Structural reactivity relationship and thermokinetic investigation of oxidation of substituted anilines by iridium (IV) in solution media

Abstract

The oxidation of para- and ortho-substituted anilines with iridium (IV) in aqueous perchloric acid has been investigated. The kinetic study reveals a first-order dependence with respect to both the oxidant and the substrates. The reaction rate increases with the presence of electron-donating groups and decreases with electron-withdrawing groups, which is explained by the linear free energy relationship (LFER) theory according to Hammett’s equation. The Hammett reaction constant (ρ) is found to be negative (ρ < 0), and it increases with temperature. Various thermodynamic parameters have been evaluated, and the validity of the isokinetic relationship has been discussed. The isokinetic temperature (β), calculated from the Van’t Hoff plot (317 K), Compensation plot (317.4 K), and Arrhenius plot (317 K), is consistent. However, the isokinetic temperature obtained from the Exner plot (439.32 K) exceeds the experimental temperature range (303–323 K), indicating that enthalpy factors likely play a more significant role in controlling the reaction. Based on the kinetic results, a probable reaction mechanism is proposed.

Introduction

Substituted anilines are widely used in various industries, including pesticide production, dye manufacturing, and pharmaceuticals^1,2. However, the extensive use of aromatic anilines has led to their increased concentration in aquatic environments^3,4. Many substituted anilines are classified as restricted contaminants by the European Union, as their release poses significant environmental concerns. Previous studies have focused on treating anilines using various effective oxidants, such as ClO₂⁵, Fenton’s reagent⁶, H₂O₂⁷, MnO₂⁸, Fe (VI)⁹, MnO₄^-10, TiO₂¹¹, and IO₄^-12,13. These pollutants often enter the environment through the use of pesticides, fungicides, and dyes in both industrial and agricultural processes, leaving behind chemical residues¹⁴. Additionally, substituted aniline residues may act as intermediates or precursors to pesticides and halogenated disinfection by-products^15,16. Given that some anilines are carcinogenic, toxic, and can induce adverse physiological effects, it is of paramount importance to develop effective transformation methods to eliminate these contaminants from the environment, particularly from aquatic systems.

Iridium (IV) is widely used both as a catalyst and as an oxidant in electron transfer processes. It undergoes reduction to iridium (III) through a one-electron transfer, and this reduced form is also employed as a catalyst^17,18. The oxidation kinetics of Ir (IV) have been studied in various reactions, where it functions both as a catalyst and an oxidant^19–21. Iridium (IV) chloride is often preferred as an oxidant due to its ease of handling and its behavior in outer-sphere one-electron transfer reactions^22–24. This oxidant has also been used in reactions involving trace metal-ion catalysis, although its activity can be inhibited by chelating agents. For example, the oxidation of thioglycolic acid in the presence of bathophenanthroline disulfonate, a chelating agent, has been studied. A review of the literature reveals that no kinetic studies have been conducted on the interaction between anilines and [IrCl₆]^2-. This work presents the results of a series of experiments investigating the electron transfer pathway from anilines to hexachloroiridate (IV) in an acidic medium, using spectrophotometric techniques.

The study of substituent effects and thermodynamic parameters is a key focus for evaluating the reaction mechanism between substituted anilines and oxidants in both aqueous and non-aqueous media^25–28. A non-linear Hammett plot has been observed in these correlations²⁹. The Hammett equation, along with linear free energy relationships (LFER) and isokinetic/enthalpy-entropy relationships, has been widely used to correlate the rate constants and equilibrium constants of reactions^30–32. In recent years, there has been significant research exploring the enthalpy-entropy compensation effect and the isokinetic relationship^33–37.

Many researchers have suggested that the enthalpy-entropy compensation effect, observed by plotting ∆H^# against ∆S^#, may arise from random experimental errors³⁸. However, the existence of both kinetic and thermodynamic compensation has been rigorously studied^39–41. In light of this, the present study focuses on the kinetics of fourteen substituted anilines reacting with [IrCl₆]^2- in an aqueous acidic medium. A detailed and plausible mechanistic scheme is proposed. Additionally, thermodynamic parameters are evaluated, and a linear free energy relationship (LFER) is constructed to describe the influence of substituents on the reactivity pattern, providing insight into the reaction mechanism.

Experimental

Materials

The substituted anilines used in this study included H, p-SO₃H, p-COOH, O-COOH, p-NO₂, p-Br, p-Cl, p-OCH₃, p-I, p-COOEt, p-COMe, p-F, p-CH₃, and 2,6-dimethyl. All reagents were of AnalaR grade (99%) (Table 1). The oxidant solution (Iridium (IV) chloride hydrate, Sigma-Aldrich) was prepared by dissolving the required amount in deionized water. This solution is stable under normal conditions and its stability is further enhanced when stored in dark-coated bottles at refrigerated temperatures (~ 5 °C). Conductivity water, distilled from an alkaline permanganate solution, was used for the reaction experiments.

Table 1. Systematic chemical name, preferred IUPAC name and initial purity of substituted anilines 

Anilines	IUPAC name	Systematic IUPAC name	Purity (%)
H (Phenylamine)	Aniline	Benzenamine	≥ 99.5
p-SO3H (Sulfanilic acid)	4-aminobenzene-1-sulfonic acid	–	≥ 99.0
p-COOH (p-Aminobenzoic acid)	4-Aminobenzoic acid	–	≥ 99.0
O-COOH (Anthranilic acid)	2-Aminobenzoic acid	2-Aminobenzenecarboxylic acid	≥ 99.0
p-NO2 (p-Nitroaniline)	4-Nitroaniline	4-Nitrobenzenamine	≥ 98.0
p-Br (p-Bromoaniline)	4-Bromoaniline	4-Bromobenzenamine	≥ 98.0
p-Cl (p-Chloroaniline)	4-Chloroaniline	4-Chlorobenzenamine	≥ 99.0
p-OCH3 (p-Anisidine/4-Aminoanisole)	4-Methoxyaniline	–	≥ 99.0
p-I (p-Iodoaniline)	4-Iodoaniline	4-Iodobenzenamine	≥ 98.0
p-COOC2H5 (Benzocaine)	Ethyl 4-aminobenzoate	–	≥ 99.0
p-COCH3 (p-Aminoacetophenone)	1-(4-Aminophenyl)ethanone	–	≥ 98.0
p-F (p-Floroaniline)	4-Floroaniline	4-Florobenzenamine	≥ 98.0
p-CH3 (p-Toluidine)	4-Methylaniline	4-Mrthylbenzenamine	≥ 99.0
2,6-(CH3)2 (2,6-Xylidine)	2,6-Dimethylaniline	2,6-dimethylbenzenamine	≥ 99.0

Spectrophotometric measurements

Absorption measurements were performed using a UV-visible spectrophotometer (PharmaSpec 1800, SHIMADZU) with 1 cm path length quartz cuvettes, controlled by UV-Probe software and digital thermostat to regulate the temperature. The reactions were monitored spectrophotometrically at intervals from 120 to 1800 s at λ_max = 485 nm (ε = 4050 dm³ mol⁻¹ cm⁻¹). To study the kinetics, the reaction mixtures were placed in black-painted glass stoppered Erlenmeyer flasks, which were suspended in a thermostated water bath. The reactions were initiated by adding a thermally pre-equilibrated solution of Iridium (IV). The reaction kinetics were tracked spectrophotometrically by periodically measuring the absorbance of the remaining Iridium (IV) at 485 nm, over the temperature range of 303–323 K, until up to 90% of the reaction was complete.

Statistical data analysis

The pseudo-first order rate constants were calculated from absorbance time data using the StatGraphics Centurion Software (19 × 64). Correlation analysis was performed using Microsoft excel (LINEST). The fitting validity was discussed by the use of standard deviation (SD) and correlation coefficient (linear regression (R²)^42–44.

Stoichiometry and product analysis

The reaction stoichiometry was established by adding sufficiently considerable amount of oxidant over that of anilines. The excess concentration of oxidant was computed after consumption the reactants by UV-visible spectrophotometer at λ_max, 485 nm (ε, 4050 dm³ mol^− 1 cm^− 1). The results represented by following equation. The oxidation product of aniline was further been identified by spectrally to be corresponding 2-keto-azoxybenzene.

Results and discussion

The concentration of Ir(IV) (0.2-1.0)×10^− 4 mol dm^-3 was varied fixed concentrations of substrates (0.2 mol dm^-3) and also keeping constant concentrations of hydrogen ion to be 0.1 mol dm^-3 at 318 K. The pseudo-first order rate constants (k’, s^-1) were found to be independent of gross initial concentrations of Ir(IV), confirming first order dependence with respect to the oxidant.

Pseudo-first order rate constants were evaluated at increasing concentrations of substrate (0.5–30.0) ×10^− 2 mol dm^-3) and a plot of these rate constants versus the concentration yielded a straight line passing through the origin confirming first order with respect to substrate.

The effect of hydrogen ion concentration was studied by employing perchloric acid. The concentration of hydrogen ion was varied from 0.1 to 0.5 mol dm^-3 at fixed concentrations of other. The rate decreases with increasing hydrogen ion concentration. The effect of ionic strength was studied using lithium perchlorate and the rate increases with increasing ionic strength.

The reaction of substituted anilines (H, p-SO₃H, p-COOH, O-COOH, p-NO₂, p-Br, p-Cl, p-OCH₃, p-I, p-COOEt, p-COMe, p-F, p-CH₃, 2, 6-dimethyl) was studied spectrophotometrically at (303–323) K in the presence of varying excess of anilines (0.2 mol dm^-3) over the oxidant (1.0 × 10^− 4 mol dm^-3) to establish pseudo-first-order kinetics. The second order rate constants, k₂, were evaluated and details are given in Table 2.

Table 2.

Second-order rate constants for the oxidation of substituted anilines by [IrCl₆]^2- in aqueous acidic medium.

Substrate	k2 (dm3 mol− 1 s− 1)
	303 K	308 K	313 K	318 K	323 K
H, (107 k2)	1.99	2.48	3.68	6.17	8.76
p-OMe, 107 k2	5.76	7.45	8.41	9.62	11.5
p-Me, 107 k2	3.35	3.80	4.56	5.43	6.66
2,6 diMe, 107 k2	3.45	4.09	4.35	5.06	5.59
p-F, 107 k2	2.45	2.92	3.33	3.71	4.20
p-Cl, 107 k2	1.52	2.16	2.95	3.68	4.37
p-Br, 108 k2	4.19	6.91	14.1	22.2	29.8
p-I, 108 k2	5.25	6.68	11.1	14.3	16.9
p-COMe, 108 k2	3.04	3.68	5.30	8.24	14.9
p-COOEt, 108 k2	1.52	2.07	3.22	4.84	8.48
p-COOH, 108 k2	0.74	1.29	1.75	2.12	3.32
p-SO3H, 109 k2	5.07	7.37	9.21	11.5	15.2
O-COOH, 109 k2	3.22	4.15	5.53	7.37	9.21
p-NO2, 109 k2	3.65	4.43	5.30	7.29	9.47

Mechanism of oxidation

Substitution of [IrCl₆]^2- complex is inactive to react and inner sphere kinetics does not occur in such reactions. In all prospect, an outer-sphere complex is built which further produced the oxidation products through decomposition. Since [IrCl₆]^3- and Cl^- don’t affect the rate of reaction by which eliminate the all probability of rate controlling step involving [IrCl₆]^3- and complex formation between substrate and oxidant by dissociation of [IrCl₆]^2-. Rate is inhibited by increase concentration of hydrogen ion, thus considering first-order respecting to reactants and inhibiting effect of acid, the following mechanism can be proposed regarding experimental outcomes.

1

2

3

Applying steady state treatment to the intermediate, such a mechanism leads to rate law (4)

4

Ion-pairing of oxidant and substrate was structured in the fast step, i.e. a slow electron transfer process is occurred through an outer sphere mechanism within the complex^45–47. Hexachloroiridate (IV) is a stable reagent for substitution reactions or hydrolysis over a broad range of acidity. Even though, a fresh solution of the oxidant was used in whole study, since the possibility of [IrCl₆ (OH)]^3- to be the reactive form is implausible under the experimental conditions. The following scheme accounts for the reaction events precisely.

Scheme 1. Reaction scheme proposed for the oxidation of anilines by [IrCl₆]²⁻

Thermodynamic parameters and the isokinetic relationship

Investigating the effect of temperature on the reaction rate is crucial for understanding the thermodynamic parameters and, ultimately, the nature of the reaction mechanism in solution. In this context, Exner’s equation and the isokinetic relationship are valuable tools for determining whether a similar mechanism governs a series of reactions⁴⁸. Additionally, the isokinetic parameter β is particularly useful for assessing whether the system is controlled by entropy or enthalpy.

In view of these observations, the activation parameters were evaluated from k₂ at (303–323) K by employing Arrhenius and Eyring Eqs. (5) and (6) through Van’t Hoff plot by method of least squares (Table 2).

5

where ‘k’ is a second order rate constant (dm³ mol^− 1 s^− 1) and other terms have their usual meaning. ΔH^#, ΔS^# and ΔG^# were evaluated from Eq. (6) to (8)

6

7

and

8

and are collected in following Table 3.

Table 3.

Activation parameters in the reaction between anilines and [IrCl₆]²⁻.

Anilines	∆H#, kJ/mol	∆S#, J/K mol	∆Ea#, kJ/mol	∆G#, kJ/mol	lnA
H	40.46 ± 0.044	− 269.91 ± 0.861	43.06	124.94	11.16
p-SO3H	54.59 ± 0.103	− 219.81 ± 2.048	57.19	123.39	17.24
p-COOH	36.54 ± 0.055	− 286.28 ± 1.109	39.15	126.15	9.11
O-COOH	47.89 ± 0.096	− 226.25 ± 1.904	50.50	118.71	16.51
p-NO2	40.93 ± 0.019	− 272.52 ± 0.369	43.53	126.23	10.84
p-Br	25.52 ± 0.029	− 284.94 ± 0.581	28.12	114.71	9.33
p-Cl	40.44 ± 0.06	− 241.58 ± 1.19	43.04	116.05	14.62
p-OCH3	60.33 ± 0.096	− 174.94 ± 1.9	62.93	115.09	22.49
p-I	23.98 ± 0.038	− 284.98 ± 0.748	26.58	113.18	9.35
p-COOC2H5	66.98 ± 0.089	− 174.26 ± 1.778	69.58	121.52	22.61
p-COCH3	61.95 ± 0.149	− 185.59 ± 2.948	64.54	120.04	21.17
p-F	16.52 ± 0.026	− 314.04 ± 0.519	19.12	114.81	5.82
p-CH3	80.41 ± 0.116	− 120.58 ± 2.29	83.01	118.14	29.24
2,6 diMe	18.85 ± 0.019	− 309.15 ± 0.372	21.45	115.61	6.45

The large (-)ve entropy of activation is arising owing to solvation effects being predominant in such reactions. Since these substituted anilines exhibit different polarities, the extent of solvation is, therefore, indicating variations in entropy of activation. A small difference in the reaction rates for various anilines may be noted for a change in the basic strength of these anilines bringing change in the availability of the nitrogen lone-pair in amino groups.

The validity of isokinetic relationship ΔH^# = βΔS^# where β is isokinetic temperature was determined by constructing a plot between ΔH^# and ΔS^# (Fig. 1, R² = 0.94, SD = 4.74) exhibiting a linear relationship and confirming validity of isokinetic relationship for these anilines apart from a common mechanism for such reactions. The isokinetic temperature β obtained from the figure is 317.7 K and free energy is 120.32 kJ/mol.

Fig. 1.

A Plot between ΔS^# and ΔH^# (Van’t hoff plot). [Substrate] = 2.0 × 10^− 2 mol/L; [IrCl₆]^2- = 2.0 × 10^− 4 mol/L; [I] = 1.0 mol/L.

If Leffler’s view⁴⁹ are taken into account, the reaction is neither isoentropic nor isoenthalpic but the almost constancy in the values of ΔG^# as mentioned in the Table 3. Such a situation normally corresponds to the fact that these anilines proceed by a common mechanism despite slight differences in their structures complies with the compensation law also known as the isokinetic relationship.

Further Exner⁵⁰ suggested a method for calculation of isokinetic temperature by employing relationships (9) and (10).

10

11

where ‘a’ and ‘b’ are constants. k₂ is second order rate constants at two different temperatures T₁ and T₂ respectively for a series of substituted anilines.

A plot of ln k₂ (at 308 K) versus ln k₂ (at 318 K) was made that yielded a straight line (Fig. 2, R² = 0.979, SD = 0.285) accounting for validity of Exner’s plot indicates that a common mechanism is operating in examination of all substituted anilines. The isokinetic effect is unlike that the compensation effect and β was then calculated from Eq. (10) employ ‘b’ from the gradient of the plot to be 439.32 K. This value of β is well above the experimental temperature range ascribing the reactions to be enthalpy controlled⁵¹.

Fig. 2.

A Plot between ln k₂ (35 °C) and ln k₂ (45 °C) (Exner’s plot). [Substrate] = 2.0 × 10^− 2 mol/L; [IrCl₆]^2- = 2.0 × 10^− 4 mol/L; [I] = 1.0 mol/L.

The facts that all the substituted anilines investigated indicate identical kinetic behavior. The activation free energies (∆G^#) do not differ significantly and also the activation energy (∆E^#) correlates with the ln A (frequency factor) (Fig. 3, R² = 0.993, SD = 1.75) and so the activation enthalpy (∆H^#) with activation entropy (∆S^#), further propound the operational of a common mechanism in these oxidation reactions. The slope of compensation plot is 317.4, which is again within the experimental temperature range.

Fig. 3.

A compensation Plot between E_a and ln A. [Substrate] = 2.0 × 10^− 2 mol/L; [IrCl₆]²⁻ = 2.0 × 10^− 4 mol/L; [I] = 1.0 mol/L.

The validity of the isokinetic relationship can be confirmed by the common intersection point of the Arrhenius lines (Fig. 4), which verifies the isokinetic relationship. The isokinetic temperature, determined to be 317 K, closely matches the temperature obtained from both the Van’t Hoff and Compensation plots. All reactions studied were governed by enthalpy⁵². Kinetic compensation was observed only when the logarithm of the frequency factor (ln A) showed a linear dependence on the activation energy (∆E^#). Thermodynamic compensation can be closely derived from the Van’t Hoff equation for equilibrium states, where changes in enthalpy are closely related to changes in entropy.

Fig. 4.

Arrhenius plots of substituted anilines. [Substrate] = 2.0 × 10^− 2 mol/L; [IrCl₆]^2- = 2.0 × 10^− 4 mol/L; [I] = 1.0 mol/L. (1) p-NO₂, (2) –H, (3) p-SO₃H, (4) p-COOC₂H₅, (5) p-COOH, (6) p-COCH₃, (7) p-CH₃, (8) O-COOH, (9) p-Cl, (10) p-OCH₃, (11) 2,6-diMe, (12) p-Br, (13) p-F, (14) p-I.

Structure-reactivity correlation

The present reaction has been studied at five different temperatures (303–323) K in aqueous acidic medium involving aniline and 13 substituted anilines. The second-order rate constants were evaluated (Table 2). The investigation of these data shows that the oxidation of anilines is enhanced by electron donating groups as substituents and inhibited by electron removing groups in the benzene ring. The reactivity order is as follows: p-OCH₃ > p-CH₃ > p-H > 2,6-dimethyl > p-F > p-Cl > p-Br > p-I > p-COCH₃ > p-COOC₂H₅ > p-COOH > p-SO₃H > O-COOH > p-NO₂ for the substituents. A linear relationship is obtained by plotting Hammett equation between σ (substituent constants) and lnk₂ for different substituents (Fig. 5, R² = 0.984, SD = 0.476) at 313 K with ρ = ˗ 4.66.

Fig. 5.

Hammett plot of ln k₂ vs. substituent constant, σ at 313 K. [Substrate] = 2.0 × 10^− 2 mol/L; [IrCl₆]^2- = 2.0 × 10^− 4 mol/L; [I] = 1.0 mol/L.

The value of reaction constant (ρ) is derived by the slope of Hammett plot. Values of reaction constants at different temperatures are shown in Table 4. As remarked by Hammett⁵³, the reaction with positive ρ values is enhanced by electron withdrawing substituents on benzene ring, while in the contrary with negative ρ values; reaction is inhibited by electron withdrawing from benzene ring. In present study, the rate of oxidation reactions was decreased by the electron withdrawal effect from benzene ring and increased by electron donating effect. These outcomes affirm the negative values of reaction constants (ρ) obtained from the Hammett plot. Negative ρ values endorse a formation of positive charge at the nitrogen atom of the aniline moiety in transition state.

Table 4.

Values of reaction constants at different temperature.

Temperature (K)	Reaction constant, ρ	Correlation coefficient	Standard deviation
303	− 5.0956	0.913	0.754
308	− 4.95	0.922	0.737
313	− 4.791	0.904	0.772
318	− 4.6472	0.876	0.817
323	− 4.387	0.847	0.863

If the slope of the Exner plot is less than unity, that means that the selectivity of the reagents diminishes with increasing temperature, however, in these oxidation series, slope is less than unity but tended towards to unity (0.9585). If slope is greater than unity than the selectivity of the reagents increases. The comparison of reaction constant (ρ) at different temperatures indicates that the negative value of reaction constant (ρ) decreases with increasing the temperature and ρ varies linearly with (1/T) (Fig. 6).

Fig. 6.

A plot between hammett reaction constant and (T)⁻¹.

This is expected that at high temperature, the selectivity of the reagent becomes less due to thermal randomization and therefore the sensitivity of the reaction towards the electron density changes decreases at the reaction site.

Conclusion

In this study, author has reported the detailed mechanism of oxidation of substituted anilines by [IrCl₆]^2-. (a) The linear relationship between the contributions of entropy and enthalpy implicit that the susceptibility of the thermal reaction of anilines to substituent effect would be inversed simply by alter the temperature (compensation effect). (b) At isokinetic temperature, structural changes do not alter the rate constants of a reaction series as the changes of ∆H are counterbalanced by changes of ∆S, so the presence of an isokinetic relationship suggests a common structure of the transition state of all oxidation reactions. (c) The isokinetic temperature is above the experimental temperature range shows that the reactions were enthalpy controlled. (d) The high negative values of entropy indicating the more ordered complex formation in a rate determining step. (e) The negative ρ values computed from the Hammett plot divulge that a positively charged reactive intermediate formation during the oxidation, indicating diminution of nitrogen lone pair and it can be facilitate when the oxidant builds a complex with substrate in which lone pair is involved in coordination with electron deficient centre of a metal ion.

Author contributions

R. S.: Conceptualization, Formal Analysis, Experimental Investigation, Calculations, Writing, Review and Editing.

Data availability

The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.