Kinetics of Oxidation of D-Galactose by Pyridinium Chlorochromate

Introduction

In recent years a number of  studies on oxidation reactions  involving Cr(VI) based oxidants and organic compounds have been made^1,2, The oxidation reaction of  carbohydrates is complex because of their multifunctional nature. Earlier^3,4 and recent⁵  reviews on the subject are noteworthy. D-Galactose plays an important role in carbohydrate chemistry because it is a constituent of glycolipids and glycoproteins. Our interest^6,7 in the use of modified Cr(VI) reagents promoted us to investigate the oxidation of D-Galactose with PCC to throw light on the mechanistic aspects of the oxidation process.

Material and Method

Pyridinium chlorochromate was synthesized by the reported procedure⁸. Aqueous solution of D-Galactose (Loba-chemie) was always freshly prepared. Perchloric acid and other chemicals (A.R., B.D.H.) were used as such without further purification. The reactions were performed under pseudo-first order conditions by keeping a large excess of D-Galactose with respect to PCC. Kinetic measurements were made using a systronics spectrophotometer 106 at 400 nm. The optical density was measured at various intervals of time.

Stoichiometry and Product Analysis

Stoichiometry of the reaction was observed as one and may be represented as:

   C₆H₁₂O₆  +  C₆H₅NHCrO₃Cl   →   C₅H₁₀O₅  + HCOOH  +  C₆H₅NHCl + CrO₂

   D-Galactose          PCC                     D-lyxose   formic acid

The end product D-lyxose was confirmed by osazone formation⁹ and by paper chromatography¹⁰. The presence of formic acid was confirmed by spot test¹¹.  

Result and Discussion

The pseudo-first order rate constants were determined at various initial concentrations of reactants. The results obtained are given in Table 1. Plots for different concentrations of PCC versus time were linear and the rate constants were independent of initial [PCC], showing first order dependence on [PCC].The reaction is first order with respect to D-Galactose also. A plot of log k against [D-Galactose] was linear with a slope of unity, thereby confirming first order dependence in [D-Galactose]. Rates of oxidation were found to increase with increase in [H⁺] and slopes of the plots of log k, versus log [HClO₄] was approximately unity, showing that the reaction is acid-catalysed and follows a first order dependence in [HClO₄]. 

Table 1: Rate Constants for oxidation of D-Galactose by PCC at 30^oC

[PCC] x 103	[D-Galactose] x 102	[H+] x 100	k1 x 104	k2  x 102
(mol dm-3)	(mol dm-3)	(mol dm-3) 1s-1)	(s-1)	(dm3mol–
0.40	0.40	2.32	2.18
0.60	0.40	2.32	2.18
0.80	0.40	2.2	2.20
1.00	0.40	2.32	2.20
1.20	0.40	2.32	2.23
0.40	0.40	2.32	2.18	5.45
0.40	0.80	2.32	4.37	5.46
0.40	1.20	2.32	6.55	5.46
0.40	1.60	2.32	8.73	5.46
0.40	2.00	2.32	10.75	5.34
0.40	0.40	1.74	1.64
0.40	0.40	2.32	2.18
0.40	0.40	2.90	2.72
0.40	0.40	3.48	3.28
0.40	0.40	4.06	3.82

The catalysis by HClO₄ suggests that protonated form of PCC is involved in the oxidation. Consequently the empirical rate law is described as follows- The reaction rate was not influenced by ionic strength when NaClO₄ was initially added to the reaction mixture over the range of 0.010 to 0.030 mol dm^-3. Similar observations were also reported in the oxidation of diols by PCC¹². 

The reaction rates at different temperatures were determined and the value of activation parameters were calculated from the slope of linear plot of log k versus T^-1. The data are presented in Table 2. The entropies of activation is largely negative as expected for bimolecular reaction.

Table 2:Temperature dependence and activation parameters of the oxidation of D-Galactose by PCC.

[PCC] = 0.40 x 10^-3 mol dm^-3 ,   [D-Galactose] = 0.40 x 10^-2 mol dm^-3 , [H⁺] = 2.32 x 10⁰ mol dm^-3,  Solvent: Acetic acid-water(50-50 % v/v)

Temperature	k1 x 104	Ea	ΔH#	ΔG#	-ΔS#
(oC)	(s-1)	(kJ mol-1)	(kJ mol-1)	(kJ mol-1)	(JK-1 mol-1)
30	2.18	–	60.16	95.50	116.65
35	2.96	60.92	60.12	96.34	117.60
40	4.72	62.42	60.08	96.73	117.10
45	6.37	64.70	60.04	97.52	117.88
50	10.19	–	60.00	97.84	117.15

The pyranoid form in its chair conformation is the stable form (⁴C₁) of aldohexoses^13,14 and predominantly exists in solution¹⁵. In the equilibrium of anomeric form of D-Galactose it is the β-galactose with equatorial orientation of glycosidic hydroxyl group that predominantly exist (scheme 1) and is supposed to be more reactive species¹⁶.

Scheme 1 Click here to View scheme

A free radical mechanism is ruled out since polymerisation of acrylonitriles was not observed. UV spectra did not show existence of the intermediate complex in the present study, however its formation in small amount cannot be ruled out in view of product identification. Hence in the present mechanism it is proposed that a chromate ester is formed in a rapid pre-equilibrium step. The proposed mechanism is outlined in scheme 2.

Scheme 2  Click here to view scheme

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