Carboxyl radical, oxidation

The anodic oxidation of the carboxylate anion 1 of a carboxylate salt to yield an alkane 3 is known as the Kolbe electrolytic synthesis By decarboxylation alkyl radicals 2 are formed, which subsequently can dimerize to an alkane. The initial step is the transfer of an electron from the carboxylate anion 1 to the anode. The carboxyl radical species 4 thus formed decomposes by loss of carbon dioxide. The resulting alkyl radical 2 dimerizes to give the alkane 3 " ... 

The decomposition of carboxyl radical occurs very rapidly, and C02 is formed with a constant rate in the initiated co-oxidation of cumene and acid [104]. 

The decomposition of carboxyl radical occurs very rapidly, and C02 is formed with a constant rate in the initiated co-oxidation of cumene and acid [104]. Cumylperoxyl radical attacks the a-CH2 group of the carboxylic acid with the formation of a labile hydroperoxide. The concentration of this hydroperoxide increases during oxidation till it reaches a stationary concentration [RCH(OOH)-COOH]st = pi2[RCH2COOH][CuOO ]/A d. This reaction produces C02 with acceleration during some period of time equal to the time of increasing the a-carboxyhydroperoxide concentration. 

There is some competing decarboxylation of the ethanoic acid, but the conversions in this kind of reaction are usually good. The key steps in the reaction probably are exchange of carboxylic acid groups on tetravalent lead, cleavage of the Pb-O bond to give the carboxylate radical, decarboxylation, oxidation... 

Hart (1952, 1954) studied the oxidation of formic acid by the radiolysis method. In the presence of oxygen, hydroxyl radicals abstract hydrogen from HCOzH. Both the carboxyl radicals and formyl radicals are formed. These radicals undergo oxygen addition and subsequently dissociate ... 

The mode of cation formation in electrochemical decarboxylation appears not to be uniform. Skcll 204 found two discrete 1 e-steps for oxidation of car-boxylates by chronopotentiometry. He attributed the second electron transfer to oxidation of the alkyl radical (path b, Eq. (94) ) as the carboxylate radical RC02 is to shortlived (r 10 10 sec) to survive for the second oxidation step. 

Although the aforementioned routes provided the desired y-amino acids, it was desirable to develop a synthesis which incorporates the carboxylic acid oxidation state prior to coupling. We hypothesized that manganese-mediated radical addition would accomplish this objective, and therefore initiated a study of manganese-mediated coupling of alkyl iodides with y-hydrazonoesters [104]. We had already shown that the manganese-mediated radical addition conditions offer excellent chemoselectivity, but it remained to be seen whether the stereocontrol model would be disrupted would an additional Lewis basic ester function in the hydra-zone interfere with the role of In(III) in two-point binding and rotamer control ... 

Oxidation of C—bonds by copper ion catalyzed reaction with an organic peroxy ester (the Kha-rasch-Sosnovsky reaction) was at one time very popular for allylic oxidation and has been thoroughly reviewed. The reaction is usually carried out by dropwise addition of peroxy ester (conunonly r-butyl peracetate or r-butyl perbenzoate) to a stirred mixture of substrate and copper salt (0.1 mol % commonly copper(I) chloride or bromide) in an inert solvent at mildly elevated temperature (60-120 C). The mechanism involves three steps (i) generation of an alkoxy radical (ii) hyttogen atom abstractitm and (iii) radical oxidation and reaction with carboxylate anion (Scheme 11). 

Alkyl radicals derived by decarboxylation of carboxyl radicals may be added m carbon-carbon multiple bonds resulting in an overall homologation of the starting acid. This reaction type is not stricdy a C— bond oxidation nevertheless, one of the key steps is C- bond cleavage by decarboxylation and it is appropriate to briefly consider the scope of such reactions here. A more complete description of inter- and intra-molecular radical C—C bond-forming reactions is given in Volume 4, Chapters 4.1 and... 

It has been suggested that the first step of reaction (6) may be the formation of a carboxylic species COOHads. Carboxyl radicals have indeed been observed by Zhu et al." for potentials lower than 0.65 V using Fourier Transform infrared Reflectance Absorption Spectroscopy with the Attenuated Total Reflection mode (ATR-FTtR). Moreover Anderson et al." made numerical simulation which indicated that the formation of an adsorbed carboxylic species was energetically more favorable. Here, it has to be noted that the electro-oxidation of CO being a stracture sensitive reaction (sensitive to the superficial stracture symmehy" and to the presence of surface defects) this species can be used to study the activity of a catalyst but also as a molecular probe to characterize the catalytic surface. ... 

H. The Oxidation of Higher-Order Hydrocarbons 109 Under this low-temperature condition the carboxylic radical undergoes attack... 

Chemical differentiation between two oxidizable sites in the same molecule can also be achieved in organic photocatalytic reactions by choice of a different semiconductor and thus adjustment of the electrochemical band-edge positions. Consistent with this idea, the photocatalytic oxidation of lactic acid on UV-irradiated plati-nized-Ti02 leads to decarboxylation, presumably through the singly oxidized carboxyl radical. In contrast, the same reagent on irradiated platinized CdS leads to pyruvic acid by oxidation of the alcohol group (Eq. 10) [96]. 

In the past, widely differing mechanisms have been proposed for the Kolbe electrolysis. The free-radical theory, which assumed acyloxy radicals as intermediates, was suggested by Crum-Brown and Wal-ker. According to Glasstone and Hickling in aqueous solution hydroxyl radicals and hydrogen peroxide were formed, which converted the carboxylic acids into dimers and alcohols or esters, the so-called Hofer-Moest products. Schall and Fichter proposed that the carboxylates are oxidatively coupled to diacyl peroxides, that subsequently decomposed. These mechanistic proposals have been reviewed. - The presently accepted mechanistic schemes have been reviewed, - and the general scheme summarized in Scheme 2 is assumed. 

Kiwifruit (Actinidla chlnensls Planch.) concentrates were prepared by vacuum distillation, followed by continuous liquid-liquid extraction and concentration of the extracts. The concentrates underwent various changes when stored at -10°C. The artifacts produced were analyzed and characterized by capillary GC, GC/MS and GC/FTIR. Carboxylic acids, probably produced by free-radical oxidation, comprised the major portion of the artifacts. Possible mechanisms of artifact formation and methods to minimize their production are discussed. 

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