Peroxide bond

Having a weak O—O bond, peroxides split easily into free radicals. In addition to homolytic reactions, peroxides can participate in heterolytic reactions also, for example, they can undergo hydrolysis under the catalytic action of acids. Both homolytic and heterolytic reactions can occur simultaneously. For example, perbenzoates decompose into free radicals and simultaneously isomerize to ester [11]. The para-substituent slightly influences the rate constants of homolytic splitting of perester. The rate constant of heterolytic isomerization, by contrast, strongly depends on the nature of the para-substituent. Polar solvent accelerates the heterolytic isomerization. Isomerization reaction was proposed to proceed through the cyclic transition state [11]. 

Oxygen-oxygen bonds (peroxides, hydroperoxides, ozonides)... 

The principal light-absorbing groups of lipids are double bonds, peroxide 0—0 bonds, and carbonyls the last two are most important. The primary mechanism by which ultraviolet radiation initiates lipid oxidation is actually indirect, mediated through homolytic scission of any preformed hydroperoxides to generate the true initiators— LO, HO, and RO —that abstract hydrogens from lipid molecules and form the ab initio L. ... 

This is called an initiation step. It may happen spontaneously or may be induced by heat or light (see the discussion on p. 279), depending on the type of bond. Peroxides, including hydrogen peroxide, dialkyl, diacyl, and alkyl acyl peroxides, and peroxyacids are the most common source of free radicals induced spontaneously or by heat, but other organic compounds with low-energy bonds, such as azo compounds, are also used. Molecules that are cleaved by light are most often chlorine, bromine, and various ketones (see Chapter 7). Radicals can also be formed... 

The prepolarization of the TiOz electrode, leading to the steady-state coverage by the surface-bonded peroxides, is shown to cause a perceptible decrease of the quantum efficiency of the photocurrent (curve A). Curve B in Fig. 16 has been obtained under identical conditions and for the same TiOz electrode made practically free from the peroxo species. As at this relatively positive potential (about 0.8 V more positive than the flat-band potential of anatase) the surface electron-hole recombination becomes negligible, the observed diminution of the photocurrent (attaining, in Fig. 16A, 15 per cent) is to be ascribed as a whole to the increase of the photoanodic overvoltage. 

The potential at which this reaction can be carried out is so high (E >1.3 V) that even noble metals are covered with an oxidic layer in fact, conducting oxides (e.g. Co-based spinels and perovskites) are often used in this process. Because of the forcing conditions, anode stability looms large, but that is another story. Many, very many mechanisms have been proposed for the reaction. Fortunately, they can be reduced to essentially two types, the same two t) es we have encountered in the case of H2 evolution vide supra), viz. the recombination of two adsorbate atoms and the reaction of an adsorbate with a solution species under the formation of an 0-0 bond ( peroxide path). The prototype of the former mechanism is that of KrasiTshikov ... 

We note that peroxy acid s corresponding carboxylic acid is also a by-product. If a compound contained a carbon-nitrogen double bond instead of a carbon-carbon double bond, peroxidation would produce a three membered ring that contained both a nitrogen and an oxygen. In this case we are dealing with an optically active peroxy acid which would give an optically active carboxylic acid. 

Oxygen to Oxygen Single Bond Peroxide/Nitro... 

Nitrogen-containing compounds (NO2 groups, adjacent N atoms, etc.) Oxygen-oxygen bonds (peroxides, hydroperoxides, ozonides)... 

Table 8.19 Bonding peroxide cured elastomers to steel with the EPDM Saret 633 adhesive strip ...

Table 8.20 Bonding peroxide-cured EPDM to steel with reactive dispersions ...

Model for Stable Three-Electron-Bonded Peroxide Radical Anions. 

An unstable substance imder the control of heat is added to the reaction, which will dissociate and produce free radicals. These are often molecules with weak bonds peroxide compounds or di-nitrogen derivatives. 

Note that we cannot describe O3 as in Structure 12.5, because this would involve expanding the octet of a second-row element. Microwave spectroscopy of O3 indicates a bond angle of 116.8° and an 0—0 bond distance of 127.8 pm this compares with the 0—0 distance of 120.7 pm in doubly bonded O2 and 149 pm in the singly bonded peroxide ion, 02 ". 

The end products are seen to consist of covalent carbon-carbon bonds, peroxide bonds, and perhaps hydroperoxides. Although any carbon-hydrogen bond might be attacked, positions that are especially vulnerable are those adjacent to a double bond, adjacent to an ether linkage, or on a tertiary carbon (indicated by arrows in Table 12.1). 

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