Fundamental Study On Electrosynthesis Of Sorbic Acid In Acetic Acid-Acetic Anhydride

The sorbic acid precusor acetoxyhexenoic acid was produced via Mn(ni) as oxident obtained from indirect electro-oxidation of acetic acid-acetic anhydride in the presence of butadiene. Sorbic acid was produced by hydrolysis with chloride acid.The results of experiment indicated that the optimization of the anodic oxidation from Mn(II) to Mn(III) on the graphite electrode was that electrolyte solution contained 0.1mol-L-1Mn(OAc)2-4H2O+1.5mol-L-1 KOAc+HOAc:Ac2O (3:1 volume ratio) and the electrolysis was carried out for 1 hour under 5mA-cm-2 and 90#. The current efficiency was greater than 80%.The electrochemical behavior of Mn(III)/Mn(II) on platinum and graphite electrode in acetic acid-anhydride was investigated. The result of equilibrium potential shown that the conditional electrode potential of Mn(III)/Mn(II) was 0.719v (vs.SCE) in the solution of l.5mol-L-1 KOAc+HOAc-Ac20(3:l volume ratio) at 25# both on the platinum and graphite electrode. The results of potential sweep and RDE on platinum electrode shown that the rate determining step of electrode process for Mn(n) anodic oxidation was controlled by charge-transfer. The anodic charge-transfer kinetic parameters were determined. The results shown that the anodic charge-transfer coefficient is 0.347, the exchange current density i0 is 5.84xl0-6Acm-2, the conditional charge ?transfer rateconstant ka is 1.35x10 m-s at 25 C. The results of linear sweep polarization curve and RDE on graphite electrode shown that the rate determining step of electrode process was controlled by mixing process of diffusion and charge-transfer. The anodic charge-transfer kinetic parameters were determined. The results shown that the anodic charge-transfer coefficient is 0.183, the exchange current density io is 5.80x10-5A-cm-2, the conditional charge-transfer rate constant ka is 9.16 10-8 m-s-1 under 25#. The reaction rate on the platinum and graphite electrode was found to be first order with respect to the concentration of both Mn(II) and OAc.Absorptiophotometric and titration method were used to determine the equilibrium constant for the disproportionation of Mn(III) into Mn(II) and Mn(IV) states in the solution of l.5mol-L-1 acetic potassium and acetic acid-acetic anhydride (3:1 volume ratio) at 25#. The resultsindicated that the molar absorptivity of Mn(ni) and Mn(IV) are 1608 L-morcnT1 and 326.9 L-morcm"1 respectively at 425nm. The molar absorptivity of Mn(II) is so small over the range of wave lengths in the measurements that its contribution to the total absorbance can be neglected. The formal equilibrium constant for the disproportion of Manganese ions is8.08xl(T3.The sorbic acid precusor acetoxyhexenoic acid was produced via Mn(HI) obtained from indirect electro-oxidation of acetic acid-acetic anhydride in the presence of butadiene. The reaction was carried out at 90癈 for 5h. The rate was found to be zero order with respect to betadiene concentration and to be first order with respect to Mn(III) concentration. The reaction rate constant of Mn(III) is 0.03 Brain'1.An absorptiophotometric method for the determination of acetoxyhexenoic acid was described. In the dermination, acetoxyhexenoic acid was first hydrolyzed to form sorbic acid at pH 3 and 30, through the catalytic action of Cu(I). The sorbic acid formed was then oxidized with potassium dichromate in a dilute sulfuric acid medium to form propanedial which was further reacted with thiobarbituric acid to form a red colored complex with its absorption maxima at 530nm. The absorbance of the red complex was directly proportional to the concentration of AA and Beer' law was kept in the range of0~10.1ng of AA per 10 mL solution, with its molar absorptivity 7.64X l0-mol'cm"1. Interference tests were made and tolerance limits of some of the coexisting ions were found as follows (mg per 10ml solution): OAc~ 21.6, Mn(II) 4.06, Mn(III) 0.17, Recoveries obtained were in the range of 92.6%-~103.2%. This method was suitable to be used in the control analysis for electrosynthesis of sorbic acid.According to quantum ch...