A kinetic and mechanistic study on the silver (I)-catalyzed oxidation of l-alanine by cerium (IV) in sulfuric acid medium

2.1 Materials

In the present work, double distilled water was used for preparing the solutions. l-alanine (E. Merck) was used as such. A stock solution of l-alanine was prepared by dissolving it in water. The Ce(IV) stock solution was obtained by dissolving cerium (IV) ammonium sulfate (E. Merck) in 0.98 mol dm⁻³ sulfuric acid and was standardized with iron (II) ammonium sulfate solution (Jeffery and G.H., 1996). Other chemicals and reagents such as sodium sulfate, potassium sulfate, potassium bisulfate, silver nitrate, sulfuric acid, acetonitrile, acetone, hydrated copper sulfate, and aluminum sulfate used were of analytical grade with 99.9% purity.

2.2 Kinetic measurements

Kinetic studies were carried out in a sulfuric acid medium in the temperature range (298–313 K) under pseudo first order conditions with a large excess of l-alanine over Ce(IV). The reaction was followed by estimating the unreacted Ce(IV) as a function of time by titrating against ferrous ammonium sulfate solution employing ferroin as indicator (Walden et al., 1933).

No precautions were taken to exclude the diffused light entering into the reaction mixture (Krishna and B., 1959). The Ce(IV) solution was thermally quite stable (Grant, 1964) in the visible region and undergoes photochemical decomposition (Heidt and Smith, 1948) only in the UV region. Since, the oxidation of (Kolp and Thomas, 1949) water even at 333 K by Ce(IV) was immeasurably slow and insignificant, no further precautions were taken to account for this.

From the titer values, plots of log[Ce(IV)] vs. time were made and from the slope of such plots, the pseudo first order rate constants k′(s⁻¹) (Table 1) were obtained. To evaluate k′, generally 8–10 values at least up to 80% completion of the reaction were used. Average values of at least two independent determinations of k′ were taken for analysis. All the first order plots were linear, with a correlation coefficient of 0.996–0.999. The observed rate constants were reproducible within the experimental error ±5%.

3.1 Effect of[l-alanine]

At a constant[Ce(IV)] (6 × 10⁻³ mol dm⁻³),[H⁺] (0.05 mol dm⁻³)[Na₂SO₄] (0.1 mol dm⁻³) and[Ag⁺] (6 × 10⁻⁶ mol dm⁻³) the kinetic runs were carried out with various initial concentrations of l-alanine, which yielded rate constants whose values depended on[l-alanine]. The pseudo first order rate constants k′(s⁻¹) thus obtained were found to increase with[l-alanine] (Table 1) over a range of[l-alanine] used (2–10 × 10⁻² mol dm⁻³). This shows that the reaction obeys first order with respect to[l-alanine]. This was confirmed by the linear plots of k′(s⁻¹) vs.[l-alanine] which yielded a straight line passing through the origin. Fig. 1. The plot of 1/k′ vs. 1/[l-alanine] exhibits an excellent linearity (Fig. 2) with a positive slope. Observed reaction order n_ap = 1.01 (r = 0.998). The values of k₂ (mol dm⁻³ s⁻¹) were evaluated from the slope of k′(s⁻¹) vs.[l-alanine] plots (Fig. 1) The k₂ (mol dm⁻³ s⁻¹) values thus obtained from such plots (Table 2) were in agreement with the corresponding values calculated from the factor k′(s⁻¹)/[l-alanine].

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Figure 1

The linear plot of k’ (s⁻¹) vs[l-alanine] confirms the first order reaction with respect to[l-alanine].

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Figure 2

Excellent linearity with a positive slope confirms the first order reaction with respect to[l-alanine].

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3.2 Effect of[Ce(IV)]

The kinetics of oxidation of l-alanine has been investigated under pseudo first order conditions, keeping the concentrations of l-alanine always in large excess (nearly 10-folds) over that of[Ce(IV)]. When the log[Ce(IV)]_T was plotted against ‘t’, very good straight line plots were obtained indicating that the reaction was of first order with respect to[Ce(IV)] (Table 1). Effect of varying[Ce(IV)] (2–10 × 10⁻³ mol dm⁻³) on the rate of oxidation has been studied at constant[l-alanine] (6 × 10⁻² mol dm⁻³),[H⁺] (0.05 mol dm⁻³)[Na₂SO₄] (0.1 mol dm⁻³), and[Ag⁺] (6 × 10⁻⁶ mol dm⁻³). The pseudo first order rate constants k′ were found to be independent of[Ce(IV)], confirming the first order dependence of rate on[Ce(IV)].

3.3 Effect of[H⁺]

The reaction rates were measured at constant[l-alanine] (6.0 × 10⁻² mol dm⁻³),[Ce(IV)] (6.0 × 10⁻³ mol dm⁻³),[Na₂SO₄] (0.1 mol dm⁻³), and[Ag⁺] (6 × 10⁻⁶ mol dm⁻³) but with various[H⁺] (3.5–7.5 × 10⁻² mol dm⁻³) (Table 3).[H⁺] was calculated ignoring the dissociation of $[{\text{HSO}}_4^{-}]$ and assuming[H⁺] ≈  $[{\text{HSO}}_4^{-}]$ . Herein, k′ increases with the increase of[H⁺]. Observed reaction order of n_ap. of[H⁺] is 1.3 obtained from the linear regression (r = 0.9928) of log k′ vs. log H⁺ indicating that this reaction is of first order with respect to H⁺. The plot of 1/k’ against 1/[H⁺] at 298 K was found to be linear (r = 0.9928) with a positive slope.

3.4 Effect of[ionic strength]

The reaction rates were measured at constant[l-alanine] (6.0 × 10⁻² mol dm⁻³),[Ce(IV)] (6.0 × 10⁻³ mol dm⁻³)[H⁺] (0.05 mol dm⁻³), and[Ag⁺] (6 × 10⁻⁶ mol dm⁻³) but with various[Na₂SO₄] (0.1–0.8 mol dm⁻³). It was observed that the rate was not influenced by the ionic strength (Table 4).

3.5 Effect of $[{\text{HSO}}_4^{-}\text{and}{\text{SO}}_4^{2-}]$

$[{\text{HSO}}_4^{-}]$ (K₂SO₄) and ${\text{SO}}_4^{2-}$ (KHSO₄) were varied in the range of (0.1–0.8 mol dm⁻³) at fixed[l-alanine] (6.0 × 10⁻² mol dm⁻³),[Ce(IV)] (6.0 × 10⁻³ mol dm⁻³)[H⁺] (0.05 mol dm⁻³), and[Ag⁺] (6 × 10⁻⁶ mol dm⁻³). It was observed that the rate was not influenced by the increasing concentration of ${\text{SO}}_4^{2-}$ and $\text{HSO}{\hspace{0.17em})}_4^{-}$ (Table 4) confirming that the reaction occurs between an ion Ce⁴⁺ and a neutral molecule[l-alanine] (Laidler, 1965).

3.6 [Ag⁺] dependence

The reaction rates were measured at constant[l-alanine] (6.0 × 10⁻² mol dm⁻³),[Ce(IV)] (6.0 × 10⁻³ mol dm⁻³),[Na₂SO₄] (0.1 mol dm⁻³), and[H⁺] (5.0 × 10⁻² mol dm⁻³) but with various[Ag⁺] (6–1.5 × 10⁻⁵ mol dm⁻³) (Table 5). A plot of rate constant k′(s⁻¹) against[Ag⁺] yields a straight line with a non zero intercept (Indu Sharma et al., 1995) (Fig. 3). Observed reaction order of Ag(I) can be obtained from linear regression of log k′ vs. log Ag(I) (order ∼0.374) at 298 K. This indicates that the reaction is of fractional order with respect to Ag(I). The plot of 1/k′ vs. 1/[Ag⁺] exhibits excellent linearity (r = 0.998) with a positive slope and positive intercept.

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Figure 3

The linear plot of rate constant k’ (s⁻¹) vs[Ag⁺] with non zero intercept confirms that the reaction is fractional order with respect to[Ag⁺].

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3.7 Effect of bivalent (Cu²⁺) and trivalent (Al³⁺) catalyst

The reaction rates were measured at constant[l-alanine] (6.0 × 10⁻² mol dm⁻³),[Ce(IV)] (6.0 × 10⁻³ mol dm⁻³),[Na₂SO₄] (0.1 mol dm⁻³), and[H⁺] (5.0 × 10⁻² mol dm⁻³) but with various[Cu²⁺] and[Al³⁺] (6–2.7 10⁻⁵ mol dm⁻³) (Table 5). The rate of the reaction has not affected much with[Cu²⁺].[Ag⁺] can alone show the catalytic activity, but[Cu²⁺] cannot show any catalytic activity in this reaction. The same type of results has been reported for the oxidation of organic carboxylic acids by Ce(IV) in aqueous sulfuric acid (Rao et al., 1978; Asim and Das., 2001). The rate of the reaction increases with increase in[Al³⁺]. It is observed that the catalytic efficiency of Al³⁺ was higher than Ag⁺.

3.8 Effect of dielectric constant

In order to determine the effect of dielectric constant (polarity) of the medium on rate, the oxidation of l-alanine by Ce(IV) was studied in acetonitrile as well as acetone mixtures of various compositions (Table 6). The data clearly reveal that the rate decreased with increase in acetonitrile and acetone content of solvent. The rate of the reaction decreased with increase in the percentage of solvents such as acetonitrile and acetone. This indicates that there is a charge development in the transition state involving a more polar activated complex than the reactants (Indu Sharma et al., 1995). The reaction is between a neutral molecule (alanine) and an ion (Ce⁴⁺) and absence of ion–ion or dipole–dipole type mechanism which is also supported by the negative ΔS^# values obtained in this work.

3.9 Test for free radical intermediates

When acrylonitrile solution (40% V/V) was added to the reaction mixture under the protection of nitrogen gas, a white deposition could be found, indicating that the reaction system can initiate polymerization of acrylonitrile and proving free radicals are formed in the reaction.

3.10 Rate and activation parameters

The effect of temperature on k′(s⁻¹) was studied in the range (298–313 K) and the results were shown in Table 7. From Arrhenius plot (Fig. 4), the value of energy of activation (Ea) was calculated. Hence, the values of ΔS^#, ΔH^#, and ΔG^# were computed from the Eyring’s plot (Fig. 5). The obtained large negative value of entropy of activation (ΔS^#) is attributed to the severe restriction of solvent molecules (electrostriction) around the transition state (Anis, 1992) and it indicates that the complex is more ordered than the reactants (Hiremath et al., 2007).

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Figure 4

Arrhenius plot shows the linear plot of log k₂ vs 1/T. From Arrhenius plot, energy of activation (Ea) is calculated.

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Figure 5

Eyring’s plot shows the linear plot of 5+log k₂ / T vs 1/T × 10⁻³. From Eyring’s plot the value of ΔS^#, ΔH^#, ΔG^# is computed.

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3.11 Stoichiometry and product analysis

Different reaction mixtures with different sets of concentration of reactants, where[Ce(IV)] was in excess over[l-alanine] at constant ionic strength, acidity, and at constant concentration of catalyst were kept for 24 h at 298 K in an inert atmosphere. The unreacted Ce(IV) was titrated against standard ferrous ammonium sulfate by using ferroin as an indicator. The main reaction products are Ce(III), aldehyde, ammonia, and CO₂ (Baryta water test).

Acetaldehyde was confirmed with an ¹H NMR and IR spectroscopy. The IR value for –C⚌O group stretching is 1745 cm⁻¹ and for the two –C–H stretchings: one at 2820 cm⁻¹ and the other at 2720 cm⁻¹.The NMR values are one –CH₃ doublet at 2.2 ppm and one quartet for –CHO proton at 9.85 ppm. Further, an aldehyde group was confirmed with qualitative test such as Tollen’s reagent (Chimatadar Shivamurti et al., 2007) and Schiff’s reagent. Nitrile test was negative, and the product is usually reported in the oxidation of amino acids. Ammonia was confirmed with Nessler’s test. Therefore, the stoichiometry of the reaction with a positive test of an aldehyde can be represented by the Eq. (1). Further work on other related amino acids with Ce(IV) is in progress.