Oxidative decarboxylation of glycine by N-bromobenzimidazole in aqueous acetic acid medium - A kinetic study

 

K. Ramakrishnan, B. Ramkumar*

PG and Research Department of Chemistry, Govt. Arts College, Trichy-620 022, India.

*Corresponding Author E-mail: rk.brchemistry@gmail.com

The oxidative decarboxylation of the glycine by N-bromobenzimidazole (BBI) has been studied in aqueous acetic acid medium. The rate shows first order dependence each on [BBI] and [glycine] and inverse fractional order on [H⁺]. The rate of the oxidation increases with decrease in dielectric constant of solvent medium indicating ion-dipole interaction. The rate of reaction is not influenced by the addition of electrolyte like sodium perchlorate. Polymerization was not observed when acrylonitrile is added to the reaction mixture. The kinetic runs were carried out at four different temperatures and thermodynamic parameters have been evaluated. Addition of benzimidazole retards the rate. A mechanism in consistent with the kinetic data has been proposed.

 

 

 

INTRODUCTION:

The chemistry of N- halo compounds has evoked considerable interest, as they are sources of halonium cations which act as oxidizing agents. Kinetics of oxidation of organic compounds by N-halo compounds have received considerable attention^1-8. N-bromo(1-bromo) benzimidazole (BBI) is gaining importance as a mild oxidant and utilized for the oxidation of various organic substrates^9-12.

 

Oxidation of glycine by various N-halo oxidants such as N-bromosuccinimide¹³,  N-chloropyrazinamide¹⁴, N-chlorosaccharin¹⁵, N-bromophthalimide¹⁶ and N-bromoanisamide¹⁷ has been reported. An extensive literature reveals that no systematic kinetic work hitherto has been done on the oxidation of glycine using BBI. In the present investigation, the reaction kinetics of glycine with BBI has been studied in aqueous acetic acid medium in presence of mercuric acetate.

 

MATERIALS AND METHODS:

BBI was prepared and purified by literature method¹⁸. Acetic acid was refluxed over chromic oxide for 6 hours and the fraction distilling at 118°C was collected and used. Glycine, perchloric acid and all other reagents used were of AR grade.

 

Kinetic measurements

All the kinetic reactions were carried out under pseudo-first order conditions, keeping [glycine] >> [BBI] in solvent system 70% (v/v) acetic acid-water medium at 308K and the courses the reactions were followed potentiometrically.

 

In a typical experiment, the required quantities of the substrate solution, perchloric acid and acetic acid-water mixture were pipetted out in a double walled beaker provided with an inlet and outlet for circulating water from the thermostat set at the desired temperature and the solution were kept in the beaker for nearly half an hour to attain the desired temperature. The reaction was started by pipetting out the required quantity of BBI solution which had also been thermostated for nearly half an hour. The total volume of the reaction mixture was always 25 ml. A stop-watch was started when half the amount of oxidant was added.

 

[Pt|BI⁽⁺⁾—BBI||^(-)SCE]       (BI-Benzimidazole)

 

The reaction was followed by setting up a cell made up of the reaction mixture into which the platinum electrode and saturated calomel electrode (SCE) were dipped. The emf of the cell was measured periodically using Equip-Tronics Digital potentiometer while the reaction mixture was continuously stirred using a magnetic stirrer. The pseudo-first order rate constant, k₁, was computed from the linear (r>0.999) plots of log (E_t- E_∞) Vs time.

 

Where, E_t - potential at time‘t’ and E_∞ - potential at infinity.

 

When the kinetic run was also done by iodometry, the same results were obtained within 2%.  Preliminary experiments showed that the rate of oxidation is not sensitive to change in ionic strength and hence no attempt was made to keep it constant.

 

Stoichiometry and product anlaysis

The stoichiometry of the reaction was determined by equilibrating mixing various ratios of [BBI] versus [glycine] at 308K for 48 hours under kinetic conditions. Estimation of unconsumed BBI revealed that 1 mole of BBI was required to oxidize 1 mole of the glycine.

 

 

 

Product analysis shows that formaldehyde was formed as the major product which was confirmed by spectral studies and TLC. The liberated CO₂ was detected by lime water test¹⁹ and ammonia was confirmed by Nessler’s reagent²⁰.

 

RESULTS AND DISCUSSION:

The oxidations of glycine were carried out in 70% acetic acid - 30% water medium (v/v) in presence of a large excess of substrate and perchloric acid of suitable concentration as the catalyzing acid source and mercuric acetate. The function of added mercuric acetate is only to fix up Br^- formed in the course of reaction as HgBr₄^2-.

The oxidation kinetics of glycine by BBI has the following salient features.

 

Effect of oxidant variation:

The kinetics of oxidation of glycine has been studied at various initial concentration of the BBI and at fixed concentration of other reactants (Table-1).The plot of log(E_t-E_∞) versus time yields a straight line indicating a first order dependence of rate on oxidant.

 

Table-1: Effect of oxidant on the reaction rate

 

[glycine]  = 6.0 x 10^-2 mol.dm^-3               Solvent    = 70% CH₃COOH

[HClO₄]   = 5.0 x 10^-2 mol.dm^-3               Temperature= 308 K

[mercuric acetate]    = 5.0 x 10^-2 mol.dm^-3

 

 

Effect of substrate variation:

The rate of reaction increases linearly with an increase in the concentration of glycine. (Table-2). The plot of 1/k_obsversus 1/[substrate] gave a straight line passing through origin indicating a first order dependence on substrate.

 

Effect of [H⁺] variation:

The dependence of the reaction rate on the concentration of H⁺ was studied at constant concentration of oxidant, substrate and other reagents and varying the initial concentration of HClO₄. The k_obs values decrease slightly with increase in the HClO₄ concentration (Table-2) and the order was found be inverse fractional.

 

Effect of acetic acid variation:

The effect of dielectric constant of reaction medium was studied by adding acetic acid in the reaction medium at constant concentration of other reactants. The rate of reaction increases with increasing acetic acid content in the solvent medium (Table-2).

 

Table-2: Effect of Substrate, HClO₄ and CH₃COOH on the reaction rate

 

[BBI]= 6.0 x 10^-3 mol.dm^-3          Temperature= 308 K                               

[mercuric acetate] = 5.0 x 10^-2 mol.dm^-3

 

 

 

Effect of temperature variation:

The reaction has been studied in the temperature range 308-323 K and the results are recorded                    (Table-3). Using Arrhenius equation, the energy of activation for the substrate has been calculated and this value was subsequently utilized in computing various other thermodynamic parameters. The results are presented in Table-4.

 

Table-3: Effect of temperature on the reaction rate

 

[BBI]       = 6.0 x 10^-3 mol.dm^-3               [glycine] = 6.0 x 10^-2 mol.dm^-3

Solvent    = 70% CH₃COOH    [HClO₄]   = 5.0 x 10^-2 mol.dm^-3

[mercuric acetate]    = 5.0 x 10^-2 mol.dm^-3

 

Table-4: Activation parameters for the oxidation of glycine

 

 

The effect of one of the products of the oxidation has been investigated by adding various [benzimidazole] keeping all other reactant concentration as constant. There is a slight decrease in reactivity with the increase in the initially added benzimidazole (BI). The retardation of rate on the addition of benzimidazole suggests a pre-equilibrium step that involves a process in which benzimidazole is one of the products.

 

Mechanism and rate law

The decrease in the reaction rate with increasing [HClO₄] and retardation of reaction rate with added benzimidazole suggest HOBr being the most probable oxidizing species.

 

The following mechanism has been proposed

 

 

 

Rate law is given by

 

(S- glycine, BI - benzimidazole)

 

 

CONCLUSION:

The kinetic study clearly demonstrates the formation of hypobromite ester followed by its cleavage to yield the products in the oxidative decarboxylation of the glycine by N-bromobenzimidazole in aqueous acetic acid medium. The rate law shows first order dependence each on [BBI] and [glycine] and inverse fractional order on [H⁺]. The mechanism proposed for this oxidation kinetics is in accordance with the observed kinetic facts.

 

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Received on 30.12.2017         Modified on 28.02.2018

Accepted on 20.03.2018         © AJRC All right reserved