Oxidation of Some Aliphatic Alcohols by Pyridinium Chlorochromate-Kinetics and Mechanism

Kinetics of oxidation of some aliphatic primary and secondary alcohols viz. ethanol, propan-1-ol, propan-2-ol, butan-1-ol, butan-2-ol and 2methyl butanol by pyridinium chlorochromate (PCC) have been studied in waterperchloric acid medium. The reaction shows first order dependence with respect to pyridinium chlorochromate [PCC] and hydrogen ion [H]. The rate of oxidation decreases with increase in dielectric constant of solvent suggests iondipole interaction. Activation parameters have been evaluated. Products are carbonyl compounds and free radical absence was proved. A tentative mechanism has been proposed.


Experimental
All chemicals were used of 'Anala R' grade.Double distilled water was used as a medium.All the alcohols were used after their distillation by proper method and purity checked by their boiling point.The solution of perchloric acid was prepared by diluting known volume of acid in water and standardized by sodium hydroxide using phenolphthalein as an indicator.PCC is prepared by improved method of Correy and Suggs described by Agrawal 19 .
The orange solid, which is collected on a sintered glass funnel dried for 1 h in vacuum and m.p. (148-150ºC) checked.The solid was not hygroscopic and highly soluble in water, acetonitrile, DMF etc. PCC solution was prepared by dissolving the known amount of this reagent in water and standardized by iodometrically using starch indicator.

Kinetic measurements
The solution of oxidant in acid-water medium obeys Lambert Beer's law i.e. absorbance versus [oxidant] is a straight line therefore reactions were followed by monitoring the decrease in oxidant concentration.
The reaction have been arranged to study under pseudo first order conditions by keeping the excess of substrate upon oxidant i.e. [Substrate]>> [Oxidant] ratio not less than 8:1 in any reaction set.The reactions were carried out in a glass stoppered cell at constant temperature ±0.1ºC.The reaction mixture was prepared by mixing the requisite amount of substrate, perchloric acid and water and allowed to stand in a thermostatic bath for a sufficient length of time.Adding the solution of the oxidant started the reaction and mixed well.The optical density of the reaction mixture was followed spectrophotometrically at 354 nm by using Simadzu U.V/Visible spectrophotometer model Jasco 7800 with recording facilities.

Product analysis and stoichiometry
Product analysis was carried out under kinetic conditions.In a typical experiment, large volume containing [alcohol] is in excess over [PCC] and kept for reaction completion.Moles of PCC consumed were determined by difference in absorbance before and after completion of reaction.The whole reaction mixture (after completion of reaction) was treated with 2,4-dinitrophenylhydrazine.A yellow-orange precipitate obtained which was filtered, washed, dried and weighed.From the weight of the precipitate, moles of carbonyl compound (product) were determined and hence stoichiometry was confirmed.Conformation of carbonyl (aldehyde/ketone) compound was done by melting point, IR and nitrogen percentage analysis of precipitate obtained.Cr(III) was confirmed by visible spectra of the reaction solution after completion of reaction.The stoichiometric equation is:

  → = + + +
There was no change in rate or absorbance on addition of stabilizer free acrylonitrile in nitrogen atmosphere.This confirms absence of free radical in these oxidations.

Effect of oxidant concentration
The reactions are of first order with respect to PCC i.e. log absorbance versus time is straight line for more than 80% reaction.Further the value of k obs is independent of the initial concentrations of PCC.The rate can be expressed as: -

Effect of substrate concentration
The rate of oxidation increased on increasing the concentration of alcohols (Table 1

Effect of pyridine and picolinic acid concentration
The addition of pyridine and picolinic acid does not affect the reaction rate.This suggests that PCC is quite stable in perchloric acid water medium in the concentration range studied and dose not dissociate to chromic acid

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

Effect of perchloric acid concetntration
The effect of hydrogen ion concentration on the rate of the oxidation was studied by varying [H + ] while keeping the concentration of other reactants constant.Since there is no effect of ionic strength on reaction rate therefore ionic strength was not kept constant.A steady increase in oxidation rate with increase in the acidity of the medium suggests the formation of protonated PCC in the rate determining step 20

Effect of solvent composition
At fixed ionic strength and [H + ] the rate of oxidation of alcohols with PCC increases with decrease in polarity of solvent.In other words a decrease in rate with increase in dielectric constant of solvent (1,4-dioxane) is observed.This is due to polar character of the transition state as compared to the reactants.According to Scatchard 22 , the logarithm of the rate constant of a reaction between ions should vary linearly with the reciprocal of the dielectric constant if reaction involves ion-dipole type of interaction (Table 3).

Effect of temperature
The rates of oxidation of alcohols were determined at different temperature (Table 4) and the reactions obey Arrhenius equation.Energy of activation was calculated by slopes of straight line obtained plotting log k versus 1/T.The activation parameters for ethanol, propan-1-ol, propan-2-ol, butan-1-ol, butan-2-ol and 2-methyl butanol were calculated (Table 5).

Discussion
Kinetics of oxidation of aliphatic alcohols by PCC was investigated at several initial concentrations of the reactants.At low concentrations of PCC and when substrates are in large excess, the reaction is found to be first order in PCC.The plot of log (a-x) ie log absorbance against time is found to be linear with a correlation coefficient 0.9982, showing first order dependence in PCC.A plot of log k 1 vs log [Substrate] gave a straight-line with slope ≈ 1 showed first order dependence over substrate.The thermodynamic parameters are mentioned in Table 5.The entropy of activation is negative as expected for a bimolecular reaction.The negative value also suggests the formation of a cyclic intermediate from noncyclic reactants in the rate determining step 23 .The large negative value of ⊗S suggests that the transition state is less disorderly than the reactants 24 .
A study increase in the oxidation rate with an increase in the acidity of the medium suggests the formation of protonated PCC in the rate-determining step.The plot of log k 1 against log [H + ] is linear with a slope of nearly two suggesting that two protons may involve in the rate-determining step.Different possibilities are one for PCC and other of substrate or both the protons are taken by oxidant.Since protonation of alcohol is less probable it is more likely that both protons are used by the oxidant.Many workers have suggested protonated PCC [25][26][27] .The protonated PCC and alcohol combine to give intermediate, which was also indicated by decrease in rate with increase in dielectric constant of reaction medium due to more polar character of the transition state as compared to the reactants which is further attacked by proton.
Although the intermediate is already positively charged hence second proton attack will be difficult and hence very slow.Energy of activation suggests C-H bond breaking in rate determining step and negative entropy of activation indicates formation of cyclic from non-cyclic or more polar than reactants structure formation.From H + effect and applying various hypothesis like Zucker-Hammett 28 , Bunnett 29 and Bunnett-Olsen 30

Figure 3 .
Figure 3. Variation of rate with solvent composition.

Table 4 .
Variation of rate with temperature [Alcohol] = 2×10 -2 M; [HClO 4 ] = 5×10 -1 M; [PCC] = 2×10 -3 M. k obs × 10 concludes water molecule is acting as proton abstracting agent or say solvent helps to remove proton.According to Zucker-Hammett hypothesis plot of log k obs against H 0 is straight line in range between slope ≈1.Bunnett has proposed that plots of log k obs + H 0 against log a H 2 O (activity of water) is linear or approximately so.The slope in such plots constitutes a parameter called Bunnett function and designated as ω.If the ω value > +3.3 suggest water to act as proton abstracting agent in rate-determining step.In case of Bunnett-oleson plots log k 1 -log [H + ] versus log a H 2 O is linear with slope ω  value > -2.0 means water act as a protonabstracting agent in rate determining step.The slopes in such plots are in good agreement with the criterian given by Zucker Hammett and Bunnett-oleson.This proves that water act as a proton-abstracting agent in oxidation reaction.B.L. Hiran et al 31 observed the same results in the oxidation of C 3 alcohols viz allyl alcohol, 1-propanol and 2-propanol by 3-methyl pyridinium bromochromate in acid medium.∆E # versus ∆S # is almost straight line indicates similar mechanism is operating in all the compounds which have been taken for study.According to the above results and data following mechanism has been proposed.
The rate law can be given as follows:Rate = k' [Oxidant] [Substrate] [H + ] 2 Rate = k obs [Oxidant] k obs = k' / [Substrate] [H + ] 2This rate law and suggested mechanism explains all the observed facts.

Table 1 .
Variation of rate with substrate concentration.
). Plot of log k obs versus log [substrate] is a straight line in all the cases with slope 1.0, 1.01, 0.89, 0.98, 1.02, 0.88 respectively for ethanol, propan-1-ol, propan-2-ol, butan-1-ol, butan-2-ol and isoamyl alcohol i.e. first order with respect to substrate.Plot of 1/k obs versus 1/[Substrate] gave linear line passing through origin or very small intercepts nearly zero suggest that the rate does not obey Michalis Mentane type kinetics.Figure 1.Variation of rate with substrate concentration.