Peroxyacids, structure

TABLE 9. PROPERTIES OE SOME ORGANIC PEROXYACIDS Peroxyacid Structure Mp. °C... 

EP 0 564 250 (1993) J. L. Coope et al., Unilever PLC Amido or imido organic peroxyacid Structured hquid with add or neutral pH, containing a pH jump system... 

Treatment of the following alkene with a peroxyacid yields an epoxide different from that obtained by reaction with aqueous Br2 followed by base treatment. Propose structures for the two epoxides, and explain the result. 

We will now explore each aspect of this two-step process. In the first step, a peroxyacid (RCO3H), sometimes called a per-acid, reacts with the alkene. Compare the structure of a carboxylic acid with the structure of a peroxy acid ... 

The rate of epoxidation of alkenes is increased by alkyl groups and other ERG substituents and the reactivity of the peroxy acids is increased by EWG substituents.72 These structure-reactivity relationships demonstrate that the peroxyacid acts as an electrophile in the reaction. Decreased reactivity is exhibited by double bonds that are conjugated with strongly electron-attracting substituents, and more reactive peroxyacids, such as trifluoroperoxyacetic acid, are required for oxidation of such compounds.73 Electron-poor alkenes can also be epoxidized by alkaline solutions of... 

Since the oxides do not have to be isolated, the sulfur solution after addition of the peroxyacid solution is simply kept in the refrigerator until S,(, has formed which is then isolated by cooling and recrystallization When both Sg and S g are dissolved in CS and the solution is cooled, then, under special concentration conditions, a new sulfur allotrope crystallizes out as orange-yellow opaque crystals of m.p. 92 °C. This compound has been shown by vibrational spectroscopy and X-ray structural analysis to consist of equal amounts of Sg and molecules in their usual conformations. In solution the mean molecular weight of 258 corresponding to 8 atoms per molecule indicates complete dissociation This is the first example of an allotrope of a chemical element consisting of molecules of different sizes. 

The two major characteristic oxidation processes of alkynes are their transformation to 1,2-dicarbonyl compounds and their cleavage reaction to carboxylic acids.710 The structure of the starting compounds has a decisive effect on the selectivity of oxidation. Since 1,2-dicarbonyl compounds proved to be intermediates in further oxidations, carefully controlled reaction conditions are often necessary to achieve selective synthesis. Certain oxidizing agents such as peroxyacids and ozone are nonselective oxidants. 

Organic peroxides can be classified according to peroxide structure. There are seven principal classes hydroperoxides dialkyl peroxides a-oxygen substituted alkyl hydroperoxides and dialkyl peroxides primary and secondary ozonides peroxyacids diacyl peroxides (acyl and organosul-fonyl peroxides) and alkyl peroxyesters (peroxyearboxylales. peroxysul-fonates, and peroxyphosphates). 

Aliphatic Peracids (Peroxyacids), called also Peroxides of the Structure RC( 0)00H... 

Peroxyacid (Section 12.7) An oxidizing agent having the general structure RCO3H. 

The Prilezhaev reaction is stereospecific, and a syn addition of the oxygen to the double bond is observed in all cases. This observation supports the assumption that the epoxidation of alkenes by peroxyacids is a concerted process. The reaction takes place at the terminal oxygen atom of the peroxyacid, and the n HOMO of the olefin approaches the o LUMO of the 0-0 bond at an angle of 180° (butterfly transition structure). 

Ketone peroxides are mixtures of various isomers, and no definite structure can be ascribed to these compounds. Nitrogen- and sulfur-containing peroxides are weU known, some of which are hazardous. Peroxyacids are discussed separately in Chapter 3. 

The last two steps are rather well understood. There exist some uneertainties encountering initiation steps due to the character and distribution of initiation centers in the polymer mass such as sensitizing impurities, charge transfer complexes (CTC) and/or participation of active environmental pollutants [3-5]. Generally this does not affeet however the structure of the oxidation products having oxidation properties formed in the eonsecutive steps to initiation [alkylhydroperoxides POOH, peroxyacids PC(0)00H] and their oxygen-centered free radieal precursors [POO, PC(0)00 ] that play an important role in the fate of polymer additives, stabilizers in particular. 

As a rule, alkenes do not react with 78-80 unless there is another reagent present—specifically, a transition metal. This reaction will not be discussed further. In sharp contrast, peroxycarboxylic acids such as 81 react directly with alkenes. Peroxycarboxylic acids 81 are named by adding the term peroxy to the name of the carboxylic acid (see Chapter 5, Section 5.9.3 and Chapter 16, Section 16.4). Using the common names, the peroxy analog of formic acid is peroxyformic acid (82), and others include peroxyacetic acid (83), peroxytrifiuo-roacetic acid (84), peroxybenzoic acid (85), and me a-chloroperoxybenzoic acid (abbreviated mCPBA, 86). Peroxycarboxylic acid 85 is a derivative of the aromatic carboxylic acid benzoic acid (PhCOOH), and the carboxylic acid precursor to 86 is clearly another aromatic carboxylic acid. (The nomenclature and structural features of benzoic acid and other aromatic carboxylic acid derivatives will be discussed in detail in Chapter 21, Section 21.2.) The salient feature of peroxyacids 82-86 is the presence of the electrophihc oxygen atom mentioned previously, which will react with an alkene. 

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