Studies in electrosynthesis; and, Studies on absorption spectra of solutions

Abstract

In spite of the large amount of experimental work dealing with the electrolysis. of organic acids and their salts, a brief resumé of which up to about 1925 has been given in the preceding section, there was no general agreement regarding the mechanism of the various reactions occurring at the anode. On the theoretical. side controversy centred mainly round the respective merits of the "oxidation" and "discharged ion" theories in. accounting for the experimental facts. In the more recent work the revival of the "peroxide"-theory by Fichter introduced an additional cause for differences of opinion.The general purpose of the investigation forming the subject of Part A of this thesis, was to make a further study of the processes of electrosynthesis and to attempt to discriminate more clearly between the various theoretical explanations of the reactions occurring at the anode.It was decided to make a more detailed investigation of the original Kolbe reaction using salts of acetic acid, mainly for three reasons.Firstly, for this acid there already existed a considerable amount of experimental data.Secondly, the main products of the electrosynthesis in this case are gaseous, and the course of the reaction lends itself to being followed by convenient analytical methods.Thirdly, the decomposition of acetyl peroxide, which is of interest for the electrosynthesis process, had not been previously studied to any extent. Indeed, the experimental evidence regarding the controlled decomposition of the peroxides of aliphatic acids in general was very meagre compared with the. data available for benzoyl and other aromatic peroxides (cf. Gelissen and Hermanns, Ber., 1926, 59, 662).The study of the decomposition of acetyl peroxide was therefore of direct interest in the general problem of the thermal decomposition of diacyl peroxides.The experimental work, which was carried out during the period 1925 - 1937, followed three main lines of investigation:1) The improvement of the gas- analytical methods and the carrying out of a more detailed examination of the gases evolved at the anode during the electrolysis of acetate solutions under various conditions. 2) A more comprehensive study of the anode potential changes during the electrolysis, and a comparison of these with -the gas- analysis datae 3) A more detailed investigation of the decomposition reactions of acetyl peroxide under various conditions and of the possible intermediate formation of this substance at the anode during the electrolysis of acetate solutions.A) LIST OF PUBLISHED PAPERS. An account of the experimental work and of the main results obtained is given in the following nine published papers, copies of which are attached. 1. The mechanism of Kolbe's electrosynthesis. (with D.A. Fairweather) J.Chem. Soc., 1926, 3111 - 3121. 2. The decomposition of acetyl peroxide. J.Chem. Soc., 1928, 2040 - 2045. 3. Formation of methane during the electrolysis of potassium acetate. (with S.N. Shukla) Trans. Faraday Soc., 1931, 35 -40. 4, The analysis of mixtures of hydrogen, methane, and ethane.; (with S.N. Shukla) J.Chem. Soc., 1931, 368 -370. 5. Discharge potential of the acetate ion. (with S.N. Shukla) Trans. Faraday Soc., 1931, 722 - 730 6. Formation of methane during the electrolysis of potassium acetate. (with S.N. Shukla) Trans. Faraday Soc., 1932, 457 - 462 7. Decomposition of acetyl peroxide, (with G.L.E. Wild) J.Chem.- Soc., 1935, 207 -210. 8. Influence of the electrode surface on anodic processes. (with J. Weiss) Trans. Faraday Soc., 1935, 1011 -1017 9, Thermal and photochemical decomposition of acetyl peroxide. (with G.L.L. Wild) J.Ohem.Soc., 1937, 1132 - 1136b) Summary of experimental results. The main experimental results can be summarized as follows: 1. The electrolytic oxidation of acetic acid is complete and not partial under ordinary conditions and occurs to a considerable extent at low anode potentials. (Paper 1) 2. The critical anode potential for the formation . of ethane in the electrolysis of Aqueous potassium acetate solutions using a smooth platinum anode iS 2.14 volts. This- potential is independent of the size of the anode and is not much affected by the concentration of the acetate, solution or by increase of temperature. (Papers 1 and 5) 3. In non -aqueous solvents the formation of ethane in general occurs more readily and is not so dependent on the nature of the anode material., (Papers 1 and 6) 4, The change in the anode process brought about by the addition of metallic salts to the acetate solution has been shown to be connected with changes in the nature of the'anode surface, probably due to the formation of oxide layers. (Paper 8) 5. Using a more accurate gas -analysis technique (paper 4) a new type of anode reaction leading to the formation of considerable amounts of methane has been found, (Papers 3 and 6) 6. The conditiöns: favourable to the formation of methane appear to be low current density and high concentration of the electrolyte, both in aqueous and in non -aqueous solutions. (Papers 3,5 and 6) 7, With a smooth platinum electrode methane commences to be formed at the same critical anode potential of 2.14 volts which is characteristic of the electrosynthesis reaction. (Paper 5) 8. The thermal decomposition of acetyl peroxide vapour at 100 deg. C follows very largely the reaction (CH3C00)2 C2H6 + 2 CO2 (Paper 9) 9. Thermal decomposition of the peroxide in the liquid state or in solution at temperatures up to 100 deg. C is a more complex process resulting in the formation of more methane than ethane even at relatively low temperatures. The decomposition in hexane solution is of the first order with respect to-the peroxide. (Papers 2, 7 and 9) 10. In the photochemical decomposition of acetyl peroxide in solution reaction according to the scheme mentioned in (8) occurs to a greater extent than in the thermal decomposition, but even so more methane than ethane is produced. The.photochemical decomposition of the solid peroxide, however, approximates more closely to the ethane reaction; (Papers 2 and 9)