The Oxidative Transformation of Substituted Mandelic Acids by Ethylenediammonium Dichromate in AcOH-H2O Medium: A Kinetic Study

Introduction

Chromium (VI) has been approved as a consequential reagent in organic synthesis and has been utilized to oxidize organic compounds in both aqueous and non-aqueous media ^1-3. It is investigated to be venomous for humans and as well as for plants and it is soluble in water ⁴. In organic chemistry, Cr(VI) reagents are often used for hydroxy acid oxidation to oxoacids or ketone. Many different oxidants are being used in various experimental setups to conduct kinetic studies on the oxidation of mandelic acids. In recent years oxidation of α-hydroxy acids by several oxidants, including benzimidazolium dichromate ⁵ quinaldinium fluorochromate ⁶, tripropylammonium fluorochromate⁷ pyridinium chlorochromate ⁸, morpholinium fluorochromate ⁹ and tetrabutylammonium tribromide ¹⁰ have been investigated. Recently a new oxidising agent called EDDC has been identified. A literature survey revealed that there is no available reports on the oxidation of mandelic acids by EDDC. Owing to the above arguments, the title reaction has been studied. The objective of this study is to perform a thorough evaluation of the substituent and solvent effects on the oxidation of various para- and meta- substituted mandelic acids in various concentrations of acetic acid-water in order to better understand the mechanism of mandelic acid oxidation.

Experimental

Materials

The current experiment employed only reagent grade chemicals (Merck-Aldrich), and their solutions were created by dissolving the necessary amounts of the samples in double distilled water.   The oxidant EDDC was made using the mentioned technique ¹¹ and its purity was assessed by iodometric analysis. Mandelic acid was a commercially accessible substance of the greatest purity and was employed as such. Acetic acid was cleaned up according to conventional procedures and the proportion that began to distil at 118°C was gathered.

Product analysis

This study proves that oxoacids were the end result of the oxidation. The 2,4-dinitrophenylhydrazine methodology was used to quantify oxoacids within dynamic environments, i.e. with an abundance of mandelic acid over the oxidant (EDDC). In a common experiment, mandelic acid (0.15 mol, 22.8 g) and EDDC (0.015 mol, 4.17 g) were dissolved in a combination of 50% acetic acid and 50% water with TsOH(0.5 mol dm^-3). To determine that the reaction had ended, the following solution was left in the dark for 24 hours. The bulk of solvent were eliminated through distillation at a low pressure. The aforementioned solution was combined with 200 ml of a 2,4-dinitrophenylhydrazine (saturated) solution in 2 M hydrochloric acid, and the reaction was then carried out overnight in a refrigerator. After being produced, hydrazone was percolated, dried, and weighed before being re-crystallized with ethyl alcohol and being weighed again. The result (mp and combined mp) was identical to an actual sample of DNP of benzoylformic acid. Before and after recrystallization, the DNP yields were 4.35 g (88%) and 4.01g (81%), respectively. After recrystallization, similar tests with additional substituted mandelic acids generated 75-83% of the equivalent oxoacid’s DNP yields. The ultimate reduction of Cr(VI) to Cr(III) was shown to occur.

Stoichiometry

Mandelic acid (0.002 mol dm^-3) and EDDC (0.01 mol dm^-3) were prepared to a volume of 100 ml in the presence of TsOH (1.0 mol dm^-3), in order to determine the stoichiometry. In the related rate measurements, the solvent compositions of 50% acetic acid and 50% water (v/v) were maintained. For 24 hours, the reaction was allowed to stand in order to ensure that it had finished. According to the findings, there are 3 moles of mandelic acid for every 1 mole of oxidant, or a 3:1 stoichiometry.

Method

The pseudo‑first-order condition were executed for all kinetic runs at constant temperature (±0.1), by keeping an excess of mandelic acids (15-fold or greater) over EDDC. The solvent was 50% AcOH- 50% water unless specified otherwise. . Residual IQDC was measured with the help of spectrophotometer (Systronics, model no. 177) at 370 nm. The progress of reaction up to 75% was studied by the help of spectrophotometer (Systronics, model no. 177) with monitoring the consumption of EDDC at 356 nm. In this investigation for all concentrations Beer’s law is applicable.  A graphic between log [EDDC] and time was linear with r² > 0.99 and rate constant (k_obs) was obtained from it and for more than two runs replicability ±3% was observed. In correlation analyses coefficient of determination (R² or r²), standard deviation (sd) and Exner’s parameter (y) ¹² was used.

Results

For all of the substituted mandelic acids, the rate laws and further experimental studies were established. The oxidation of mandelic acid and substituted mandelic acids, the equivalent oxoacids are produced by EDDC. A stoichiometric determination indicates the general reaction that follows-

 RCH(OH)COOH + Cr₂O₇^-2 + 8 H⁺ → 3 RCOCOOH + 2Cr⁺³ + 7 H₂O            (1)

R is a phenyl or a substituted phenyl group in this particular case where substituents are m-Cl, m-Br, m-NO₂, p-OMe, p-Me, p-F, p-Cl, p-Br, p-NO₂.

Induced polymerization of cyanoethylene

Cyanoethylene did not polymerize when mandelic acid was oxidised by EDDC in a nitrogen environment. The rate of oxidation was also unaffected by the addition of cyanoethylene (Table 1). The reaction was performed with the addition of 0.1M butylated hydroxytoluene (BHT) in order to further verify a chemical route without free radicals (BHT).  BHT was found to be essentially quantitatively unaltered.

Rate laws

In terms of EDDC, the reaction was observed to be of first order. Plotting log (EDDC) vs time throughout individual kinetic runs revealed linear behaviour (r² > 0.99). Additionally, EDDC initial concentration had no impact on the pseudo first-order rate constants (Table 1). Regarding MA, the order ranged from one to two (Table 1)

Table 1: Rate constants for oxidation of mandelic acid by EDDC at 298K

[Mandelic acid] (mol dm-3)	103 [EDDC] (mol dm-3)	[H+] (mol dm-3)	104 kexp (sec-1)
0.05 0.1 0.15 0.25 0.4 0.6 1.0 2.0 1.0 1.0 1.0 0.6 0.6	1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.5 2.5 4.0 1.0 1.0	0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5	0.53 1.69 4.5 9.6 16.7 23.9 29.3 33.5 30.2 28.9 29.8 a22.5 b23.9

0.002^a M and 0.006^b M of cyanoethylene are present in the reaction mixture.

Based on the downward curvature of the plots it appears that a complex relationship exists Plotting 1/k_exp versus 1/[Mandelic acid]² revealed a linear relationship with an intercept on the rate-ordinate. (Figure 1).

Figure 1: Plot of 1/(104 kexp) vs 1/[Mandelic acid]2 at 298K [H+] = 0.5 mol dm-3, 103[EDDC] = 1.0 mol dm-3 Click here to View Figure

As a result, the following general mechanism and rate equation are postulated.

Rate =  k_dK_eq [Mandelic acid]² [EDDC]_t /(1+ K_eq[Mandelic acid]²)                     (3)                                                                 

Or, (Rate/ [EDDC]_t  )^-1 = 1/k_exp = 1/ k_dK_eq [Mandelic acid]²   +   1/k_d                             (4)                                                                            

Here, [EDDC]_t  =  [Complex]+ [EDDC]

At various temperatures, equation (4) was used to examine the influence of k_exp on the concentration of Mandelic acid, the double reciprocal plots were used to compute the values of K_eq and k_d. Using the corresponding values of K_eq and k_d at various temperatures, the activation components for the complex breakdown and the thermodynamic components for the complex creation were computed. (Tables 2 and 3).

Kinetic isotopic effect

It was investigated that the cleavage of the α-C-H bond had a role in the rate-determining phase during the oxidation of deuteried mandelic acid by EDDC. The findings (Tables 2 and 3) revealed that while the rates of their breakdown had a significant kinetic isotope effect (k_H/k_D= 6.01   at 298 K), the formation constants of the complexes of ordinary and deuterated mandelic acids were quite similar. In the mandelic acid oxidation by water based chromic acid, the value of a primary kinetic isotope effect of ca. 6 was found to be substantially comparable ¹³. In the current investigation, the kinetic isotope effect’s value also dropped as temperature rose.

Table 2: Formation constants and thermodynamic parameters for [Mandelic acid – EDDC] complexs

Substituents	Keq (mol-2 dm6)				∆Hf (kJmol-1)	∆Sf (Jmol-1K-1)	∆Gf (298K) (kJmol-1)
	298K	308K	318K	328K
H	6.33	3.90	2.80	1.6	-38.6±1.8	-106±5.8	-7.10±1.4
m-Cl	6.19	4.03	2.85	1.55	-39.0±2.1	-107±7.7	-7.10±1.8
m-Br	5.92	3.82	2.56	1.49	-39.3±1.5	-109±4.9	-7.00±1.1
m-NO2	6.25	3.75	2.62	1.7	-37.1±0.8	-102±2.8	-6.90±0.7
p-OMe	6.45	3.79	2.65	1.69	-38.0±1.1	-105±3.2	-7.06±0.8
p-Me	6.08	3.86	2.64	1.56	-38.6±1.4	-107±4.4	-7.01±1.07
p-F	6.81	3.86	2.89	1.72	-38.3±1.9	-105±6.1	-7.20±1.5
p-Cl	6.59	4.10	2.76	1.51	-41.5±1.9	-116±6.2	-7.24±1.5
p-Br	6.25	3.95	2.70	1.42	-41.6±2.4	-116±7.9	-7.14±1.9
p-NO2	6.64	3.96	2.91	1.81	-36.6±1.4	-100±4.5	-7.14±1.1

 

Table 3: Rate constants for decomposition of [Mandelic acid- EDDC] complexes   and their activation parameters.

Substituents	103 kd (s-1)							∆H* (kJmol-1)	∆S* (Jmol-1K-1)	∆G* (298K) (kJmol-1)
	298K	308K		318K		328K
H	3.43		7.12		15.6		34.7	60.1±1.4	-91±4.6	87.2±1.10
m-Cl	0.71		1.70		3.80		0.80	65.3±0.7	-87±2.3	91.0±0.56
m-Br	0.65		1.60		3.50		8.20	65.6±0.8	-86±2.7	91.2±0.65
m-NO2	0.17		0.42		1.03		2.60	71.2±1.1	-79±3.7	94.6±0.91
p-OMe	10.7		22.1		43.2		94.0	55.8±1.3	-96±4.3	84.3±1.00
p-Me	7.05		15.0		29.7		65.1	57.1±1.2	-95±3.9	85.3±0.96
p-F	2.64		5.60		13.1		28.6	62.4±1.3	-86±4.1	87.8±1.00
p-Cl	1.32		3.10		6.70		15.6	64.0±1.0	-86±3.1	89.5±0.75
p-Br	1.28		2.82		6.40		14.5	63.2±1.1	-89±3.6	89.6±0.87
p-NO2	0.12		0.32		0.80		2.10	74.6±1.0	-70±3.1	95.4±0.70
Deuteriated mendelic acid	0.57		1.26		2.93		6.73
kH/kD	6.01		5.72		5.32		5.15

 

Effect of acidity

On increasing acidity, the rate of oxidation is also increasing (Table 4). log-log plot between k_exp against [H⁺] has a slope of 1.62±0.02 and r² > 0.99, indicating that it is linear. It shown that the order in relation to [H⁺] is less than two.

As a result, a plot of k_exp^-1 vs [H⁺]^-2 was made, and it was discovered to be linear with a clear intercept (Figure 2). In the absence of TsOH, there was no noticeable effect.

Table 4: Dependence of reaction rate of mandelic acid on hydrogen-ion concentration.

[H+](mol dm-3) 104kexp (s-1)

0.05    0.23

0.15  1.35

0.35  5.4

0.6  13.2

0.75  20.1

1.0  29.3

[Mandelic acid] = 1.0 mol dm^-3, 10³ [EDDC] = 1.0 mol dm^-3, temperature = 298K

Figure 2: Plot of 1/(104 kexp) vs 1/[H+]2 at 298K [Mandelic acid] = 1.0 mol dm-3, 103[EDDC] = 1.0 mol dm-3 Click here to View Figure

Polarity of the solvent and reaction rate

Acetic acid’s content is increased from 30% to 70%, investigations were conducted to determine how the solvent content affected the reaction rate (Table 5). The plot of log k_d vs 1/D [Dielectric constant (D)] is linear and has a positive slope, which suggests that the oxidant and substrate have a dipole-dipole or ion-dipole interaction (Figure 3). The curvature in the log(k_d) vs (D-1)/(2D+1) picture reveals the lack of dipole-dipole interaction in the rate regulating step¹⁴. Similar kind of solvent polarity effect on rate observed for oxidation of diols by IQDC¹⁵.

Table 5: Effect of solvent polarity on the rate of reaction at 298K

%AcOH-H2O (v/v)	D	1/D	104kexp(s-1)
15-85 25-75 50-50 65-35 85-15	67.46 60.23 42.17 31.33 16.88	0.0148 0.0166 0.0237 0.0319 0.0592	33.4 31.5 29.3 27.6 20.5

[Mandelic acid] = 1.0 mol dm^-3, [H⁺] = 0.5 mol dm^-3, 10³[EDDC] = 1.0 mol dm^-3

Figure 3: Plot of  logkexp vs 1/D ,showing the effect of solvent polarity for the oxidation of mandelic acid. Click here to View Figure

Isokinetic Temperature

The entropies and enthalpies of activation of the oxidation of mandelic acid and substituted mandelic acid demonstrated a satisfactory correlation (r² = 0.9716). Exner’s criteria ¹⁶ was used to test the correlation and determine its authenticity. Exner’s plot of log k_d values at 298 K and 328 K for the substituted mandelic acid was linear (r² = 0.9997). The isokinetic temperature value based on Exner’s method, was 825±74 K. The validity of the free energy linear relationship is dependent on the existence of a linear isokinetic relationship ¹⁶. This suggests that there is a common mechanism shared by all such correlated reactions.

Reactive oxidising species

Following proton transfer, it appears that EDDC is an ionic molecule. Conductivity measurements were taken at 298K to assess the state of EDDC under our reaction parameters. The observed proton concentration relationship suggests that [EDDC] is protonated, resulting in enH₂[Cr₂O₇H₂]⁺² species (Q)  that is more effective oxidising agent  and active electrophile [eqⁿ (6)].

Further, The following is how the rate relates to [H⁺]:

k_exp = α[H]² / 1+ β[H]²                  (6)

Similar kind of [H⁺] dependency  on rate observed for oxidation of amino acids by BIDC ¹⁷.

Discussion

Correlation analysis of reactivity

A review of the values in Table 2 and 3 indicates that the formation constants, K_eq, of the substituted Mandelic acid-EDDC complexes are not very responsive to substitution in the aromatic ring. Similar findings have already been reported in the oxidation processes of various Cr(VI) complexes^18-19. However, the complex rates of decomposition, k_d, vary significantly with the substitution. Hammett values were correlated with the mandelic acid and substituted mandelic acid k_d values ²⁰.

log k_d  =  log k₀  +  ρ σ          (7)    

here, the reaction constant is ρ, the substituent constant is σ, the rate constant for mandelic acid is k₀, and the rate constant for substituted mandelic acid is k_d.

log(k_d) vs (σ) is linearly plotted (Figure 4). The analysis revealed excellent correlations with negative reaction constants (Table 6). When the temperature rises, the value of the ‘ρ’ falls. This states that when the temperature rises, the reaction’s specificity reduces.

Table 6: Temperature dependence of the reaction constants in the oxidation of mandelic acids by EDDC.

T(K)	ρ	R2	sd	ψ	Data points
298 308 318 328	-1.85±0.006 -1.74±0.012 -1.66±0.001 -1.59±0.006	0.9998 0.9995 0.9997 0.9992	0.006 0.014 0.0093 0.0091	0.015 0.023 0.018 0.029	10 10 10 10

 

Figure 4: A plot of  (4+ logkd) vs Hammett σ  values,  for the substituted mandelic acid oxidation by EDDC at 298K Click here to View Figure

Mechanism

Since cyanoethylene has so little influence on the reaction rate, oxidation via single electron resulting to free radicals is unlikely in this process ²¹. BHT works well as a free radical trap. The fact that BHT was retrieved intact further opposes the presence of a single-electron oxidation. In the reaction the great magnitude obtained of reaction constant that validates the absence of single-electron oxidation. The majority of hydrogen abstraction processes have relatively tiny reaction constant values ²². Prior protonation of EDDC to produce enH₂[Cr₂O₇H₂]⁺² was made possible by acid catalysis (Q). The solvent’s significant cation-solvating capacity serves as evidence for this. This type of kinetics led us to suggest the formation of 2:1 complex (R) in a rapid pre-equilibrium. A nucleophilic attack of hydroxyl oxygen on enH₂[Cr₂O₅(OH)₂]⁺²  results in the complex (R) is postulated. According to the observed sequence of reactivity, the oxidation process was accelerated by electron-releasing groups. This is accounted for by a rise in the oxygen’s hydroxyl oxygen’s electron availability, which promotes complex formation. Complexes of regular and deuterated mandelic acid have essentially comparable formation constants. However, there was a noticeable kinetic isotope effect on the rates of their decay. In other words, the α-C-H bond is broken at the rate-determining phase. The emergence of a polar transition state with characteristics that are close to carbocationic is indicated by the minimum negative reaction constant value and the isotope effect. In order to produce the corresponding oxoacid, the complex (R) then goes through a rate-determining hydride-ion transfer from substrate to oxidant (Scheme1). The cation-solvating ability of the solvents, which plays a significant role, further promotes the hydride-ion transfer process. But in some reports on the oxidation of substituted mandelic acid, the slow step-by-step formation of a carbonium centre is hypothesized ^23,24.

To transfer hydride ions, one may employ either by a chromate ester or an acyclic procedure.  The loss of hydrogen happens by a coordinated cyclic process, according to a Kwart and Nickle method ²⁵ analysis of the kinetic isotope effect that occurs temperature dependence. The known equation was employed to analyze the information for mandelic acid and deuterated mandelic acid.

k_H/k_D = A_H/A_D e^-(∆Ea/RT)                    (8)

The findings show that the entropy of activation of the various processes is strikingly comparable and that the activation energy difference for k_H/k_D is 4.36 kJ mol^-1, which is in line with the respective C-H and C-D bond’s zero-point energy differences. (approximately 4.50 kJ mol^-1). This closely resembles a symmetrical transition state’s characteristic ^26,27. When secondary alcohols are oxidised by BIDC, similar results has previously been noted ²⁸. Bordwell has vehemently rejected the existence of a concerted one-step bimolecular hydrogen transfer process²⁹. It is obvious that the current reaction does not transfer hydrogen through an acyclic bimolecular mechanism.

Only genuinely concerted sigmatropic reactions with a cyclic transition state are fully symmetrical processes requiring linear hydrogen transfer ³¹. As a result, it is safe to conclude that the hydride-ion transfer happens via a cyclic transition state during the oxidation of mandelic acid by EDDC.

The experimental findings are explained by a mechanism shown in Scheme 1. The technique suggested in Scheme 1 may be used to get the rate law, which is shown below:

The equilibrium treatment may be used to calculate EDDC:

-d[EDDC]/dt =  k_d [R] = k_d K_eq [Mandelic acid]² [Q]              (9)

= k_d K_p K_q [Mandelic acid]² [EDDC] [H⁺]²                             (10)

Here, [EDDC]_t = [EDDC] + [Complex], where K_p is small and [Q] formed will undergo reaction with mandelic acid.

Or, 1/k_exp=1/k_d K_pK_q [Mandelic acid]²[H⁺]²  + 1/k_d          (13)

Scheme 1: Oxidation process of substituted mandelic acid by EDDC Click here to View Figure

The reaction system is well described by equations (11) or (12).

Comparing eq.ⁿ (4) with (13), we get

K_eq = K_pK_q [H⁺]²                                                           (14)

The observed negative entropy(-ΔS) of activation provides additional evidence for the proposed mechanism. As charge separation occurs in the transition state, the two ends extensively solvate. As a result, multiple molecules of solvent are immobilised, which is depicted by the entropy loss. The solvent effect is further explained by the -ΔS value. First, Cr(VI) is transformed into Cr (IV). This is intended to combine with another Cr(VI) to generate Cr (V), which will then be promptly reduced to the final product Cr (III). It is generally accepted that these steps take place during Cr(VI) oxidations in this manner ³¹.

Conclusion

The oxidation of mandelic acids by EDDC in acetic acid (50%)-water (50%) resulted in the production of the corresponding oxoacid. The reaction has first order kinetics according to EDDC, while for mandelic acids order is less than two. The reactive oxidizing species is the doubly protonated EDDC, and hydrogen ions are used to catalyze the reactions. Cleavage of the α-C-H bond is observed during the rate-determining phase. It is suggested that the complex goes through rate-determining oxidative breakdown by the transfer of hydride ions from substrate to oxidant, producing the corresponding oxoacid as a product.

 Acknowledgement

We acknowledge the University Grant Commission (UGC), government of India, for financial assistance.

Conflict of Interest

No known financial or personal conflict of interest that might have affected the study’s conclusion has been found.

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