Research On Electrochemical Chlorination Of P-toluic Acid And Methyl 4-methylbenzoat

3-chloro-4-methylbenzoic acid(methyl ester)is one kind of important chemical intermediates with many reactive sites on the benzene ring,this results in it can be used to synthesize many functional derivatives which are widely used in medicine,spices,fungicides and other fields,but its synthesis embraces some problems,such as high cost?long reaction time?low selectivity?pollution of aluminum salt or iron salt,and the chlorine atoms are difficult to be reused.In order to find an approach to solve them,a novel way,electrochemical chlorination was proposed to synthesize it.Electron as a clean catalyst,could electro-oxidize chlorine anion into chlorine cation,and then the cation,through electrophilic substitution reaction,react with reactant to get target products.On the basis of cyclic voltammetry,the effects of reaction factors on the electrochemical chlorination process were investigated by single factor experiments and orthogonal experiments to obtain optimum technological parameters,the concrete structures of materials and products were analyzed by means of nuclear magnetic resonance,infrared spectroscopy and mass spectrometry,and the relative electrochemical parameters or feasible technical approaches for the industrial synthesis of 3-chloro-4-methylbenzoic acid(methyl ester)had been explored.In the synthesis process of 3-chloro-4-methylbenzoic acid,the effect of graphite electrode and platinum electrode on the electrochemical oxidation reaction of 4-methylbenzoic acid was preferentially studied by cyclic voltammetry firstly,and the change rule came from peak potential and peak current which corresponding to chlorine anions and chlorine atoms was analyzed.In the reaction,it was necessary to generate chlorine cation which had electrophilic activity to attack the benzene ring,although the peak potential corresponding to platinum electrode was negative to graphite electrode,the oxidation peaks corresponding to chlorine atom under graphite and platinum were both overlapped with the oxygen evolution peak,which result in low current density of two peaks.Considering a larger cost of platinum electrode,especially the degree of oxygen evolution of platinum electrode was much stronger at higher current density than graphite electrode,so the main research with graphite as the working electrode of chlorination reaction.Through the optimization of conditions such as organic solvent,supporting electrolyte,reaction temperature,current density,acid concentration and others based on single factor experiments and orthogonal experiments,when yield of target product as priority,anode liquor consisting V(acetonitrile): V(methanol)= 3 : 1?6.7 mol·L-1 of HCl in 25.0 m L aqueous solution?2.0 g of tetraethylammonium chloride(TEAC)and 0.5 g of 4-methyl benzoic acid,cathode liquor consisting hydrochloric acid solution,temperature was 45?,current density was 0.2 A·cm-2,and reaction time was 4 h,the yield for electrochemical synthesis of 3-chloro-4-methylbenzoic acid was 94.6%,and its current efficiency was 23.3%.When platinum was used as working electrode,adjusting the TEAC was 1.0 g,the maximum yield in platinum electrode system was 96.6% and the current efficiency was 23.8% with other conditions fixed.This shows the yield and current efficiencies at the two electrodes are similar,respectively.Similarly,the cyclic voltammetric behavior of methyl 4-methylbenzoate at graphite electrode and platinum electrode was preferentially studied before the concrete synthesis of 3-chloro-4-methylbenzoic acid methyl ester,it was found that the oxidation peak of chlorine atom corresponding to two electrodes both overlapped with oxygen evolution peak in different extent,and the oxidation peak of platinum electrode system was almost completely covered by oxygen evolution peak,which resulting in the corresponding peak current density were bad.Considering the strong oxygen evolution of platinum electrode at higher current density,the electrochemical chlorination reaction of the graphite electrode was chiefly studied,and the reaction parameters such as organic solvent,supporting electrolyte,reaction temperature,current density and acid concentration were optimized.On the basis of single factor experiments and orthogonal experiments,the optimum reaction conditions of yield priority were obtained,with anode liquor consisting 30.0 m L of acetonitrile?8.6 mol·L-1 of HCl in 25.0 m L aqueous solution and 1.0 g of methyl 4-methylbenzoate,cathode liquor consisting hydrochloric acid solution,temperature was 50?,current density was 0.3 A·cm-2,the yield for electrochemical synthesis of 3-chloro-4-methylbenzoic acid methyl ester was 96.8%,and its current efficiency was 28.8% in 4 h.Using platinum electrode and keeping the other conditions unchanged above,the maximum yield in platinum electrode system was 98.1% and the current efficiency was 29.2%.This shows that the yield and current efficiencies at the two electrodes are close,respectively.The results indicated that chlorinated aromatic compounds can be prepared by electrochemical method using hydrogen chloride as electrolyte,methanol or acetonitrile with water as mixed solvents.For the different dissolving ability of aromatic compounds,a small amount of tetraethylammonium chloride can be added to promote the dissolution.This method not only avoid the security risks from the use of chlorine,thionyl chloride and other highly toxic reagents,but also improve the utilization of chlorine atoms and achieve a controlled response process.Graphite replace platinum for the electric synthesis of 4-methylbenzoic acid(methyl ester)is feasible,and graphite electrode present good applicability with low cost to the electrochemical chlorination of similar substrate.