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KINETICS OF OXIDATION OF AROMATIC ANIL BY PYRIDINIUM DICHROMATE – STUDY OF THE EFFECTS OXIDANTS AND SUBSTRATE

DR.V.Krishnasamy1, S.Rosy christy2
 1Professor of Chemistry , Bharath university ,Chennai – 73, India 2Assistant Professor of Chemistry , Bharath university ,Chennai – 73, India
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Abstract

The kinetic study of simple and substituted anils ware studied under various conditions viz., solvent ,oxidants , electrolyte , catalysts and mixture of catalysts and temperature; simple and substituted anils , pyridinium Chloro Chromate (PCC) ,Pyridinium Dichromate ( PDC) were prepared in the laboratory and kinetic study were carried out. The graphs were drawn at all relevant places and results were obtained in full satisfaction.

Keywords
anils ,Prydinium Chlorochromate[ PCC] ,Pyridinium Dichromate[PDC], Osmiumtetraoxide , oxidation

INTRODUCTION
Among oxo derivatives of variable valence metals , Chromium compound play the most important role , in oxidative reaction .A number of Chromium reagents are readily available . Almost every oxidisable fuctional group may undergo Chromium oxidation .Chromium ( VI ) containing reagents include Chromium acid , dichromate ion , Chromyl chloride , chromyl acetate , t – butyl Chromate , Chromylnitrate and Co – ordination complexes of Chromium trioxide .1-2
Chromium ( VI oxidations are usually performed under acidic condition . Co – solvents like ( Jone’s reagent ) benzene , methylenechloride ( Two Phase system ) are often added in order to deal with water insoluble organic complexes . The oxidation rate is generally high under acid catalysed conditions , however the low pH of the reaction medium and the presence of water favouring hydrolytic condition exclude the use of this class of reagents for the oxidation of molecules containing acid sensitive groups.
Polymer supported Chromium ( VI) reagents have also been developed. These reagents offer the advantage of reducing the work of procedure to more filtration . Recently some neutral or almost neutral Chromium (VI) reagents have been developed to effect oxidation under mild conditions.
Pyridinium Chlorochromate (PCC) introduced by Covey et al3 is widely used in the oxidation of alcohols. Pyridinium flourochromate has a less pronounced acidity and is an effective agent for the oxidation of Plycyclic organic substitutes . PFC was developed by Bhatachargee and coworker . 4
In 1986 , Narayanan and Balasuramaniam5 introduced Pyridinium bromochromate . This is an efficient oxidant for alcohols and a brominating agent as well .Firouzahadi and Coworkers used Zinc chromates. This is a very useful reagent for the oxidation of variety of organic compounds , including alcohols , oximes , olefins and aromatic hydrocarbons.
They have also shown chromium peroxide complexes as versatile , mild and efficient oxidants in organic synthesis.Chlorotrimethyl Silone , Imidazolium dichromate , 2,2’ – Bipyridyl chlorochromate (BPCC) and the corresponding chromate (BPC) have been proposed as oxidants for the oxidation of the hydroxyl group to thr carbonyl group.4 – ( dimethyl amino ) – pyridinium chlorochromate and chromyl nitrate have been shown to be efficient and mild oxidant in aprotic media.Naphthyridium chlorochromate , Pyrazinium chlorochromate , tetrabutyl ammonium chlorochromate , dimethylpyrazole chromate , tetrabutyl ammonium dichromate , Pyridine complex of oxodiperoxochromium and the dipyridine complex of chromium trioxide are some of the Chromium (VI) oxidants introduced in the last twenty years.Quinolinium Chlorochromate was used by Bhavani et al to oxidize methyl phenyl sulphoxides.Quinolinium dichromate was introduced by Balasubramanian and Prathiba 6 as an effective oxidant under non – aqueous conditions.
Pyridium Chlorochromate (PCC) is irreplaceable in the generation of functional groups in highly unsaturated carbinols and pyrols . yet mildly acidic character of Pyridinium chlorochromate precludes its use with acid sensitive substrates or products. Pyridium Chlorochromate oxidation has been investigated under various condition on different substrates .Despite the introduction of numerous oxidants based on chromium (VI) not much work has been done to investigate the mechanism of oxidation in these cases.
1.1.PYRIDIUM DICHROMATE AS AN OXIDANT
Pyridium dichromate in acetic anhydride was shown to be an efficient oxidant for alcohols by Anderson and coworkers . Pyridium dichromate (PDC) ( C3H5NH)+ 2 Cr2O7 a stable bright orange solid ( mp. 144 – 1460 c) is simply prepared in quantity by dissolving CrO3 in a minimum of water, adding Pyridine and collecting the precipitated product. The composition and structure of the product was confirmed by elemental analysis and infrared spectrum ( nujoil null ) which showed bands at 930 , 875 , 765 and 730 cm -1( characteristic of dichromate ion 27 ). PDC is very soluble in water , dimethylformamide , dimethyl sulphoxide or dimethyl acetamide and the solutions are relatively stable. It is sparingly soluble in methylnechloride , ethanol – free chloform of acetone , and not noticeably soluble in hexane , toluene ether or ethylacetate . Although PDC dissolves in acetonitrile , the solution is not stable .Most of the experiments have been carried out with solution of PDC in DMF or in methylene chloride suspensions.It liberates iodinefrom an aqueous acidic solution as follows:
( PYH)2 Cr2O7 + 6KI + 3H2SO4 ------------------ 2PY + H2O + Cr2O3 + 3K2SO4 + 3I2
1.2.REACTIONS OF PDC
Pyridium dichromate (PDC) in N,N – dimethyl formamide or dichloromethane is nearly neutral non – aqueous chromium (VI) oxidant. This species probably present in the Sarret and Conferth oxidizing mixtures. Although specific reference has been made to oxidation of alcohols by Pyridium dichromate itself in a short note 28 , there is no indication of any unusual effectiveness , utility , or advantage to be gained by the preformed pyridium dichromate as a discrete oxidizing species before Corey and Schnuidt prepared it in 1979.Solutions of Pyridium dichromate in DMF rapidly oxidize allylic alcohols44 to alpha , beta – unsaturated carbonyl compounds at O 0 C. Kinetics and mechanism of oxidation of cyclic ketoximes by Pyridium dichromate and Quinolinium dichromate was investigated by Chidambaranathan et al. 7
Oxidation kinetics of Cyclanols by Pridiniumdichromate was studied by Padma et al Kinetics and mechanism of oxidation of some ortho- substituted benzyl alcohl by PDC was studied by Kabilan and Thenmozhi.Kinetics and mechanism of oxidation of some substituted phenylmethyl sulphoxide using PDC was reported by Meenakshisundaram et al. 8 Oxidation of S – phenylmercaptoacetic acid and phenoxyacetic acid by PDC was studied by Kabilan and Krishnasamy.9Gurumurthy et al 10 investigated a comparative study on the kinetics of oxidation of some secondary alcohols by 1 – chlorobenzotriazole and PDC .

2.EXPERIMENTAL
2.1PREPARATION OF ANILS
2.1.1.General method of preparation of Anils :
image
The anils were prepared 87a by refluxing equimolar quantities of benzaldehyde and aniline or substituted anilines in alcohol for about 2 to 3 hrs. The resulting solution was cooled and poured into the cold water . The precipitated anil was filtered , washed , dried and recrystallised from alcohol. The purity of anils were checked by determining their melting points ( Table 1.1).
2.1.2.Preparation Of Pyridinium Dichromate (PDC)
Pyridine ( 80.6 ml ) was gradually added to a cooled solution of 100 gm ( 1 mol ) of Cr2O3 in 100 ml water at 30 C . The solution was distilled with 400 ml of acetic acetone and cooled to 20 c . After 3 hrs. the orange crystals were collected , washed with acetone and dried in vacuuo.
2.13.Purification of Acetic acid
Acetic acid of AR grade was first refluxed with chromium trioxide ( 20g per litre ) for 4hrs. and then distilled . It was refluxed again with Chromium trioxide again with chromium trioxide ( 2g per litre ) for 2 hrs. Then it was fractionated through a Dufton column and the fraction at 117 – 118 0 was collected.
2.1.4.Purification of water
De – ionized water was distilled over Potassium Permanganate in an all glass corning vassel . All the aqueous solutions were prepared using this double – distilled water .

3.Kinetic measurements
The reactions were carried out in 60% (v/v) acetic acid - water under pseudo - first order conditions taking always the substrate in excess. The solutions of the described concentrations were prepared.
1. Anil in acetic acid
2. PCC in water
3. Sodium perchlorate in water
4. Perchloric acid in water
All the solutions were thermostated atleast one hour before use .The progress of the reaction was monitored by withdrawing aliquots from the reaction mixture at regular intervals of time and estimating the unreacted PDC by iodometric method.

4.Product Analysis
The reactants in the ratio anil : PDC ( 1 : 10 ) in the 70 % acetic acid were mixed under kinetic conditions . The products of oxidation were identified as the corresponding benzaldehyde and nitrosobenzene . benzaldehyde was characterized as the 2, 4 – dinitropheynyl hydrozone derivative . The remaining solution on evaporation yielded nitrzobenzene , which was identified by its characteristic UV spectrum .

5. Stoichiometry
The stoichiometry of the reaction was detrmined by carrying out several sets of experiments with varying amounts of [PDC] largely in excess over [anil]. The estimation of unreacted PCC showed that one mole of anil reacts with two mole of PDC [ ie ., 1 :1)

6.RESULTS AND DISCUSSIONS
Kinetics and mechanism of several organic substrates by PDC 1- 12 were documented. But there are no systematic kinetic reports of the oxidation of Osmium ( VIII ) . Catalysed oxidation of benzaldehyde anils by Os ( VIII ) complexes . This prompted us to undertake the title for investigation .
6.1.Effect of varying the concentration Oxidant
The oxidation of aromatic anil by PDC was investigated at several initial concentrations of the oxidant in 70 % acetic acid media. The plot of log [PDC] Vs time ie., the plot log titre Vs time was linear indicating a first order dependence of rate on PDC ,; values of the pseudo first order rate constants kobs were calculated from the plots , according to the first order equation by the method of least squares .. reaction was found to be First order in PCC as shown by the linear plot of log titre value vs time (Table – 2.1 ).
Table – 2.1
image
6.2.Effect of varying the concentrations of Substrate
The reaction was carried out under with varying concentration s of Anil , always kept in excess , with constant concentrations of PDC. The rate constant in k increased linearly with substrate concentration ( Fig. 1 ) with a slope of 0.62 indicating fractional order dependence on the substrate concentration ( Table – 2 .2)
image

Tables at a glance
Table icon Table icon Table icon
Table 1.1 Table 2.1 Table 2.2
 

Figures at a glance
Figure 1
Figure 1
 

References
  1. K.B. Wilberg , “ oxidation in organic Chemistry ” Part ‘A’ 69 Academic Press , New York , (1965 )
  2. G.Cainelli and G.Cardillo , “ Chromium Oxidatio in Organic Chemistry ,” 5 Springer – Verlag , Perling Hiedelberg ( 1984)
  3. E.J. Covey and J.W.Suggs , Tetrahedron Lett., 2647 ( 1975)
  4. M.N.Bhattacharjii , M.K.Chaudri , H.S.Dasgupta and N.Poy , Synthesis , 588 , ( 1982 )
  5. N.Narayanan and T.R.balasubramanian , Indian J.Chem. 25 B , 228 ( 1986)
  6. K.Balasubramanian and Prathiba , Indian J.Chem.., 25 B , 326 (1986)
  7. V.Chidambaranathan , M.Phil, Thesis , Annamalai University ( 1989 )
  8. K.S.P. Meenakshisundaram , Ph.D., Thesis , Annamalai University ( 1991 )
  9. K.Krishnasamy , , M.Phil, Thesis , Annamalai University ( 1991 )
  10. R.Gurumurhty , G.Mangalam , M.Gopalakrishnan , A.Mohanakumar , P.Sankar and K.Sathyanarayanan, Asian J.Chem. 4 , 903 ( 1992)