INTRODUCTION:

In the carbohydrate field, many researchers have investigated the oxidative degradation of monosaccharides and their derivatives. This was done by different oxidants such as chromium (VI), vanadium (V), cerium (IV), cobalt (III) and iron (III) in aqueous sulfuric, perchloric acids and alkaline medium^[1-13]. However, despite the extensive study made on the kinetics of the oxidation of monosaccharides by these oxidants, the use of surfactants in similar studies is not so common^[14-18]. The primary aim is to ascertain the catalysed reaction of D-ribose by titrimetric method in view of the analytical, synthetical and biological importance of this sugars. Thus it is beneficial to explore the kinetics of oxidation of sugars by cerium(IV).

MATERIALS AND METHODS:

An aqueous solution of cerium(IV) and D-(+) ribose (E. Merck) was prepared afresh by dissolving a weighted amount in double distilled water. The solution was standardized iodometrically against standard solution of sodium thiosulphate using starch as an indicator.

The solution of lanthanum trichloride (E. Merck) was prepared by dissolving the sample in sulfuric acid of known strength. Cerium(IV) [0.1M] acidified with sulfuric acid in the presence of La(III) and a known concentration of KHSO₄(salts) solution is also taken in a 250 ml iodine flask and placed in a thermostate for an hour to attain the temperature of the bath. Cerium(IV) is stable in acidic solution and do not show photochemical decompose. Hence, the rates could be measured in daylight^[19]. Aliquots of the reaction mixture were withdrawn quickly at known intervals of time and poured into another iodine flask containing a drop of 4% potassium iodide solution to arrest the reaction. Librated iodine was titrated against standard sodium thiosulphate solution upto a starch end point. A micro burette was used for this purpose. From the titer value, the amount of cerium (IV) present in the aliquot could be easily determined.

Product Identification

Formic acid formation and respective aldopentose were confirmed by spot test^[20] and also by paper chromatography and high performance liquid chromatographic method. Formation of intermediate carbon centered aldopentose free radicals were confirmed by induced polymerization reaction with acrylonitrile and EPR spin trapping method^[21].

RESULTS AND DISCUSSION:

Under the conditions [S]>>[CeIV]>>[La(III)], the reaction is studied at different concentrations of oxidant cerium(IV) at constant concentrations of other reactants. The order of reaction with respect to oxidant cerium(IV) is determined at constant concentration of substrate D-(+)ribose for different concentration of cerium(IV). The results are given in Table(1). The results show that the rate constant is inversely proportional to the concentration of cerium(IV) for catalysed system. In the presence of catalyst La(III) the plot of k₁v/s Ce(IV) concentration are found to be linear(Figure1). This indicates that first order kinetics with respect to cerium(IV).

In order to study the behavior of substrate D-(+)ribose reaction rates, different sets of the experiments were carried out at different concentration of D-(+) ribose keeping concentration of other reactants constant. The observations are given in (Table 2).The result shows that the graphical plot for the pseudo first order rate constant k₁v/s D-ribose concentration is found to be a straight line (Figure 2a), which indicates thatthe rate of the reaction is directly proportional to the substrate concentration. The plot of log k₁v/s log[D-ribose] is linear(Figure 2b). This indicates that the order with respect to substrate D-(+) ribose is one. In order to see the effect of H⁺ion concentration on the reaction velocity, the reaction has been carried out at various initial concentration of sulphuric acid, while fixed concentration of other reactants constant. The results so obtained are represented in (Table 3). Form the table 3 it was found that the rate of reaction decreases with the increase of sulphuric acid concentration in La(III) catalysed oxidation. The plot of k₁v/s 1/[H⁺] and log k₁ v/s log [H⁺] are linear (Figure 3a and Figure 3b). The result indicates that the order with respect to [H⁺] is inverse first. In order to see the effect of catalyst lanthanum(III)on the reaction velocity, the reaction has been carried out at various initial concentration of lanthanum trichloride. The result so obtained are given in (Table-4). The table 4 indicates that the rate is dependent on the catalyst concentration. When a graph is plotted between La(III) concentration and the rate constant, a linear curve is obtained indicating that the rate is linearly related to LaCl₃ concentration. The plot of logk₁v/s log[La(III)] is linear (Figure 4). The reaction rate increases with increase in La(III), suggesting that rate is directly proportional to the La(III).

The reactions were studied at different concentration of [KHSO₄], while keeping all reactants constant. The observations are given in (Table-5). The graphical plot of log k₁v/s log[KHSO₄] is found to be a straight line (Figure 5),Which indicates that the rate of the reaction is inversely proportional to the HSO₄^-ion concentration.

To observe the effect of temperature on the reaction rate, the reaction was studied at six different temperatures from 308K to 333K, while keeping all other reactants are constant. The observations are given in(Table-6).

The kinetic data shows that the velocity of reaction increases with rise in temperature, showing the validity of the Arrhenius equation in figure 6. The plot of logk₁ vs 1/T is linear. So an attempt has been made to correlate the various activation parameters on the reaction mechanism.

Table-1; Effect of variation of [Cerium(IV)], on the reaction rate at 308K

10²[D-ribose]=5.00 mol dm^-3; 10⁷×[La(III)]=4.90 mol dm^-3; 10²[H₂SO₄]=3.00 mol dm^-3; 10³[KHSO₄]= 5.00 mol dm^-3

Run No	10³[Ce(IV)] mol dm^-3	10⁴× k₁ sec^-1
1	1.00	7.21
2	3.00	5.23
3	5.00	4.64
4	7.00	4.12
5	9.00	3.51
6	11.00	2.87
7	13.00	3.90

Figure 1 Plot of k₁ v/s [Ce(IV)]

Table-2; Effect of variation of [D-ribose] on the reaction rate at 308K

10³[Ce(IV)]=3.00 mol dm^-3; 10⁷×[La(III)]=4.90 mol dm^-3; 10²[H₂SO₄]=3.00 mol dm^-3; 10³[KHSO₄]= 5.00 mol dm^-3

Run No	10²x[D-ribose]mol dm^-3	k₁x10⁴sec^-1
1	1.00	4.91
2	2.00	5.76
3	4.00	8.79
4	6.00	11.53
5	8.00	14.64
6	10.00	17.58
7	12.00	21.21

Figure 2(a) Plot of k₁ v/s [D-ribose]

Figure 2(b) Plot of k₁ v/s log[D-ribose]

Table-3;Effect of variation of [H⁺]on the reaction rate at 308K

10³[Ce(IV)]=3.00 mol dm^-3; 10⁷×[La(III)]=4.90 mol dm^-3; 10²[D-ribose]=5.00mol dm^-3; 10³[KHSO₄]= 5.00 mol dm^-3

Run No	10²x[H₂SO₄] mol dm^-3	k₁x10⁴sec^-1
1	2.00	7.76
2	3.00	6.61
3	4.00	5.76
4	5.00	5.50
5	6.00	5.25
6	7.00	4.90
7	8.00	4.68
8	9.00	4.48

Figure 3 (a) Plot of k₁ v/s [H⁺]^-1

Figure 3(b) Plot of log k₁ v/s log [H⁺]

Table-4;Effect of variation of [La(III)]on the reaction rate at308K

10³[Ce(IV)]=3.00 mol dm^-3; 10²×[ H₂SO₄]=3.00 mol dm^-3;10²[D-ribose]=5.00mol dm^-3; 10³[KHSO₄]= 5.00 mol dm^-3

Run No	10⁷ x [La(III)] mol dm^-3	k₁x10⁴ sec^-1
1	4.90	7.76
2	5.90	8.92
3	7.90	12.03
4	8.90	12.88
5	9.90	13.80
6	10.90	15.49
7	11.90	17.38
8	12.90	18.55

Figure 4 Plot of logk₁ v/s log[La(III)]

Table-5;Effect of variation of [KHSO4]on the reaction rate at 308K

10³[Ce(IV)]=3.00 mol dm^-3; 10²×[ H2SO4]=3.00 mol dm^-3;10²[D-ribose]=5.00 mol dm^-3;10⁷ ×[La(III)]]=4.90 mol dm^-3

Run No	10³x[KHSO₄] mol dm^-3	k₁x10⁴ sec^-1
1	0.00	7.76
2	2.00	9.55
3	3.00	11.23
4	5.00	12.31
5	8.00	14.80
6	9.00	15.49
7	10.00	16.22
8	12.00	17.24

Figure 5 Plot of log k₁ v/s log [KHSO4]

Table-6;Effect of variation of [Temperature]on the reaction rate

10³[Ce(IV)]=3.00mol dm^-3;10²×[ H₂SO₄]=3.00mol dm^-3;

10²[D-ribose]=5.00mol dm^-3;

10⁷×[La(III)]=4.90 mol dm^-3; 10³[KHSO₄]= 5.00 mol dm^-3

Temperature in Kelvin	1/T x10^-3	K₁x10⁴sec^-1
308	3.24	7.76
313	3.19	9.34
318	3.14	11.23
323	3.09	13.81
328	3.04	16.99
333	3.00	19.57
Kinetic and activation parameters for La(III) catalysed reaction
Parameter		D-(+)ribose
E_a* (kJ mol^-1)		31.55
ΔH* (kJ mol^-1)		28.91
ΔS* (J mol^-1)		-126.13
ΔG* (kJ mol^-1)		69.02
log A		6.23

Figure 6 Plot of log k₁ vs 1/T

Energy and Entropy of Activation

The result shows that the average value of energy of activation energy (E_a) was found to be 31.55 kJ/mol for lanthanum(III) catalysed oxidation. The value of frequency factor at 318K is 6.23 min^-1and entropy of activation at 318K is -126.13 J mol^-1 and free energy of activation(ΔG*) 69.02 kJmol^-1.The value of entropy of activation is found to be negative. The fairly high value of negative ΔS*suggests the formation of more order activated complex, whereas the high positive value of the free energy of the activation (ΔG*) and enthalpy of activation (ΔH*) indicate that the transition state is highly solvated. Energy of activation, free energy of activation and entropy parameters suggest that La(III) forms the activated complex more easily compared to the others. Mechanism consistent with observed rate laws have been suggested.

Reaction Mechanism

The kinetic data fit well with the Michaelis-Menten model, suggesting that 1:1 type complex of substrate D(+)ribose and La(III) catalysed is formed in the first equilibrium step. The kinetics of this reaction were studied and showed that the D-(+)ribose, cerium(IV) and catalyst La³⁺ion interact in two equilibrium steps to form an intermediate complex^[22-24]which is assumed to disproportionate forming a free radical and reduced to Ce⁺³ ion. It is believed to involvement of both C₁ and C₂ hydroxyls ^[25] in a complex.

Substrate is easily protonised in acid media in the presence of catalyst, indicating involvement of H⁺ in the pre equilibrium step. Cerium(IV) has been found kinetically active in this study with generation of free radicals in the reaction. Thus a mechanism consistent with the above kinetics is proposed (Scheme 1).

Scheme 1 Mechanism of oxidation of ribose in the presence of lanthanum(III) catalyst

On the plot of 1/k_obs against 1/[S] is made from which the constants1/ k_sk₁ and k₂/k_sk₁k₃ are determined from the slope and intercept respectively. According to the equations mentioned in the above; when plots are made between 1/k_obsand 1/[S] a positive intercept would be observed which confirms the validity of the mechanism and also the rate law. Equation (9) also suggests that the plot of 1/k_obs versus 1/[H⁺] at constant[La(III)] and [S] should also be linear. 1/k_obs versus 1/[La(III)] at constant [S] and [H⁺] should yield good linear plots through the origin. The values of k_sk₁k₃ and k₂ for [S] can also be calculated from the double reciprocal plots as shown in the graphs.

Since La(III) is inert in the proposed mechanism, it may bond to[Ce⁴⁺] to form an outer-sphere complex(La⁴⁺…..Ce³⁺), which is rapidly reduced into an inner-sphere complex by D-(+)ribose. As La⁴⁺ is unstable, the free radicals can be generated through an inner-sphere electron transfer process between La⁴⁺ and ribose. Thus the oxidation of ribose occurs through the La³⁺/La⁴⁺ catalytic cycle.

CONCLUSIONS:

The oxidation of D-ribose by cerium (IV) in sulfuric acid medium is to take place between the positively charged species of cerium (IV) and pyranose form of the monosaccharide. The reaction occcurs through the formation of an intermediate complex, which undergoes slowly unimolecular decomposition to yield a free radical. The free radical then reacts with cerium(IV) species to form the product. The high negative value of ΔS^*suggests the formation of more activated complex, whereas, the positive value of free energy of activation(ΔG^*) and enthalpy of activation (ΔH^*) indicate that the transition state is highly solvated. Energy of activation, free energy of activation and entropy parameters suggest that La(III) forms the activated complex more easily compared to the others. Mechanism consistent with observed rate laws have been suggested.

ACKNOWLEDGEMENT:

The authors are thankful to Principal and Head, Department of Chemistry, Govt. Nagarjuna PG College of Science for providing Lab facilities. Authors are also thankful to Dr. Sanjay Ghosh Assistant Professor for helpful discussions.

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Received on 09.10.2012 Modified on 18.10.2012

Asian J. Research Chem. 5(10): October, 2012; Page 1271-1277