Ruthenium (III) Catalyzed Oxidation of Calcium Dobesilate Monohydrate by Chloramine-B in Alkaline Medium. A Kinetic and Mechanistic Approach

 

Diwya¹, R. Ramachandrappa², Pushpa Iyengar¹*

¹East Point College of Engineering and Technology, Bidarahalli, Virgo Nagar Post Bangalore-560049, Karnataka, India.

²Department of Chemistry, Jyoti Nivas College, Koramangala Industrial Layout, Bangalore - 560095, Karnataka, India.

*Corresponding Author E-mail: pushpaiyengar@yahoo.co.in

ABSTRACT:

Kinetics and oxidation of Calcium Dobesilate(CDM) (calcium 2,5-dihydroxybenzenesulfonate) by sodium – N- chlorobenzene sulphonamide(Chloramine-B or CAB) using Ru(III) catalyst in NaOH medium has been studied at 303K. Oxidation reaction follows kinetics of first order with [CAB], positive first order dependence on [CDM], positive fractional order slope with [NaOH] and [RuCl₃]. Effect of halide ions and added benzene sulphonamide has negligible effect on the rate of reaction. Variation in ionic strength has no effect on the rate of the reaction indicating that non-ionic species are involved in the rate limiting step. Positive effect of Dielectric constant was observed. Kinetic parameters were evaluated by studying the reaction at different temperatures. Addition of reaction mixture to aqueous acrylonitrile did not initiate polymerization showing the absence of free radical species. Oxidation products were identified. PhSO₂NCl^— of CAB, the reactive species which combines with the substrate to give product in presence of catalyst. Based on kinetic results, reaction stoichiometry and oxidation products, a suitable mechanism have been proposed.

 

 

INTRODUCTION:

Redox reactions have been the subject of detailed mechanistic and kinetic studies as they are most commonly encountered in chemical analysis. Investigation reveals that oxidation of compounds using other oxidants have been carried out^(1—8). Organic haloamines resemble hypohalites in their oxidative behaviour, but are more stable than hypohalites and are developing rapidly as oxidants, disinfectants and antiseptics. Diverse nature of chemistry of haloamines is the consequence of their ability to act as oxidizing and halogenating agents in acidic and alkaline media. They react surprisingly with a wide range of industrially and biologically important functional groups affecting an array of molecular transformations. Halo amines furnish halonium cations in the +1 oxidation state and are generally known to undergo a two electron change in its reaction.

 

Chloramine-B(Sodium N-Chlorobenzenesulfonamide or CAB; C₆H₅ClNO₂S·Na) a prominent member of the series of haloamines was first proposed by Alfanas’ev as a substitute for Chloramine-T in volumetric work and can be prepared in the lab. Literature survey shows that lot of work have been carried out using CAB as oxidant^(9-14).

 

Calcium Dobesilate Monohydrate (CDM; calcium 2,5-dihydroxybenzenesulfonate), is a vasoactive drug^(15,16)  with presumed effects on endothelial integrity, capillary permeability and blood viscosity. It is often recommended for venous disorders, diabetic retinopathy and other microvascular disorders. It selectively acts on capillary walls regulating their physiological functions of resistance and permeability. Since the reaction was fast when Bromamine – T reacted with CDM, kinetics of oxidation of CDM (calcium 2,5-dihydroxybenzenesulfonate) by CAB in alkaline medium using ruthenium (III) chloride as catalyst has been studied at 303K.

 

MATERIAL AND METHODS:

Chloramine – B was prepared by action of chlorine on benzenesulphonamide in presence of NaOH. Benzene sulphonamide was added gradually to 4-5 N NaOH solution (2-3 moles) at 25ºC with constant stirring. Homogenous solution was heated at 65-70ºC and chlorine was bubbled slowly for a period of 1 hour. The mass was stirred for one hour at same temperature then subsequently heated to 85ºC and was then filtered through a schotts funnel. A 99% yield was produced and was confirmed by spectral studies. It was preserved in glass stoppered colored bottles at room temperature.

 

Table 1: Effect of varying [CAB], [CDM], [RuCl₃] and [NaOH] on the reaction rate at 303K:

104 [CAB]o mol dm -3	103 [CDM] mol dm -3	103 [NaOH] mol dm -3	106  [RuCl3] mol dm -3	104k/ s-1
5.0 10.0 15.0 20.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0	10.0 10.0 10.0 10.0 5.0 10.0 15.0 20.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0	50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 1.00 5.00 10.0 15.0 50.0 50.0 50.0 50.0	2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 1.0 2.0 4.0 6.0	2.60 2.56 2.55 2.58 1.36 2.60 3.80 5.08 1.00 1.69 2.00 2.17 0.81 2.60 6.62 9.28

Calcium dobesilate monohydrate (Biocon, Bangalore, India) and RuCl₃ (Arora-Mathey, Bombay, India) was used without further purification. An aqueous solution of CAB was standardized iodometrically and preserved in brown bottles to prevent photochemical degradation. All other chemicals used were of analytical grade. Double distilled water was used in preparing all aqueous solutions. Permittivity of the reaction medium was altered by addition of methanol in varying proportions (v/v) and values of permittivity of methanol – water mixtures reported in literature were employed¹⁷.

 

Kinetic procedure:

All reactions were performed under pseudo-first order conditions ([CDM]_o >> [CAB]_o) at 303K, in a glass stoppered borosil boiling tubes coated black on outside to prevent photochemical effects. For each run, requisite amounts of solution of CDM, NaOH, RuCl₃ and H₂O (total volume kept constant) were introduced into the tube and thermally equilibrated at 303 ±1K. Reaction was initiated by rapid addition of measured amount of CAB solution to the above mixture and was shaken intermittently. Progress of the reaction was monitored by iodometric determination of unreacted CAB in a measured (5 cm³) aliquot of the reaction mixture at different time intervals. Reaction was studied for more than two half-lives. Pseudo-first order rate constants (k^/), calculated from the linear plots of log [CAB] versus time were reproducible to ± 4%.

Stoichiometry

Reaction mixtures containing a known excess of [CAB] over [CDM] were kept in the presence of NaOH and RuCl₃ at 303K for 72 hours. Estimation of unreacted CAB showed that one mole of CDM was consumed per mole of CAB to give corresponding p-benzoquinone.

 

Ca²⁺[C₆H₅SO₃]₂.H₂O + C₆H₅SO₂NClNa                  2C₆H₄O₂+ C₆H₅SO₂NH₂ + NaCl + Ca²⁺ + H₂O + SO₂ + SO₄^2-…….(1)

 

Fig 1: Effect of [CDM] on the reaction rate. Plot of log[CDM] vs logk^/

 

Fig 2: Effect of [NaOH] on the rate of the reaction. Plot of log[NaOH] vs logk

 

Product analysis:

Oxidation product p-benzoquinone was detected by spot tests¹⁸ ie. 2, 4- dinitrophenyl hydrazine test and sodium nitroprusside test and was confirmed by IR spectroscopy. Strong peak for CHO group was observed around 1725   cm^-1. Reduction product of the oxidant,                       p-benzenesulphonamide was detected by thin layer chromatography¹⁹ using light petroleum-chloroform-butan-1-ol (2:2:1v/v/v) as solvent and iodine as reducing agent (R_f=.0.88). The reported R_f value is consistent with that given in the literature²⁰. Further it was confirmed by its melting point 150-151^oC (melting point: 149-151^oC) and IR spectra.

 

 

Fig 3: Effect of [RuCl₃] on the rate of the reaction. Plot of log[RuCl₃] vs log k^/

 

RESULTS:

1.       Effect of reactants on reaction rate :

Under pseudo-first order conditions of [CDM]_o>> [CAB]_o,  at constant [CDM]_o, [NaOH], [RuCl₃]and temperature, plots of log k^/ vs log[CAB] were linear indicating a first order dependence of the reaction rate on [CAB]. Rate constant was unaffected with change in [CAB](Table1). Increase in [CDM] lead to increase in the k^/ values (Table1). Plot of logk^/ vs [CDM] (Figure 1) was linear with a positive slope of 1.0 indicating first order dependence on [CDM].Reaction was studied with varying [NaOH] at other fixed experimental conditions. Rate increased with increase in [NaOH] (Table 1) and a plot of log k^/ vs log [NaOH] (Figure 2) was linear with positive fractional slope of 0.56. As [Ru(III)] was increased, rate also increased(Table 1). In presence of catalyst, the reaction was three times faster than uncatalyzed catalyst. Plot of logk^/ vs log[Ru(III)] (Figure 3) was linear with a positive slope of  1.102 showing first order dependence on [Ru(III)]. Catalytic constant K_c was found to be 3.65x10¹.

 

2.       Effect of halide ions and benzene sulphonamide:

At constant [OH^–-], addition of NaCl or NaBr did not affect the rate of the reaction (Table 2). Hence the rate is independent of [NaCl] and [NaBr] and depends only on [OH^–-]

 

Addition of reduction product of the oxidant i.e., benzene sulphonamide did not affect the rate of reaction(Table 2), indicating its non involvement in pre-equilibrium step with the oxidant.

 

Table 2: Effect of [Cl^-], [BSA] and [NaClO₄] on the reaction rate:

103[Conc.] mol dm-3	[NaCl] 10-4 k/(s-1)	[BSA] 10-4 k/(s-1)	[NaClO4] 10-4 k/(s-1)	[NaBl] 10-4 k/(s-1
1.0 4.0 5.0 10.0	2.58 2.26 2.29 2.36	2.25 2.26 2.37 2.25	2.27 2.26 2.37 2.35	2.58 2.60 2.47 2.45

 

3.       Effect ionic strength and dielectric constant of the medium:

Variation of ionic strength using NaClO₄ solution did not affect the reaction rate indicating that non-ionic species are involved in the rate limiting step (Table 2).

 

Dielectric constant (D) of the solvent was varied by adding methanol to the reaction mixture. Addition of methanol resulted in increase in reaction rate supporting the rate limiting step with no charge dispersal (Table 3). Plot of (10²/D) vs log k^/ (Figure 4) was linear with positive slope of 2.6. Values of dielectric constant (D) of water-methanol system of different compositions are reported in the literature. Blank experiments with methanol showed that MeOH was oxidized slightly (3%) by the oxidants under experimental conditions. This was corrected for calculation in the net reaction for the rate constant for the oxidation of CDM.

 

Table 3 -Effect of Dielectric constant of the medium on the reaction rate.:

MeOH (% v/v)	D	102/D	104k/(s-1)
0 10 20 30	76.73 72.37 67.38 62.71	1.30 1.38 1.48 1.60	2.6 4.4 7.7 15.1

 

Fig 4: Effect of dielectric constant on the rate of the reaction. Plot of 10²/ D vs logk^/

 

4. Effect of temperature:

Reaction was studied over a range of temperature from 293K to 323K (Table 4) by varying the [CDM] and keeping other experimental conditions constant. It was found that the rate of the reaction increased with increase in temperature (Figure 5). From the linear Arrhenius plots of logk^/ vs 1/T (Figure 6), the activation parameters were evaluated (Table 5).

 

Table 4: Effect of [CDM] on the reaction rate at different temperatures:

103 [CDM]		104k/ (s-1)
mol dm3	293K	303K	313K
5.0 10.0 15.0 20.0	0.58 1.19 2.41 3.31	1.36 2.60 3.80 5.08	2.88 5.69 7.23 11.6

 

ig 5: Effect of [CDM] on the rate of the reaction at different temperatures. Plot of log[CDM] vs logk^/

 

Table 5: Effect of varying temperature and the values of activation parameters:

Temperature (K)	104k/(s-/)	Activation parameters
293 303 313 323	1.19 2.60 5.69 10.2	Ea =  65.100 kJ mol-1 ∆H≠  = 62.58 kJ mol-1 ∆G≠ = 95.01 kJ mol-1 ∆S≠ = - 107.00 JK mol-1 logA = 7.6585

 

5. Test for free radicals:

Addition of acrylonitrile to the reaction mixture did not initiate polymerization indicating absence of free radicals in the reaction mixture.

 

6. Catalytic activity:

Moelwyn- Hughes⁽²¹⁾ have pointed the relationship between catalyzed and uncatalyzed rate constants.

 

k_T = k_u + K_c[Ru(III)]^x

Where, k_T is the observed rate constant in presence of Ru(III), k_u is the rate constant in absence of catalyst, K_c is the catalytic constant, x is the order with respect to Ru(III). Catalytic constant was found to be 3.65x10¹. Values of K_c were evaluated at different temperature. Plot of logK_c vs 1/T(plot not shown) was linear and thermodynamical parameters were calculated (Table 5). 

 

Fig 6: Effect of Temperature on the rate of the reaction. Plot of 10³/T vs logk^/

 

DISCUSSIONS:

Investigations by Pryde and Soper²², Morris et.al²³, Bishop and Jennings²⁴ and Hardy and Johnston²⁵ on sodium-N-haloarenesulfonamides have shown that Chloramine-B is similar to Chloramine - T and behave as strong electrolyte in aqueous solution forming different species as shown in equations.  (2 - 4)

PhSO₂NClNa                   PhSO₂NCl ^—  +  Na⁺…(2)

PhSO₂NCl ^— + H₂O   PhSO₂NHCl + OH⁻    ....(3)

PhSO₂NHCl + H₂OPhSO₂NH₂  + HOCl              (4)

 

Possible oxidizing species in alkaline Chloramine-B solution is PhSO₂NHCl, HOCl and PhSO₂Cl^—.Oxidation potential of haloamine sulfonamide system is pH dependent and decreases with increase in pH of the medium. In alkaline solution of haloamine, dihaloamine does not exist and the possible species is the anion PhSO₂Cl^—. If HOCl is the oxidizing species, a first order retardation of rate by added benzene sulfonamide (PhSO₂NH₂) will be observed which is contrary to the experimental observations. If PhSO₂NHCl is the reactive species, retardation of rate by [OH^—] is expected, which is contradictory to the obtained experimental results. Hence, PhSO₂NCl^— can be considered to be as a reactive species.

 

In aqueous solution of [RuCl₃], Ru(III)^(26-29) can exist as [RuCl₆]^3—, [RuCl₅(H₂O)]^2—, [RuCl₄(H₂O)₂]^— , [RuCl₃(H₂O)₃], [RuCl₂(H₂O)₄]⁺, [RuCl(H₂O)₅]²⁺ and [Ru(H₂O)₆]³⁺.  [RuCl₆]^3- complex ion is predominant in high [Cl^-]. As the number of Cl^- ions decreases in the complex, the rate of Cl^- ion replacement by H₂O decreases. Therefore the aquation of [RuCl₆]^3-→ [RuCl₅(H₂O)]^2- occurs within seconds while the half reaction time for the conversion of [RuCl(H₂O)₅]²⁺→ [Ru(H₂O)₆]³⁺ is approximately one year. Since [RuCl₆]^3- was dissolved in HCl, the rate of aquation of [RuCl₆]^3-→[RuCl₅(H₂O)]^2- is fast, the active catalyst species may be assumed to be either [RuCl₆]^3— or [RuCl₅(H₂O)]^2— ion as shown in the following equilibria [Eqs. (5) and (6)]:

 

RuCl₃ + 3HCl  [RuCl₆]^3— + 3 H⁺                   …   (5) 

[RuCl₆]^3— + H₂O    [RuCl₅(H₂O)]^2—  +  Cl^—         (6)

 

Ramachandrappa et.al³⁰ used the above equilibria in Ru(III) catalyzed reaction in HClO₄ solution. In present case since there was no effect of chloride ion on the reaction rate, Eq 6 does not play a role in the reaction and hence the complex ion, [RuCl₆]^3— is assumed to be the reactive catalyst species.

 

Ultraviolet spectral measurements showed that Ru(III) and CAB had absorption bands around 204 and 224nm respectively in presence of 0.1M NaOH. Mixtures of CAB + Ru(III) in presence of 0.1M NaOH showed no change in λ_max. When CAB and CDM solutions were mixed in presence of 0.1M NaOH, a single sharp absorption band appeared at 210nm indicating an intermediate formation between CAB and CDM. Based on preceding discussion, a general mechanism(Scheme 1) is proposed for RuCl₃ catalyzed oxidation of Calcium Dobesilate by CAB to account for observed kinetics.

 

Based on the preceding discussion, following scheme of rate law is proposed for the oxidation of CDM in NaOH medium in presence of RuCl₃ homogeneous catalyst system

TsNHCl + OH^―       Ts NCl^―  + H₂O          fast (i)

Ts NCl^― + [Ru(III)]  X                         fast (ii)

X +  CDM         X^/                 slow and rds (iii)

X^/               products                               fast (iv)

 

 

(Scheme 1)

In Scheme 1, X and X^/ represents the intermediate species, whose structures are shown in Scheme 2 in which a detailed mechanistic interpretation of CDM oxidation by CAB in alkaline medium is proposed. The reactive TsNCl^― species interacts with the catalyst in a fast step (ii) to form X. Furthermore, X reacts with CDM, in a fast pre – equilibrium step (iii) to form complex X^/. Finally X^/   hydrolyses in the slow, rate limiting step forming end products.

 

If [CAB]_t represents total concentration of the oxidant, then from the above scheme,

[CAB]_t  = [PhSO₂NHCl]  +  [PhSO₂NCl ^―]  + [X]………(7)

 

From the above, the following rate law can be derived:

(8)

From the slow step (iii) of Scheme I,

Rate = k₃[CDM][X]……………………………(9)

 

 

 

Scheme 2: Mechanism of oxidation of  Calcium Dobesilate by Chloramine - B

 

 

By substituting [X] from the above equation in Eq 8, the following rate law is obtained:

…(10)

 

The above rate law is in good agreement with the experimental results, where in a first order dependence of rate on both [CAB] and [CDM]_o and fractional order dependence on each [OH^―] and [RuCl₃] is observed. A plot of 1/[OH^―] vs 1/k^/ gives the value of the constant K₁ which was found to be 5.4. Similarly a plot of 1/[CDM] vs 1/k^/ gives the value 3.65 x 10^-3 which is k₃ and a plot of 1/[RuCl₃] vs 1/k^/ gives the value 0.044 which is K₂.

 

Effect of varying solvent composition on the rate of reaction has been described in several publications for the limiting case of zero angle of approach between dipoles or an anion – dipole system. Amis³² has shown that a plot of logk^/  vs 1/D gives a straight line with a negative slope for reaction between a negative ion and a dipole or between two dipoles, while a positive slope indicates a reaction between a positive ion and a dipole. Since the dielectric effect is positive in present studies, it supports with no charge dispersal interaction in the rate limiting step(iii) of Scheme 1.  Addition of halide ions has no effect on the rate indicating no interhalogen or free chlorine is formed. Variation of ionic strength of the medium does not alter the rate indicating the involvement of non ionic species in the rate limiting step. Proposed mechanism is further supported by moderate values of energy of activation and other activation parameters. Fairly high positive values of free energy of activation and enthalpy of activation indicate that the transition state is highly solvated, while the large negative entropy of activation suggests the formation of compact activated complex with less degrees of freedom.

 

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Received on 01.06.2012        Modified on 05.07.2012

Accepted on 09.07.2012        © AJRC All right reserved