Peracids and their salts

The electrochemical formation of all these peroxyanions requires a high positive potential and to facilitate such conditions, it is normal to use an acidic medium and cither a Pt or Pb02 anode. Because platinum is often superior it is usual to use a platinized titanium, tantalum or niobium surface or a thin platinum foil spread on a base-metal support. 

The largest of these processes remain those for ammonium and sodium persulphate and there are several plants operating on the 2000-10000 ton year scale. The processes involve the oxidation of the sulphate in a sulphuric acid medium at a Pt-based anode and the medium must be free of heavy-metal ions which catalyse the decomposition of persulphate. The current density is high--about 1 A cm -and the current efficiency 60 80%, Two cell technologies are used (1) a cell with an asbestos diaphragm and (2) an undivided concentric tube cell. To avoid reduction of the persulphate at the cathode, the conversion per pass is kept low and the persulphate is crystallized between passes through the cell. 

Perchlorate is always made as the sodium salt potassium and ammonium perchlorate are then prepared by double decomposition. The electrolyte is sodium chlorate (300-700 gdm ) at a pH between 0 and 1 and sodium perchlorate is also oflen present in the cell feed to ease isolation of the product. 

The anode is a platinized titanium or platinum-covered base metal and the cathode is steel or another cheap metal a separator is not necessary as the reduction of perchlorate is strongly kinetically hindered and the cathode reaction is hydrogen evolution. Reported energy consumptions lie in the range 2400-3500 kWh ton The energy efficiencies are as low as 20 %, suggesting that cell designs could well be improved. 

Potassium permanganate is widely used as an oxidizing agent, especially for oxidation in the fine organic chemicals industry. World production is about 40 000 ton yr , by far the largest plant being 15 000 ton yr sited in the USA. The electrochemical step is the oxidation of manganate 

The electrochemical oxidation is carried out with an electrolyte which is caustic potash (1—4 m) and potassium manganate (100—250 g 1 ) at 60°C at an anode made from nickel or monel (Ni/Cu). The cathode is iron or steel. The anode reaction requires an unusually low current density between 5 and 150 mA cm but usually at the lower end of this range. Even so some oxygen evolution occurs and the current yields are between 60 and 90% the material yield generally exceeds 90%. 

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Inorganic peracids and their salts (common examples which are particularly hazardous)... 

E. Searles, Preparation, Properties, Reactions and Uses of Organic Peracids and Their Salts. F.M.C. Corporation, Inorg. Chem. Div., New York, 1964 B. Phillips, Peracetic Acid and Derivatives. 2nd ed. Union Ckrbide Chemicals Co., New York, 1957. 

Epoxidation is carried out by four different procedures (i) epoxidation with peracids such as peracetic acid or perbenzoic acid in the presence of an acid catalyst (ii) epoxidation with organic and inorganic peroxides, including transition metal catalysts (iii) epoxidation with halohydrins using hypoha-lous acids and their salts and (iv) epoxidation with molecular oxygen. 

Use of the imonium group for protection of enones was explored. Stability to peracids, lead tetraacetate, bromine, and acetic anhydride was claimed (727). The usual resistance of enamines (but not their salts) to additions of Grignard reagents was used for selective addition to a 3,17-diketosteroid by formation of the usual 3-monoenamine 728). 

Krief et al. have shown that selenium ylides behave as their sulfur analogues and convert a variety of carbonyl compounds to oxiranes <89H(28)1203>. The latter compounds can be directly obtained by using R2Se=CHR /i-hydroxyalkylselenides (available from carbonyl compounds by addition of RSeCH2Li) may serve as suitable precursors as well, either in a two-step protocol, via the selenonium salt by alkylation with magic methyl (MeS03F), or directly by treatment with thallous ethoxide in chloroform. Oxidation of the /t-hydroxyalkylselenides with peracid, followed by treatment of the resulting selenone with base, results in oxirane formation (Scheme 60). 

The epoxy sulfones were prepared by exhaustive peracid oxidation of the corresponding alkene sulfides. These were generated by ring expansion of cyclic 1-methyl-2-vinyl sulfonium salts 1 via 2,3-sigmatropic rearrangement of the methanides. From the five-membered sulfonium ylide la, (Z)- and ( )-thiacyclooct-4-enes 2 were obtained as an 85 15 mixture. Their chromatographic separation turned out not to be feasible due to concomitant EjZ isomerization on the silica gel column. Since the separation of these epoxy sulfone mixtures obtained by exhaustive oxidation also proved unsuccessful, it was found expedient to first oxidize the mixture of sulfides to an alkene sulfone mixture ( )-3, which could be separated and eventually epoxidized into epoxides cis- and trans-4 and cis- and trans-5. 

Most frequently the polymerization process is initiated by free radicals obtained through the decomposition of hydroperoxides, alkyl peroxides, dialkyl peroxides, acyl peroxides, carboxylic ester peracids, salts of (tetraoxo)sulphuric acid, hydrogen peroxide, aliphatic azo compounds and bifunctional azobenzoin initiators. The rate of decomposition of different initiators into free radicals depends on their stmcture and on temperature. A measure of the efficiency of the initiator in the pol5mierization process is the half-decomposition period. 

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