Radicals RCOO

The heats of formation of the radicals RCOO, where R — CH3J CgHg, n-CgH and CgHg, have been determined from dissociation energies i)(RC0-0—O-CO-R), obtained from pyrolysis measurements on the respective peroxides. These dissociation 

The heat of formation of gaseous acetic acid is — 109-9 kcal/ mole, which refers to one half of the heat of formation of the dimer, so that one half of the heat of dimerization must be subtracted from this heat of formation to give that of gaseous, monomeric acetic acid. The heat of dimerization has been measured as — M-5 kcal/ mole, so that we arrive at a value of — 102 8 kcal/mole for the heat of formation of the gaseous moiiomer. Using the value for the heat of formation of the radical CH OOO of 45 zb 2 kcal/mole we obtain the value D(CH3COO—H) — 110 ib 2 kcal/mole from the relationship 

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For a homologous series of acids, RCOOH, the ionization heat wdll, therefore, be affected by the dissociation energy of the oxygen-hydrogen bond (b), the electron affinity of the radical RCOO (c), and the hydration energy of the ion RCOO (d). From a study of the variations of ionization heats, within a homologous series, it may be possible to suggest which of these factors is important. 

These decompose to give alkyl radicals RCOO- R. + COs... 

Diacyl peroxide decomposition can yield two types of radicals (RCOO, R ) ... 

Kolbe electrolysis The electrolysis of sodium salts of carboxylic acids to prepare alkanes. The alkane is produced at the anode after discharge of the carboxylate anion and decomposition of the radical RCOO- RCOO + e-RCOO —> R + CO2 R + RCOO —> R — R + CO2... 

Thus, the RCOO radicals produced initially are unstable, and before (or while) undergoing dimerization, split up into simpler radicals and CO2 molecules. 

Reactions involving a crossed anodic condensation are of practical interest when they combine organic radicals of different type. In solutions containing anions RCOO and R COO, condensation products of the type RR are formed together with the standard products RR and R R by the reactions described. 

Routes C, D, and E. In these routes the C-3 ester function is cleaved under forms of RCOO% RCO-, and RCOOH, respectively. In the last case, ionized tropidine is formed. Concomitant losses of ethylene and hydrogen radical lead again to the /V-methylpyridinium ion (cf. Route A). 

Figure 11.11 The resonance structure of a generalized organic acid RCOO-, where R is any organic group and -COO- is the acid radical, (a) shows two possible equivalent forms for this structure, and (b) the resonance structure, which is a mixture of the two.

The differences in the rates of decomposition of the various initiators are related to differences in the structures of the initiators and of the radicals produced. The effects of structure on initiator reactivity have been discussed elsewhere [Bamford, 1988 Eastmond, 1976a,b,c Sheppard, 1985, 1988]. For example, k,i is larger for acyl peroxides than for alkyl peroxides since the RCOO- radical is more stable than the RO radical and for R—N=N—R, kd increases in the order R = allyl, benzyl > tertiary > secondary > primary [Koenig, 1973]. 

The electrochemistry of RH-Nu" systems is well established (Eberson and Nyberg, 1976 Eberson et al., 1991 Childs et al., 1991). The radical cation mechanism has been shown to prevail for most situations where Nu" = F", Cl , RCOO", OCN", CN", NOJ, Py and triethyl phosphite, all of them nucleophiles that are difficult to oxidize (Table 5). The initial formation of Nu" is indicated for the redox-reactive SCN", N3, I- and N02, with Br and (N02)3C occupying a somewhat indeterminate position. 

The situation is similar in the case of oxygen evolution, where and are postulated as adsorbed intermediates, even though these radicals are very unstable in the bulk of the solution. The same type of argument has been employed to justify the existence of adsorbed intermediates in some complex organic reactions, such as RCOO and... 

The anodic oxidation of RCOO produced by deprotonation of RCOOH occurs at about 2 V (vs. SCE), known as the Kolbe reaction, producing the radical coupling products of the alkyl groups of RCOO, R-R and CO2 [350]. Since the one-electron reduction potential of the singlet excited state AcrH+ (2.3 V vs. SCE) [351] is positive enough to oxidize RCOO, photoinduced electron transfer from RCOOH (R = H, Me, Et, /-Pr, /-Bu, C11H23, C15H31) to AcrH+ occurs in the... 

When a chemical intermediate step in an overall electrochemical reaction sequence is rate determining, for example, an adsorbed radical recombination step or a first-order dissociation step involving an adsorbed intermediate [e.g., of RCOO in the Kolbe reaction (75)], then the general principles of heterogeneous catalysis do apply more or less in the usual way. However, even then, at an electrode, it must be noted that its surface is populated also and ubiquitously by oriented adsorbed solvent molecules (2, i) and by anions or cations of the electrolyte (7). The concentrations and orientational states of these species are normally dependent on electrode potential or interfacial field (7-i). 

Reductive alkylation of N-methylacridinium (87) occurs when it is irradiated with carboxylic acid salts. The reaction is thought to proceed by electron transfer from the carboxylate to the excited acrldinium ring followed by decarboxylation of RCOO coupling of the alkyl radical produced with the acridinyl radical then gives (88). A very similar sequence probably occurs in a reaction proposed as a synthetic procedure for decarboxylation of carboxylic acids.In this case an aza-aromatic compound such as acridine is irradiated with a carboxylic acid in benzene in the presence of tert-butyl thiol. The authors propose that a hydrogen bonded acridine-acid complex is excited and that adiabatic proton transfer is followed by electron transfer. This produces RCOO which decarboxylates, and reduction of the alkyl radical then ensues. The major fate of the acridine is coupling to (89) if the reaction is perfonned in the absence of oxygen. 

OH ------> RCOO -b H2O. Hydroxyl radical is more toxic to living... 

Kolba s method A method of making alkanes by electrolysing a solution of a carboxylic acid salt. For a salt Na+RCOO , the carboxylate ions lose electrons at the cathode to give radicals ... 

Gaseous mixtures of N O, and aldehydes give as a major reaction product peroxyacyl nitrates RCOOjNO [21, 37]. In view of the potential role of these reactions in the formation of photochemical smogs, the kinetics and mechanism of the + aldehyde system have been studied. The RCOO NO formation involves the reactions of NO and NO radicals in equilibrium with N O ... 

However, it was suggested that if the residual anions could be completely removed from the organoclays, the primary degradation pathway would switch to an elimination-type mechanism [16]. The Hofmann elimination of ammonium compounds was most probably the source of additional amounts of vinyl-type unsaturation found in melt-processed OMMT-PE relative to both the polymer control and the Na+MMT-PE samples [28]. On the other hand, the presence of alkenes was also explained by three possible routes of decomposition (i) pyrolysis of alkanes derived from the major component of the organic part, i.e., hydrogenated tallow (HT) (ii) pyrolysis of the tallow (unsaturated fatty acids used for the preparation of the quaternary ammonium salt) and (iii) decarboxylation of RCOO and RCO radicals [17, 29]. 

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