Kinetics and Mechanism of Oxidation of Aliphatic Secondary Alcohols by Benzimidazolium Fluorochromate in Dimethyl Sulphoxide Solvent

Introduction

Organic reaction mechanism not only deals with the relationship between the molecular structure and chemical properties but tells us about the rate constants. Thus, the organic reaction mechanism is a multidisciplinary one which involves organic chemistry, physical chemistry and if some metal compound is involved also inorganic chemistry. Correlation analysis of structure and reactivity is of great importance as nature of the substituent can cause field effect, steric effect or resonance, which directly affects reaction rates in the redox process.It also gives us information about the reaction mechanism, transition state, nature of the intermediates, oxidizing and reducing species formed during the course of a chemical reaction.

Chromium (V1) is a versatile oxidant which is usually found in the chromic acid, metal dichromates and metal halo chromates. It is normally soluble in water and insoluble in most of the organic solvents. Many chromium oxidants have been used in both aqueous and non-aqueous media for the study of oxidation of various organic compounds ¹. Cr (VI) reagents are known to be multi functional and can oxidize almost all the organic functional groups. Many chromium complexes have been prepared and used in the synthetic organic chemistry ^2,3. Oxidation of secondary alcohols to the carbonyl compounds ^4-7 by the oxidants containing chromium (VI), has been widely studied ^8-11.BIFC is also a very mild and selective oxidizing agent¹². Most of the oxidation by BIFC has been carried out in an aqueous medium. In this paper we report the oxidation of different secondary alcohols by BIFC in non-aqueous medium.We feel our study on correlation analysis of different reactions will not only establish mechanism of the reaction but also determine kinetic parameters of the reaction, nature of reactive species and effect of solvent. The literature survey has also revealed that the oxidation of secondary alcohols by BIFC in the non-aqueous medium has not been done and presented so far thus we have used dimethyl sulphoxide as a solvent for our study. The main purpose of our research was to study rate constants, the effects of solvent for oxidative surfaces and propose an appropriate mechanism for oxidation

Experimental

Materials

All the substrates (alcohols) taken were the commercial products of fluka.¹³ Anhydrousmagnesium sulfate was used to dry the liquid alcohols, which were then subjected to fractional distillation. Diphenyl methanol was purified by recrystallizingfrom ethyl alcohol. BIFC was preparedby the reported method⁸. Iodometric titration was usedto check its purity. Deuterated benzhydrol (PhCDOHPh) was prepared by the method available in the literature¹⁴. NMR spectra confirmed its purity of 96±5%. We have used P- toluene sulphonic acid(TsOH)asa hydrogen ions source. Purification of solvents was done by the methods available in the literature¹⁵.

Scheme 1: Structure of Benzimidazolium Fluorochromate Click here to View figure

Product Analysis

We have taken0.1 mol secondary alcohols and 0.01 mol of BIFC and prepared its 100 mL solution using DMSO solvent. 0.5 mol dm^-3of P-toluene sulphonic acid (PTS) was also added for the supply of H⁺to prevent photo-oxidation.Abovemixture was covered with a black paper and placed in the dark place for a day, till the reaction was complete. Then non-aqueous layer was separated, rinsed with H₂O and was dehydrated using anhydrous magnesium sulphate⁸.The yields of the products were approximately 87%. The chromium oxidation state¹⁶ wasfound to be 3.92±0.14, in entirely reduced reaction mixture, which was ascertained by the iodometric titration method.

Kinetic measurement

For our experiments we used pseudo-first-order conditions, for which we took excess of alcohol over BIFC. DMSO was used as solvent, if other than specified.

All the studies were done at a constant temperature^16,17 (± 0.1 K), and up to the 80% of reaction was observed by observing the decrease in the concentration of BIFC spectrophotochemically at a wavelength of 364 nm. Linear least squares method was used for the calculations of pseudo-first-order rate constant and a graph between log [BIFC] versus time was plotted. The reactions were repeated and it was observed that the rate constants were consistent with in ±3% limit¹⁸.

Results

Oxidation of secondary alcoholsby BIFC resulted in the production of ketones¹⁹. In general, overall reaction can be expressed as shown in the eqn. 1 (where BI = benzimidazolium).BIFC undergoes a 2-electron transformation which is clear from the below equation-

Rate law

The reaction order was found to be first with respect toBIFC.^20,21 Figure (1) illustrates a typical kinetic run & Figure (2) represents order of reaction. Moreover, the rate constant was independent of the initial concentration of BIFC.The reaction rate increased directly with the increasing concentration of the alcohols²² (Table 1). The plot of 1/k_obs versus 1/[alcohol] plot was linear with the slop on the rate ordinate²³ suggesting the Michaelis – Menten type of kinetics with respect to the alcohols (Figure 3). This is represented by the Eqn. (2). Equation (3) is referred to the mechanism and Eqn. (4) assigns the rate law.

Induced polymerization testof Acrylonitrile

In the nitrogen atmosphere, the alcohols oxidation was ineffective to induce the acrylonitrile polymerization¹⁹. Moreover, there was no effect of addition of acrylonitrileon the oxidation rate. This clearly indicates that no free radical formation is observed during reaction. (Table1).

Table1: Rate constants obtained for the secondary alcohols oxidation by BIFC at 298 K

[Sec.alcohols] (mol L-1)	103[BIFC] (mol L-1)	[H+] (mol L-1)	2-hydroxyPropane 104kobs/S	2-hydroxyButane 104kobs/S	2-hydroxyPentane104kobs/S	2-hydroxyHexane 104kobs/S
0.10	1.0	0.1	6.41	10.9	14.8	16.4
0.20	1.0	0.1	11.2	19.2	32.2	36.3
0.40	1.0	0.1	22.1	37.5	58.6	69.1
0.60	1.0	0.1	33.6	56.8	87.9	97
0.80	1.0	0.1	44.5	75.2	116	131
1.00	1.0	0.1	54.8	94.8	143	162
0.20	2.0	0.2	12.4	20.3	33.3	37.2
0.20	4.0	0.4	10.6	18.7	31.6	35.4
0.20	6.0	0.7	11.8	19.8	32.7	36.8
0.20	8.0	1.0	10.8	18.9	31.8	35.6
0.40*	1.0	0.1	23.9*	38.9*	59.6*	70.4*

* contained 0.001 mol dm^-3acrylonotrile.

Figure 1: A typical kinetic run shown by the oxidation of 2- hydroxy propane by BIFC Click here to View figure

Figure 2: Order of the reaction plot for the secondary alcohols oxidation by BIFC Click here to View figure

Figure 3: Secondary Alcohols oxidation by BIFC: The plot of 1/kobs versus 1/[Alcohol] Click here to View figure

Effect of Hydrogen ions

PTS was used for the supply of H⁺needed to increase the rate of the reaction. The rate of secondary alcohols oxidation was increased with the increase in the (H⁺) acidity as shown in the (Table 2)and Figure (4). The dependence of hydrogen ions can be represented by the following equation

Table 2: Dependence of rate on [H⁺]

[Sec.Alcohol] 0.10 L-1   [BIFC] 0.001 mol L-1      Temperature 298 K
[H+]/mol L-1	0.1	0.2	0.4	0.6	0.8	1.0
2-hydroxy propane/(104kobs/s-1)	6.41	9.34	11.5	13.3	15.9	18.6
2-hydroxy butane/(104kobs/s-1)	10.9	15.2	19.6	22.4	25.7	29.8
2-hydroxy pentane/(104kobs/s-1)	14.8	19.2	25.5	31.7	35.9	41.6
2-hydroxy hexane/(104kobs/s-1)	16.4	22.8	30.1	36.6	43.7	49.9

 

Figure 4: Effect of Hydrogen ion. Click here to View figure

Kinetic isotope effect

Primary kinetic isotopic effect^24,25 was exhibited by the deuterated benzhydrol (PhCDOHPh) and k_H/k_Dvalue of 5.76 was obtained at a temperature of 298 K^20,21.A significant initial kinetic isotopic effect was demonstrated by the PhCDOHPh oxidation. This confirmed the fact that the cleavage of α-C-H bonds is taking place in the rate-determinationstep²⁶. A significant initial kinetic isotopic effect was demonstrated by the PhCDOHPh oxidation (Table 3).

Influence of temperature on the reaction rate

The kinetic study of ten secondary alcohols oxidation has been done at different temperatures viz. 318,308,298 and 288K in DMSO solvent and the calculations of activation parameters has been done (Table 3). The log k₂ at different temperatures was linearly related^20,21 to 1/T in all the alcohols as shown in the (Figure 5), which verifies the Arrhenius equation²⁷ for these oxidations.

Table 3: Enthalpy, entropy, free energy and rate constants obtained for the secondary alcohols oxidation by BIFC.

Rate constants at K          Activation parameters
Sec. alcohol	104k2/mol-1dm3s-1				∆H*	∆S*	∆G*
(R)	288	298	308	318	(kJ mol-1)	(J mol-1 K-1)	(kJ mol-1)
Methanol	15.9	33.6	75.4	152	55.2±0.7	-31±2	64.2±0.6
Ethanol	26.5	54.8	115	221	51.6±0.4	-39±1	63.1±0.3
Propanol	46.0	87.9	167	289	44.4±0.4	-59±1	61.9±0.3
i-Propanol	75.0	142	295	480	45.5±1.3	-51±4	60.7±1.0
i-Butanol	124	223	412	673	40.8±0.6	-64±2	59.6±0.5
Butanol	53.2	97	205	385	48.3±1.3	-45±4	61.5±1.0
ClCH2	0.24	0.69	1.64	4.20	69.4±0.9	-16±3	74.0±0.7
MeOCH2	2.02	4.85	11.2	25.4	61.6±0.4	-25±1	69.1±0.3
Phenol	106	223	421	780	47.9±0.4	-40±1	59.7±0.3
Benzhydrol	124	230	466	843	46.6±0.9	-44±3	59.4±0.7
Benzhydrol-a-d	20.7	39.9	82.1	159	49.5±0.9	-48±3	63.8±0.7
kH/kD	5.99	5.76	5.68	5.30

Figure 5: Influence of temperature. Click here to View figure

Influence of solvents

Oxidation of different secondary alcohols was studied in different organic solvents²⁸. We have used many organic solvents as listed in the table (4). Due to the solubility ^20,21 of BIFC and its reaction with alcohols the solvents choice was restricted ¹⁹. There was no reaction found with the selected solvent. The kinetics¹⁹ of all solvents was similar. Therefore, we have taken 2-hydroxy propane as the representative secondary alcohol. The value k₂at 308 K is listed in Table 4. (cf. Fig.6)

Figure 6: Secondary alcohols oxidation by BIFC:  Effect of Solvent. Click here to View figure

Table 4: Effect of Solvents on 2-hydroxy propane oxidation by BIFC at 308 K

Solvent	104k2(s-1)	Solvent	104k2(s-1)
Chloroform	40.5	Ehanoic acid	8.24
1,2-Dichloromethane	45.3	Hexanaphthene	1.46
1,1-Dichloromethane	35.9	Methylbenzene	10.9
Dimethyl sulphoxide	75.4	Phenylethanone	47.8
Acetone	32.1	Tetrahydrofuran	16.7
DMF	66.1	Tert-butyl alcohol	12.4
Butanone	27.4	1,4-Dioxane	13.2
Nitrobenzene	50.1	1,2-Dimethoxyethane	11.8
Benzene	11.8	Carbon disulfide	6.26
Ethyl acetate	12.2

 

Discussion

The correlation²⁹ within the activation parameters like enthalpies and entropies²⁹ of the ten secondary alcohols oxidation was not satisfactory (r² = 0.9391), indicating that the compensation effect was not operative in this reaction³⁰. The correlativity was in accordance with the Exnerʼs criterion^31-32. An Exnerʼs graph³² was plotted between log k₂ at two different temperatures 298 K and at 318 K. The graph was linear and (r²) value was 0.9989 (Figure7). The isokinetic temperature 30 has the value 717±84 K, which was evaluated by using the Exnerʼs plot. The isokinetic relationship³⁰ was determined and it suggested that all the secondary alcohols are oxidized by the same mechanism^20,21 and the change in the reaction rate was controlled by both the changes in the enthalpy and entropy of the activation³⁰.

Figure 7: Exner’s Isokinetic Relationship. Click here to View figure

The oxidation rate constants, k2 in all the solvents used were correlated in the term oflinear solvation energy relationship (LSER) of Kamlet³³ and Taft³⁴.

In this equation, π* representedthe solvent polarity^20,21, β the hydrogen bond acceptor basicityand α is the hydrogen bond donor acidity^20,21. A₀ is the intercept as per the standard line equation. It can be stated here that 12 out of 18 solvents had zero value for α. The correlation analysis results with respect to Eqn. (5), a parametric equation involving π* and β, and separately with π* and β are given below as equations (6) – (9).

Here nis the number of data points and ψ is the Exnerʼs statistical parameter³²

However, the correlation was not satisfactory by Exner’s criterion³² (cf. eqn. 6). The main contribution was due to polarity of thesolvent²⁹. It singly accounted for ca. 85% of the data²⁹.Both β and α played relatively minor roles²⁹. Solvent effect data were analyzed in relation to Swain’s equation³⁵ (10), which included the cation and anion solvating^36,37 conception of the solvent.

Where A denotes the anion-solvating power of the solvent and B denotes the cation-solvating power, C denotes the intercept term and (A+B) indicates the polarity of solvent³³. The rate was determined in different solvents in terms of eqn. (10), by using A and B and separately with (A + B)^36,37.

The rate of oxidation of 2-propanol and other secondary alcohols in different solvents¹⁶ showed excellent correlativity in terms of Swain’s equation³⁵ (cf. eqn. 11), which plays a major role with cation solvating strength. In fact, cation solvating power accounts for 99% of the data. The correlativity with the anion solvating power was minimal. The solvent polarity indicated by (A + B), was also considered for 89% of the data.

Correlation analysis of reactivity

The rate of alcohols oxidation failed to give any important correlation separately with Taftʼs³⁴ σ* and E_s values.There was no significant co-linearity (r² = 0.2332) between σ* and E_s of the 8 substituents.

Therefore, the rates were correlated in terms of Pavelich-Taftʼs dual substituent-parameter equation³⁸ (17).

The correlativity was excellent as the reaction constants were negative as shown in the (Table 4). This supports the fact that in the intermediate state³⁹ of the rate determination step, there is a carbon center with the electron deficiency. This negative steric hindrance indicated a steric acceleration of the reaction¹⁵. It can be described as based on the high ground state energy^20,21 of highly crowded alcohol. The rapid oxidation of 1-phenyl ethanol and benzhydrol happened due to the stabilization of the carbon centre with electron-deficiency in the transition state by the phenyl group by resonance¹⁹.

Table 4: Correlation of rates of Oxidation in terms of Taftʼs equation

Temperature / K	ρ*	δ	r2	sd	ψ
298	-1.75±0.02	-0.69±0.02	0.9998	0.02	0.015
308	-1.63±0.01	-0.60±0.01	0.9999	0.01	0.012
318	-1.56±0.01	-0.52±0.01	0.9998	0.01	0.014
328	-1.47±0.01	-0.44±0.01	0.9999	0.01	0.014

Mechanism

Scheme 2: Mechanism. Click here to View figure

In the present study due to the presence of considerable primary kinetic isotope effect¹⁴ (k_H / k_D = 5.76) at 298 K, it was confirmed that the cleavage of α-C-H bond was taking place in the rate-determining step^24,25. The negative value of reaction constant and the considerable deuterium isotope effect both are indicative of the presence of an electron-deficient carbon centre in the transition state⁴⁰. Therefore, the transfer of hydride-ion from secondary alcohol to the BIFC is suggested. The bigger role of cation-solvating power of the solvents also supports the hydride ion transfer mechanism⁴¹. The hydride ion transfer can occur either via an acyclic bimolecular reaction or may occur due to the involvement of a cyclic, planner and symmetrical transition state of chromate ester. The reaction is a two-step process, which was duly supported by the kinetic studies. Therefore, the whole mechanism is suggested to involve formation of the cyclic chromate ester¹⁵ formation in the rapid first phase. The slower second phase involves decomposition of the cyclic ester^21,22 unit via a cyclic concerted symmetrical transition state via sigmatropic rearrangement leading to the formation of the product as represented above. The polar transition state is also supported by the observed negative value of entropy of activation²⁸. The solvent effect has also indicated that the intermediate state is more polarized than the reactant^40-48.Finally, we can say that BIFC is a very effective, mild oxidant, which can be used for the synthesis of ketonic compounds.

Acknowledgement

We are thankful to our HOD, Professor Kailash Daga, for providing laboratory facilities and chemicals needed for the conduct of this work, executed at the department of Chemistry, Jai Narain Vyas University, Jodhpur, India.

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