Mediated electrosynthesis using Ce:H-/Ce4+ redox system olfers a clean and facile route for the production ofseveral important aromatic carbonyl products. Use of methane sulphon ic acid medium for the generation of ceric ion has made cerium mediated electrosynthesis practical for the commercial production or aromatic carbonyl products. In this paper, results are presented for the electrochemical generation of Ce(lV) in methane sulphonic acid medium and the oxidation of p.xylene to p-tolualdehyde using dilTerent solvent media. Under optimised conditions, 90·95 % selectivities have been achieved for the synthesis of p-tolualdehyde

Chidambaram, S.

Devadoss, V.

Jayaraman, K.

Kanakamsrinivasan, R.

Krishnamoorthy, S.

IR@CECRI

Bulletin ofElectrochemistry 14 (10) October 1998, pp 326-328 0256-1654/98/$ 3-50 3~ 19QR ('ECRI MEDIATED ELECTROSYNTHESIS OF p-TOLUALDEHYDE FROM p-XYLENE USING CERIe METHANE SULPHONATE K ]AYARAMAN, S KRISHNAMOORTHY, V DEVADOSS, R KANAKAM SRINIVASAN AND S CHIDAMBARAM Central Electrochemical Research Institute, Karaikudi - 630 006, INDIA [Received: 4July 1998 Accepted: 12 September 19981 Mediated electrosynthesis using Ce:H-/Ce4+ redox system olfers a clean and facile route for the production of several important aromatic carbonyl products. Use of methane sulphon ic acid medium for the generation of ceric ion has made cerium mediated electrosynthesis practical for the commercial production or aromatic carbonyl products. In this paper, results are presented for the electrochemical generation of Ce(lV) in methane sulphonic acid medium and the oxidation of p.xylene to p-tolualdehyde using dilTerent solvent media. Under optimised conditions, 90·95 % selectivities have been achieved for the synthesis of p-tolualdehyde. 3+ 401­Keywords: p-tolualdehyde, mediated eleclrosynthesls, Ce ICe • redox system. Aromatic aldehydes find wide app! ications as intennediates solubility of eerie and cerous ions and poor nucleophilicity in the production of phamlaceuticals, Oavouring chemicals [2,3 ]. etc. and the development of an electrochemical process for p-tolualdehyde is widely used in the perfumery industry and the production of aromatic aldehydes is of considerable also finds application as polymer-additive. The objective of inte.rest in industrial electro-organic synthe.sis. While the the present studies is to conduct preliminary voltammetric direct oxidation process is hampered by poor product studies on the oxidation to optimise process condition for the selectivity, difficulties associated with product isolation, generation of ceric from cerous and chemic.al oxidation of deactivation of electrodes etc., indirect electrochemical p-xylene using cerie methane sulphonate. oxidation using electrogenerated metal oxidants offers a The reactions involved in the indirect electrochemicalclean and facile route for the production of aromatic oxidation process are as follows aldehydes. CH CHO Mediated or indirect electrosynthesis is basically a cyclic process involving electrochemical generation of a redox + 4Ce(IV) -+ (9 + 4Ce(llI) (1)toagent and the use of the agent to effect a chemical reaction. Cf1 CJ.4S3 Anode 4Ce(TII) -+ 4Ce(IV) + 4e (2) Several mediators such as chromous/chromic, manganous/manganic, cerous/ceric etc., have been employed in the oxidation of alkyl aromatics and the electrolytic regeneration of these redox reagents have also been Cathode (3) genr.rally the reagent of choice due to its higher stability and solubility at all acid concentrations and yields excellent investigated. Among the metal oxidants, cerium(IV) is EXPERIMENTAL selectivities to aromatic carbonyl compounds (1 J. Among the cerium salts, use of cerium methane sulphonate has severat Linear sweep voltammetric studies were carried out on the advantages over othr.r anions, such as high stability towards electrochemical generation of eerie methane sulphonate using oxidation, high selectivity for a wide range of substrates, high a single compartment 50 cc capacity glass cell with platinum 326 .... JAYARAMAN et al - Mediated Electrosynthesis of p-Tolualdebyde from p-Xylene using eerie methane sulphonate. electrode of area 0.19 cm 2 at a sweep rate of 2 mY/sec. Platinum foil and a saturated calomel electrode were used as counter and reference eleclrodes respective!y. Cerium concentration was varied from 20-500 mM in 2 M - methane sulpbonic acid. All readings were recorded at 303 :t: 1 K. Wenking potentiostat (Model 75L, FRG), Scan generator (VSG 72 FRG) and Rikadenki recorder (X-Yt) were used. Preparative studies on tbe generation of cerie methane sulpbonate were carried out in a divided cell using c.ation excbange membrane (Nafion 423) as tbe diapbragm. Platinum was used as anode and stainless steel as catbode. The cell contents were stirred by a glass stirrer. Electrolysis was conducted at constant current conditions. Current efficiency for the Ce(lU) oxidation was calculated by estimating total cerium and cerie content in the electrolyte before and after electrolysis. All chemicals used were of technical grade and de ionised water was used for pre.paring tbe electrolyte. Total cerium and ceric content were estimated periodic.ally during the course of elcctrolysis using FeS04• and ferroin as indicator. Cbemic.al oxidation studies were carried out in a glass reactor kept in a thermostatically controlled beating bath. Oxidation of tbe p-xylene was conducted with the ceric ion fully dissolved or with suspensions of ceric methane sulpbonate complex. p-xylene was dissolved in an organic solvent and the organic phase was added to the cerie solution. Reaction was' 1 -2·31 mAcm d ~ c ~ w a ~ bz ~ Q::> u 0·6 Fig.I: Linear sweep voltammetry (LSV) of Pe in (a) 2M CH; SO; 11 (b), (c), (d), (e) & (j) are (a) + 50, 200, 300, 400 & 500 mM Ce(lIl) solution. v - 2mV.S- 1 conducted till there was no more ceric in tbe aqueous pbase. After the reaction, the pbases were separated and the aqueous pbase was extracted with the organk solvent The combined organic pbases were dried over anhydrous sodium sulpbate and distilled to recover the solvent and separate the product. The c.oncentrations of the product and tbe uncoverted substrate were analysed by HPLC using Sbimatzu LC-8A. HPLC analyser with CLS-ODS column with methanol as mobile phase witb UV ddector. The flow rate was 2 ml/minute. The retention time. for p-toluadldebyde was around 3.643 minutes. After tbe product isolatioll, tbe aqueous phase was cbarged to tbe electrochemic.al reactor for regeneration of cerium(lV). RESULTS AND DISClTSSION Voltammetric studies Platinum oxide formation is a well known occurrence at bigh anode potentials and can affect tbe study of reaction kinetics of Ce(III) oxidation. The procedure of Kuhn and Randle [41 was bence followed to obtain a constant reproducible oxide layer. The electrode was initially cleaned by polarizing it negatively to remove any existing oxides. The electrode was tben beld at a positive potential (+ 1.6 V) for 15 minutes to create a reproducible oxide layer. Steady state polarisation studies were carried out on this platinum surface. Fig. 1 Shows the linear sweep voltammograms (LSV) obtained for the oxidation of Ce(lIl) to Ce(lV) in methane sulpbonic acid (MSA) medium. It is seen tbat in the absence of any Ce(lII) tbe oxide film fonllation takes place between 1.1 V to 1.2 V and oxyge.1l evolution takes place at 1.5 V TABLE I: Electrochemical oxidation of Ce(IIij to Ce(lV) in methane sulphonic acid Anode: Pt Cathode: Stainless steel Anolyte : 2M methane sulphonic acid Catholyte : 1M methane sulphonic acid Temperature: 303 - 313 K Con. of c.n Output Current Ce(III) Ce(IV) Ce(IIl) efficiency M A.dm-2 M M (%) 0.35 3.5 0.27 0.08 56 0.47 3.5 0.39 0.08 76 0.80 9.0 0.55 0.25 65 0.80 9.0 0.70 0.10 65 1.20 10.0 0.83 0.35 68 1.20 10.0 1.01 0.12 72 327 JAYARAMAN et al - Mediated Electrosynthesis of p-Tolualdehyde from p-Xylene using cerie methane sulphonate. TABLE II: Chemical oxidation of p-xylene using Ce(IV) methane sulphonate Temperature: 303 • 333K ConCD Coover Selec Expt. Ce(lV) of Solvent sion tivity No. M MSA % % M 1 0.60 1 Dichloro ethane 89.9 97.8 2 0.55 2 -do- 87.4 95.0 3 0.61 4 -do- 81.0 88.0 4 0.70 2 Acetonitrile 82.8 90.0 5 0.53 2 Cyclohexane 79.1 86.0 Conc: Concentration in M MSA: Methane sulphonie acid vs SCE. Fig. (b), (c), (d), (e) etc. show the LSV in presence of varying concentration of Ce(III) and it is seen that Ce(III) oxidation takes place from 1.3 V to 1.5 V vs SCE which is more pronounced at higher concentration of Ce(III) and is followed by oxygen evolution beyond 1.5 V. However the peaks are not well defined. This explains the high current efficiencies acbieved with higher concentration of Ce(III) in preparative scale studies. However the oxidation of cerous to eerie is not a simple reaction and is reported to proceed through a complex involving the formation of an intermediate species (4). Electrochemical oxidation of Ce(UI) to Ce(lV) Electrochemical oxidation of Ce(III) to Ce(IV) was conducted at varying current densities and Ce(III) concentrations, with both fresh solution and solution obtained after chemic-al oxidation of p-xylene. The results are reported ill Table I. It is seen that the current efficiency for oxidation was 60-70 % and cerie eoncentration upto 1.0 M can be generated at higher current densities with good current efficiencies. Chemical oxidation of p-xylene Oxidation of p-xylene was conducted with homogeneous solution of cerie methane sulphonate or with suspensions of the eerie salt in MSA. Studies were conducted with varying concentrations of MSA and different solvents and the results are reported in Table IT. It is seen that 90-95 % selectivities were obtained for p- tolualdehyde at 80-90 % conversions. After chemical oxidation, the aqueous phase containing cerous methane sulfonate was regenerated in the electrochemical reactor and the regenerated eerie solutions were used for chemical oxidation. CONCLUSION An indirect electrochemical method has been developed for the oxidation of p-xylene to p-tolualdehyde using eerie methane sulfonate. Preliminary voltammetric studies were conducted. to study the oxidation of cerous to eerie in methane suIphonic acid medium. Process optimisation studies were conducted to optimise the conditions for the electrochemical generation of cerie and 65-75 % current efficiencies were achieved for the oxidation. Chemical oxidation of p-xylene using ceric methane sulphonate yielded p-tolualdebyde with 80-90 % conversions (based on isolated yield of product). As compared to direct electrochemical oxidation or catalytic oxidation technique, the indirect e1~trochemieal process will be a facile and cleau technology for the production of p-tolualdehyde. Further developmental studies are in progress. REFERENCES 1.	 Robert P Kreh, R M Spotnitz and 1 T Lundquist,] Org Chem, 54 (1989) 1526-31 2.	 R M Spotnitz, R P Kreh and 1 T Lundquist, Tetrahedran Letters, 28(10) (1987) 1067 3.	 H 0 Law and F M Perkin, Trans Faraday Soc, 1 (1905) 31 4.	 T H Randle and A T Kuhn, Electrochim Ada 31, 7 (1 ()86) 739 328 

Mediated electrosynthesis of p-tolualdehyde from p-xylene using cerie methane sulphonate

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