A Novel Study of Kinetics and Mechanism of Iridium (III) Catalysed Oxidation of Alcohol by Cerium (IV) In Aqueous Acidic Media

 

A.G. Jadhav1, V.V. Gaikawad1, N.K. Halikar2 and N.T. Patel2*

1Department of Chemistry, Mahatma Gandhi College, Ahmedpur. (MS) India.

2P.G. Department of Chemistry, Yeshwant Mahavidyalaya, Nanded. (MS) India.

*Corresponding Author E-mail: aanil_kumar2006@rediffmail.com

 

ABSTRACT:

Alcohols have many applications in pharmaceutical research and are used extensively at various phases during the drug development process. Alcohol is used in prediction of drug solubility from structure as solubility of drug is an important factor affecting its bioavailability.

In view of such varied importance of different alcohols, in the present study the Kinetics and mechanism of iridium (III) catalyused oxidation of alcohol by cerium (IV) in aqueous sulphuric acid medium have been investigated at different temperatures [Alcohol] >> Ce (IV) >> (IR) (10–5 mol/dm3) in 1 mol dm–3 sulphuric acid medium. The reactions are found to be the zero order with respect to Ce (IV) ion. From the [HSO4]  dependence, Ce (IV) has been found kinetically active the proposed mechanism involves a pre-equilibrium interaction between the catalyst and substrate. The process is acid catalysed. Activation parameters have been determined to investigate the effect of temperature; and probable rate law and mechanism has been proposed.

 

KEYWORDS: Alcohol = 4-methyl 2-pentanol.

 

 


INTRODUCTION:

The use of Ir (III) and Ru (III) in trace amounts as an efficient catalysts in the homogeneous reactions involving Ce (IV) as an oxidant has been reported1,2 several kinetic investigations on redox reactions involving Ce (IV) and different organic and inorganic compounds including different alcohols have been carried out3-8.

 

Kinetic study on the oxidation of ethanol by Ce (IV) in perchloric acid media have been reported by glycols by Ce (IV) and oxalic acid9, malonic acid10, aliphatic ketones and aldehydes11, isobutylic acid and 3-bromopropanoic acid12,13 have been studied. Various investigations are also made on Ce(IV) aqueous H2SO4, where it forms strong sulphato complex14,16. Hence we reported here kinetics and mechanism of Iridium (III) catalysed oxidation of alcohol and glycol by Ce(IV) in aqueous acidic medium.

 

The kinetic study shows that oxidation of Alcohol and Glycol by Ce (IV) in aqueous sulphuric acid media is slow but gets catalysed by Ir (III) at trace concentration (10-5 mol dm-3). It prompted us to explore the kinetic behaviour of the title reaction in detail on metal ion catalysis in Ce (IV) oxidation.

 

MATERIALS AND METHODS:

Cerium (IV) stock solution was obtained by dissolving Ce (IV) (Himedia A.R.) in 1.0 mol dm-3 sulphuric acid. The stock solutions were kept at room temperature for more than 48 hours to attain equilibrium and was standardized with ammonium iron (II) sulphate solution using ferroin as an indicator Glycols were of Across Organics grade. Doubly distilled water was used throughout the experiment. Ir (III) solution was prepared in 1 mol dm-3 H2SO4 solution.


Procedure and Kinetic Measurements:

Kinetic measurements were carried out on a Elico (CL 157 Colorimeter). The progress of the reaction was monitored by measuring absorbance of  at 350 nm. The kinetic study was carried out under pseudo-first order conditions using [Glycols] >> [Ce (IV)]. To a thermally equilibrated solution containing Ce (IV), H2SO4, Ir (III) and glycol solutions were added to form the reaction mixture and progress of the reaction was monitored at regular time intervals by observing variation of optical density with time. H+ ion concentration was maintained by the addition of required amounts of H2SO4. The pseudo first order rate constants were computed by the graphical method and the experimental values were reproducible within +3%.

 

RESULTS AND DISCUSSION:

Under varying concentration of [Alcohols], [Glycols] and [Ce(IV)] kinetics of Ir (III) catalysed oxidation of Alcohol and Glycol by Ce (IV) in aqueous sulphuric acid media  was investigated at several concentrations of the reactants indicating first order kinetics with alcohol and first order with respect to glycol. The results are shown in Table 1.

 

The rate of the reactions were studied under varying [Ce (IV)]. Plots of log [O.D.] versus time were linear indicating a first order dependence in [Ce(IV)] from the pseudo-first order rate constants were computed. Under varying [Ce (IV)]. Plots of log [O.D.] versus time were indicating first order dependence in [Ce(IV)] with respect to alcohol and zero order with respect to glycol.

 

The rates were increased with increase in [Glycols]. From the plots of log [O.D.] versus time, the Pseudo-first order rate constants (Kobs) were evaluated at, different values of [Glycols]. The order in [Glycols] was found to be first order indicating the formation of a complex between Ir(III) and [Glycols].  The fact is supported by the linearity of plot of 1/KObs Versus 1/[Glycols] giving intercept. Under the kinetic conditions, at fixed [Ir] the plot of KObs. Vs [substrate] indicating first order for substrate.

 

Keeping [Ce(IV)], [Alcohol] and [Glycols] constant, increase in Ir (III) increased the rate and the order in Ir (III) was found to be first order  indicating the complex formation between them which has been confirmed by reciprocal plots of 1/KObs Versus 1/Ir (III).

 

There results could be explained by proposing the following equilibrium in which the neutral Ce (IV) was assumed to be the reactive species. The reactions were studied at different temperatures and rate constants at 15-45 0C (Where used to calculate K (specific rate constant)). DH# and DS# for the oxidation of each alcohol and glycol and their values are reported in Table 2.

 

Under the experimental conditions, from an independent experiment, in the absence of substrate, it has been found that Ir (III) catalysed oxidation of water by Ce (IV) is insignificant. Hence, this path was not taken into consideration to calculate the kobs.

When Ir (III) is mixed with excess of Ce (IV) it is  oxidized11-13 rapidly to Ir (IV) by Ce (IV) as shown in equation (1).

 

The effect of temperature has been studied from the range of 150C to 450C, under the experimental conditions. The energy of activation was computed by the Arrhenius plots. The calculated pseudo-first order rate constants at 298.15 k are shown in Table 2.

Table 1 : Pseudo-first order rate constants at 298.15 K for the Ir (III) catalysed oxidation of alcohol and glycol by Ce (IV) in aqueous sulphuric acid medium 0.25 mol/dm3, [H+] = 0.1 mol dm-3 [Ir (III)] = 1 x 10-5 mol dm-3.

 

[Substrate] x 10-2 mol dm-3

Kobs x 102 (min-1)

4-methyl 2-pentanol

2-methyl 2,4-pentane glycol

6.48

0.342

02.21

7.40

0.388

02.737

8.33

0.431

03.066

9.25

0.475

03.46

10.18

0.546

03.896

10.61

0.666

04.418

11.38

0.864

04.788

 

Table 2 : Activation parameters at 298.15K for the Ir (III) catalysed oxidation of Alcohol and Glycol by Ce (IV) in aqueous H2SO4 medium.

Substrate

KObs 10-4       Sec-1

Ea                 J Mol-1

H#                   J-1 Mol-1

S#              J-1 Mol-1

4-methyl, 2-pentanol

1.153

2136.9879

1544.862

-49.8226

2-methyl 2,4-pentane glycol

7.782

9933.0736

9340.948

-24.1311

 

Based on experimental facts such as the rate data, induced polymerization, orders of the species of activation parameters, the most probable mechanism, proposed is as shown in Scheme 1.

 

Scheme 1:

 

Scheme 1 leads to rate law which is in the form of the experimentally observed one –

Where, k obtained from experimentally from the effect of [substrate] on kobs.

At constant [Ir (III)] the plot of  1/KobsVersus 1/[substrate] was found to be linear and from the slope and intercept of which the values of formation constant K and bimolecular rate constants k were calculated.

 

Constancy in the calculated values of DG# for these oxidation reactions indicates that the same type of the reaction mechanism could be operative for the oxidation reactions. The complex formation between Substrate and Ir (III) takes place and finally the complex decomposes into products as per detailed proposed mechanism of oxidation as shown in Scheme 1.

 

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Received on 15.03.2011        Modified on 13.05.2011

Accepted on 09.06.2011        © AJRC All right reserved

Asian J. Research Chem. 4(8): August, 2011; Page 1272-1274