Kinetics and Mechanism of the Oxidation of Lactic Acid and Mandelic Acid by Pyridinium Chlorochromate in Aqueous Acidic Medium

: Pyridinium chlorochromate (PCC) act as two electron oxidant in kinetic studies of the oxidation of lactic and mandelic acid in acidic medium. The influence of several factors such as acidity, salts, solvent composition and temperature on the reaction rate has been studied. The reaction shows first order dependence with respect to [PCC] and [H + ]. The various thermodynamic parameters have been evaluated. Products as oxoacids and absence of free radical were confirmed. Suitable reaction mechanism has been proposed.


Introduction
Pyridinium Chlorochromate (PCC) 1,2 has been used as a mild and selective oxidizing reagent in synthetic organic chemistry.Oxidations of α-hydroxy acid by other pyridinium and quiniolinium halochromate 3 , have been reported.These seem to be no report on the oxidation of α-hydroxy acid by PCC and no report on its mechanism.Hydroxy acid may be oxidized either as alcohol, yielding corresponding oxoacids 4 may undergo oxidative decarboxylation to yield a ketone 5 .The catalytic effect of some bidentate ligands on the oxidation of lactic acid by Cr(VI) 6 has also been reported.

Experimental
The hydroxy acid was a commercial product of the highest purity available and used as such.PCC was prepared by reported method and its purity checked by iodometric method.Double distilled water used as a medium.The solution of perchloric acid is prepared by diluting known volume of acid in water and standardized by sodium hydroxide using phenolphthalein as an indicator.
Product analyses were carried out under kinetic condition i.e. with an excess of the reductant over PCC.For both the acid, reaction mixture was allowed to stand in a dark for ≈ 24 h to ensure the completion of the reaction.It was then treated with an excess of a freshly prepared 2,4-dinitrophenylhydrazine (DNP).The hydroxy acids yielded the DNP of the corresponding oxoacids (85-91% yields) after crystallization.The oxidation state of chromium in completely reduced reaction mixture was 3.

Kinetic Experiments
Keeping excess of the hydroxy acid over PCC attained the pseudo first order conditions.The temperature was kept constant to ± 0.1 K.The reaction was followed by monitoring the optical density of the reaction mixture spectrophotometrically at 354 nm up to 70% of the reaction by using a digital UV/ visible spectrophotometer model.No other reactant or product had any significant absorption at this wavelength.

Results and Discussion
The rate and other experimental data were obtained for both the hydroxy acids studied and similar results were obtained.The oxidation of hydroxy acids resulted in the formation of the corresponding oxoacids.Product analysis and stoichiometric determinations indicates that the overall reaction may be written as- PCC undergoes a two electron change.

Effect of substrate concentration
The reaction increases linearly with an increase in the [hydroxy acids] (Table 1).The plot of 1/k obs versus 1/[subsrate] gave linear line passing through origin suggests that the rate does not obey Michaelis-Menten type kinetics.The hydroxy acids also show first order dependence over substrate in quinolinium bromochromate (QBC) 7 and pyridinium hydrobromide perbromide 8 .
Table 1.Variation of rate with substrate concentration

Test for polymerization
The oxidation of hydroxy acids by PCC in an atmosphere of nitrogen failed to induce the polymerization of acrylonitrile.

Effect of ionic strength
There was no effect of SO 4 2-and CH 3 COO -observed on the reaction rate in the Debye Huckle limit.It proves that interaction in rate determining step is not ion-ion type 9 and one of the reactant molecules is neutral.

Effect of solvent composition
The rate of oxidation of lactic and mandelic acid was determined in different solvent composition [1,4-dioxane].The kinetics is similar for both the acids in the solvent.
According to Scatchard 10 the plot of log k obs versus 1/D was liner and indicates ion dipole type of interaction 11 (Table 2) in rate determining step.

Effect of perchloric acid
Rate of oxidation increased with increase in hydrogen ion concentration.The plot of log k obs against log [H + ] is linear with slope for lactic acid 1.20 and mandelic acid 1.06.Therefore order with respect to [H + ] ion is one.In presence of acid, PCC may be protonated.A protonated Cr (VI) species is likely to be better electrophile and oxidant compared to the neutral one (Table 3).

Effect of temperature
The rate of oxidation of acids was determined at different temperature and the activiation parameters were evaluated (Table 4).Representative Figures 1-6 are given for lactic acid and similar behaviour was observed in case of mandelic acid also.

Discussion
The rate of oxidation increased on increasing the concentration of hydroxy acid (HA).Plot of 1/k obs versus 1/[HA] is linear and passing through origin means no Michaelis-Menten type kinetics were observed and no reversible complex formation between acids and PCC.The rate of reaction increases with increasing H + concentration.Plot of log k obs Vs log [H + ] is straight line with slope ≈ 1 supports the idea that may be protonated PCC enhance the rate of oxidation.There is no effect of ionic strength proves that in rate determining step ions are not involved.Increase in the percentage of 1,4-dioxane increase the rate, suggest that medium of low dielectric constant favors the reaction.Linear plots of log k obs versus 1/D for both the acids confirm the ion-dipole type of interaction in rate determining step.Log k versus (D-1)/(2D+1) is a straight line 12 , with negative slope indicates that activated state is less polar, while positive slope indicates more polar activated state.In our study we get negative slope indicate that activated state is less polar.Therefore, by increasing percentage of dioxane should increase which we have observed in both the acids.Rate of reaction increased by increasing temperature.Various thermodynamic parameters are evaluated.
Energy of activation and large and negative value of entropy suggest that α-C-H bond and formation of corresponding oxoacids as product.Which fits in the criterion suggested by Narain and Bakore 13 .The energy of activation ranges between 37 and 57 kJ mol -1 .Energy of activation does not correspond to carbon-carbon bond fission in the decomposition of organic substrates by Cr(VI) in rate-determining step.Calculation of activation parameters showed that these reactions are not enthalpy controlled.Lowest energy of activation is not associated with highest rate or vice versa.Vazirani et al 14 also observed similar value of energy of activation for chronic acid oxidation of mandelic acid.
The observed negative value of entropy of activation also supports it.As the charge separation takes place, the charged ends become highly solvated.This results in an immobilization of a large number of solvent molecules, reflected in less of entropy 15 .
It is of interest to compare the mode of oxidation of hydroxy acids by QBC 7 , PFC 3 and PCC.The oxidation by PFC exhibited Michaelis-Menten type kinetics with respect to hydroxy acids.While the oxidation by PCC and QBC present a similar kinetic picture.The rate laws and hydrogen ion dependence are similar in both the cases.
According to above data, suitable mechanism is proposed as follows -

Table 2 .
Variation of rate with solvent composition.

Table 3 .
Variation of rate with perchloric acid concentration

Table 4
Figure 2. Variation of rate with substrate concentration.

Table 5 .
Thermadynamic parameters for lactic acid and mandelic acid