Peracids, formation

Figure 3. Acid and peracid formation in ozonolyses at 0°C. Average composite of two runs...

Analysis of the total acid titration data indicates that 33-39 mmoles of acid and peracid have been formed from aldehyde oxidation. Furthermore, 34-36 mmoles of aldehyde should have been formed in the first stage and oxidized in the second stage. If aldehyde had been oxidized solely to peracid, the active oxygen determination should have showed 34-36 mmoles of peracid instead of at the most 13 mmoles (meq./2). Enough acid but not enough peracid has been formed. This means that there are at least two paths by which aldehyde can be oxidized to acid, one of which involves peracid formation. 

The lipase method of in situ peracid formation is relatively safe, and the selectivity to epoxide, i.e. the amount of epoxide formed compared with the... 

Figure 3.7 Epoxidation of alkenes by lipase catalysed peracid formation in a two-phase system.

The approaches to anhydrous or essentially anhydrous solutions include reaction of the acid anhydride with hydrogen peroxide in the presence of a solvent,44 oxidation of the analogous aldehyde45 and azeotropic removal of water during peracid formation.46 By far the easiest and safest method is to simply extract the equilibrium peracid into an appropriate solvent.47 The solvents usually employed are ethyl acetate or isopropyl acetate. A range of substrates has been epoxidized using such extracted peracids (Figure 3.12). 

This section describes the oxidation in the temperature range below that at which appreciable peracid, peracyl or acyl radical decomposition occurs. This is not an entirely satisfactory basis for classification since some peracid decomposition must occur if autocatalysis — a characteristic feature of aldehyde oxidation — is to take place. Furthermore, the proportion of RCO radicals generated which decompose depends not only on the temperature but also on the oxygen pressure. Nevertheless, provided that the oxygen pressure is sufficiently high and the temperature below about 150 °C, peracid formation is almost quantitative, at least over the first 50 % or so of reaction. 

Peracid formation is proportional to irradiation time t. The slope of the lines in Figure 2 which is the rate of peracid formation relative to IQ, is definitely increasing with decreasing... 

Figure 2. Peracid formation in isooctane photooxidation against time of irradiation, for different rates of radical initiation I. ...

Figure 4. Fit between kinetic model of isooctane photooxidation and experimental data. (A) Relative rate of peracid formation against -log I. (B) Relative rate of iodometrically...

Figure 5. Consumption of > -H and formation of >N0- in isooctane photooxidation I 5 x 10 M isooctyl radicals/h, corresponding (according to Scheme II) to a rate of peracid formation of...

This method has been used by numerous researchers and has become the simplest and most cost-effective method for the synthesis of PAN, PPN, and PBN calibration standards. For the synthesis of other important PANs this method has some problems. Low aqueous solubilities of some anhydrides, as for PBzN, make the generation of the corresponding peracids problematic. For the unsaturated analogs (e.g., MPAN), polymerization reactions in the acidic solutions used for the synthesis compete with peracid formation. The photochemical procedures remain more successful for the production of these higher analogs. 

Minning, S., Weiss, A., Bomscheuer, U. T., and Schmid, R. D., Determination of peracid and putative enzymatic peracid formation by an easy colorimetric assay. Anal. Chim. Acta, 378, 293-298, 1999. 

Both methods of biocatalytic peracid formation are extremely useful for epoxi laation in oleochemistry because both the necessary carboxyl group and the C=C-... 

For most carboxylic acids, a strong acid catalyst must be added in order to achieve an acceptable rate of reaction sulphuric, or sulphonic acids, the latter in resin-bound form, or phosphonic acids can be used. A few acids are strong enough to catalyse their own peracid formation, notably formic and trifluoroacetic. The reaction may be speeded up by increasing the temperature, subject to safety considerations (see section 9.4). 

Electrode modification can be carried out by methods that vary greatly. A reaction can be affected simply by addition to the electrolysis solution of a substance that is readily adsorbed onto the electrode surface. Thus, additimi of a thiocyanate salt to the medium diverts the anodic oxidatimi of carboxylates frran decarboxylative dimerization (Kolbe reaction) to peracid formation [1]. Often, a polymer solutimi containing an electrocatalyst is placed on a surface, and the solvent evaporated or a monomer is electrochemicaUy polymerized in situ from solution mito the surface. Electrocatalysts deposited in this manner include organometallic electrocatalyst complexes such as vitamin B12 [2], oxidizable heterocycles such as pyrrole or thiophene, or metal ions [3]. Successive layers of complementary materials may be laid down on an electrode to achieve the desired immobilization effect. Thus, a polymer (PDAA polydimethyldiallyl ammonium chloride) bearing... 

Maleic anhydride monomer cont.) oxime adducts, 228 palladium complexes, 213 peracid formation, 246 phenols photochemical reaction, 180 phosphine reactions, 230 phosphite reactions, 206 photo-adduct applications, 213 photo-adduct sterochemistry, 182, 183, 192-194 photochemical dimerization, 188 photochemical polymerization, 195, 239, 241-243, 247, 248... 

Acidic resins can be used as heterogeneous catalysts for in situ peracid formation. Protonic ion-exchange resins based on sulfonated polystyrene were used for epoxidations [119]. Acetic acid was, thus, used successfully as epoxidation catalyst in combination with resins such as Dowex 50W-X8 [104,120,121], Dowex 50X-12 [95], or Amberlite IR-120 [118,122] to epoxidize PBD. The formation of products from side reactions of the epoxides are to be expected and in some cases have been reported [122]. A more advanced approach uses the immobilized enzyme Novozym 435 to catalyze the in situ formation of peracetic acid from acetic acid and hydrogen peroxide [123]. 

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