxidant (Bhattacharjee et al., 1984) similar to that of chromium trioxide oxidation (Wiberg, 1965). Protonated PFC is likely to be a better electrophile and a better oxidant compared to a neutral one. The effects of the dielectric constant of the medium and the ionic strength indicate the reaction to be of ion–dipole type. Thus in the reaction mechanism the protonated PFC species behaves as a dipole.

3.5 Effect of temperature

The reactions were studied in the temperature range from 298 to 328 K to calculate the rate constants and various thermodynamic parameters (Tables 1 and 2). A plot of log k₁ versus 1/T (inverse of absolute temperature) is a straight line with negative slope. This shows that Arrhenius Equation is valid for this oxidation. The large negative value of entropy of activation ΔS^# suggests slow bimolecular reaction in the rate determining step and that the rate-determining transition state is less disorderly than the reactants, i.e., a rigid transition state by an associative process (Glasstone et al., 1941). This transition state complex is unstable and the chromate ester bond breaks to form the product. According to Pearson (Basolo and Pearson, 1973) also the large negative entropy value obtained in the present study, suggests that the solvent molecules are strongly oriented or ‘frozen’ around the ions thereby resulting in the loss of entropy and accounts for the lowering of rate coefficient values with increase in the polarity of the medium.

3.6 Effect of ionic strength

The effect of ionic strength was studied to observe the effect of salt on the rate of oxidation in Debye–Huckel limit by varying concentration of NaClO₄ from 0.001 to 0.01 mol dm⁻³ provided other conditions being constant (Table 3). The rate of reaction remains almost unchanged while increasing the NaClO₄ concentration. It proves that interaction in rate determining step is not of ion–ion type (Laidler, 2005).

3.7 Effect of solvent composition

The influence of solvent polarity has been studied in acetic acid–DMF mixture. The acetic acid % (v/v) has been varied from 0% to 90% at fixed ionic strength, [TMBA], [PFC], [H⁺] and temperature (Table 4). The rate of oxidation increases with decrease in polarity of solvent. In other words, a decrease in rate with increase in dielectric constant is observed, suggesting that a medium of low dielectric constant favors the oxidation process. This is due to polar character of the transition state as compared to that of reactants. The plot of log k₁ versus 1/D (dielectric constant) is linear with positive slope suggesting the presence of either dipole–dipole or ion–dipole type of interaction between the oxidant and the substrate (Scatchard, 1932, 1939). Plot of log k₁ versus (D − 1)/(2D + 1) is a curvature indicating the absence of dipole–dipole interaction in the rate determining step. Positive slope of log k₁ versus 1/D plot indicates that the reaction involves a cation–dipole type of interaction in the rate determining step. 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. In this case there is a possibility of a positive ionic reactant, as the rate decreases with the increasing dielectric constant of the medium. Due to the polar nature of the solvent, transition state is stabilized, i.e., the polar solvent molecules surround the transition state and result in less disproportion.

3.8 Effect of added MnSO₄ concentration

In oxidation process to find out effect and involvement of various oxidation states such as [Cr(V)] and [Cr(IV)], we have studied the effect of [MnSO₄] on the rate of oxidation by varying in the range from 0.001 to 0.005 M (Table 5). It has been observed that addition of low concentration of Mn(II) ion can be explained by the disproportion of intermediate valence states of Cr(IV) and Cr(V). The addition of Mn(II) fixes the Cr(IV) species as Cr(III) and thus the total concentration of Cr(IV) available in a given time is lowered by the addition of Mn(II) and hence observed deceleration with increasing addition of Mn(II) ion. This may be taken as evidence for the formation of Cr(IV) species and hence PFC acts as a two-electron transfer oxidant (Chaudhary, 2002).

3.9 Induced polymerization of acrylonitrile

The reaction mixture in kinetic conditions to which a known amount of acrylonitrile (free from stabilizer) scavenger had been added initially, was kept for 24 h in an inert atmosphere of N₂ fails to induce the polymerization of acrylonitrile. Then on dilution of the reaction mixture with methanol, no turbidity occurred, indicating the absence of free radical intervention.

4.1 Rate law

$$\text{Rate}=-\mathrm{d}[\text{PFC}]/\mathrm{d}t=k[C]$$