Equilibrium peracids

Where unsaturated compounds cannot be epoxidized via in situ techniques then equilibrium peracids or solvent extracted peracids are frequently used. The equilibrium peracetic acid is often used stoichiometrically and is the favoured method for unreactive alkenes. Kinetics favour the use of equilibrium peracids over in situ techniques. Ring opening is still, however, a major side-reaction but... 

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). 

The most common commercial equilibrium peracid is PAA. Its preparation is as follows ... 

In contrast to equilibrium peracids, preformed peracids refer to percarboxylic acids that were isolated from the reaction mixture and purified for use in formulations. The development of these peracids offers several challenges due to their susceptibility to alkaline hydrolysis, risk of spontaneous combustion, possible spotting of colored fabric, and poor cost/performance. 

The following consequences of the equilibrium reactions were supposed as predecessors of the final step of S(IV) oxidation by protonated peracid [25]. 

In acidic solution, dihydrothiazin-3-ones are in equilibrium with their enol form and susceptible to electrophilic attack at the 2-carbon. They can be oxidized at this site with peracids or diacyl peroxides (Scheme 25) <19828312, 1982S424>. 

Oxidation with Stoichiometric Oxidants. Certain peracids reacting with alkanes yield alcohols. Peracetic acid,68 69 perbenzoic acid,70 m-CPBA,71,72 and nitroper-benzoic acids may be used. Alcohols or an equilibrium mixture of the alcohol and the trifluoroacetate77 are formed on the action of pertrifluoroacetic acid. A high degree of regioselectivity (better than 97%), specifically, preferential attack at the tertiary C—H bonds, is usually observed ... 

The equilibrium is shifted by removal of the water (134) or removal of the peracid by precipitation (135,136). Peracids can also be generated by treatment of an anhydride with hydrogen peroxide to generate the peracid and a carboxylic acid. 

Peracids are also available as aqueous solutions that contain the peracid in equilibrium with hydrogen peroxide and the parent acid. Peracetic acid [79-21-0] is commercially available as a 40% solution in dilute acetic acid. The water and dilution of the peracid make these solutions easier to handle than their solid counterparts, but they still require careful handling and protection from heat. 

Preformed Peracids. Peracids can be generated at a manufacturing site and directly incorporated into formulations without the need for in situ generation. Two primary methods are utilized for peracid manufacture. The first method uses the equilibrium shown in equation 7 to generate the peracid from the parent acid. 

Conjugated unsaturated ketones are unreactive to peracids because of the depletion of electronic charge in the olefiinic bond, but epoxyketones are readily prepared by using the nucleophilic hydroperoxide ion (H02"") [284], In simple aliphatic compounds the epoxidation follows kinetics first order with respect to hydroperoxide ion and unsaturated ketone [28s]- According to House [286], the reaction should be represented as a reversible addition of hydroperoxide ion, followed by closure of the epoxide ring in a slow step, with expulsion of hydroxide ion. The over-all rate will be determined both by the equilibrium constant in the first step and by the rate constant for the second. 

In this way, for the particular case of the oxidation of ethanal (R=CHs) in liquid phase, it has been shown -that an equilibrium occurs between primary peracid, excess of aldehyde, and an additional per-oxidic compound known as peroxide X. ... 

Smit followed the equilibration of acetic acid and aqueous hydrogen peroxide by titrating the hydrogen peroxide with permanganate, which does not react with peracetic acid. In dilute solution at 0° the hydrolysis of peracetic acid is so slow that one can titrate the peracid by iodimetry. At room temperature, equilibrium is reached only after several days. The reaction is markedly accelerated by 1% sulfuric acid or by warming to 70°. Thus Fernholz heated a mixture of 30 ml. of 30% hydrogen peroxide and 300 ml. of acetic acid for 5 hrs. at 70° and then let the solution stand for 2 days at ambient temperature and obtained a solution containing 2-2.5% of peracetic acid. 

NH2— —NO2. A first communication notes that trifluoroacetic acid, a very strong acid (pKa 0.3) reacts with aqueous hydrogen peroxide much more iiipidly than formic or acetic acid, and that the equilibrium probably is much more liivorable to the peracid. For oxidation of 0.05 mole of p-aminobenzonitrile to p-nitrobenzonitrile, a solution of peracid is prepared by adding 5.1 ml. (0.2 mole) of... 

In reality, the phenomena are complex. Thermal decomposition is superimposed on catalytic decomposition and, at the same time, both homolytic and heterolytic mechanisms occur. The fast equilibrium that is established between peracid and X peroxide also adds to the complexity of the system, viz. 

The most important route to the peroxy acids (formerly called peracids) requires establishment of the equilibrium shown in the following equation ... 

Hydrophilic peracids with less than four carbon atoms are effective bleaches, oxidizers, and disinfectants. The most important is peroxyacetic acid, which is often used in equilibrium mixture with acetic acid and hydrogen peroxide as a 5,10, or 15% active solution. Its characteristic odor and safety aspects, however, prevent its widespread use in homecare applications. 

Lower aliphatic acids. The two peracids most relevant to industry are performic and peracetic. Performic acid is always generated in situ by the above reaction, as it is not sufficiently stable to isolate as an equilibrium mixture. Peracetic acid can, however, be used in a variety of forms including in situ generation, pre-formed equilibrium mixtures, distilled aqueous solution and solvent-extracted products. 

This refers to mixtures containing a peracid, the corresponding carboxylic acid, hydrogen peroxide and water. These are produced by loading a reactor with the carboxylic add and water to achieve the desired concentration, and optionally, sulfuric acid to speed up the reaction. Hydrogen peroxide is then added and the mixture is stirred and left to equilibrate. Equilibration time varies depending on the desired peracid concentration, ratios of the raw materials, and process conditions. The concentration and molar ratio of the acid and hydrogen peroxide determine the position of the equilibrium. 

Most aliphatic carboxylic acids show a very low rate of conversion into peracids under the action of oxygenated water at temperatures below 40°C and in the absence of catalysts [211]. Acid catalysts substantially shorten the time to achieve the reaction equilibrium. These catalysts thus become indispensable for the process of epoxidation by the in situ method. 

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