Carboxylate radical

Diacyl peroxides are sources of alkyl radicals because the carboxyl radicals that are intitially formed lose CO2 very rapidly. In the case of aroyl peroxides, products may be derived from the carboxyl radical or the radical formed by decarboxylation. The decomposition of peroxides can also be accomplished by photochemical excitation. 

The reaction is likely to proceed by a radical-chain mechanism, involving intermediate formation of carboxyl radicals, as in the related Kolbe electrolytic synthesis. Initially the bromine reacts with the silver carboxylate 1 to give an acyl hypobromite species 3 together with insoluble silver bromide, which precipitates from the reaction mixture. The unstable acyl hypobromite decomposes by homolytic cleavage of the O-Br bond, to give a bromo radical and the carboxyl radical 4. The latter decomposes further to carbon dioxide and the alkyl radical 5, which subsequently reacts with hypobromite 3 to yield the alkyl bromide 2 and the new carboxyl radical 4Z... 

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 current-potential relationship indicates that the rate determining step for the Kolbe reaction in aqueous solution is most probably an irreversible 1 e-transfer to the carboxylate with simultaneous bond breaking leading to the alkyl radical and carbon dioxide [8]. However, also other rate determining steps have been proposed [10]. When the acyloxy radical is assumed as intermediate it would be very shortlived and decompose with a half life of t 10" to carbon dioxide and an alkyl radical [89]. From the thermochemical data it has been concluded that the rate of carbon dioxide elimination effects the product distribution. Olefin formation is assumed to be due to reaction of the carboxylate radical with the alkyl radical and the higher olefin ratio for propionate and butyrate is argued to be the result of the slower decarboxylation of these carboxylates [90]. 

Esters of (V-hydroxyphthalimide can also be used for decarboxylation. Photolysis in the presence of an electron donor and a hydrogen atom donor leads to decarboxylation. Carboxyl radicals are formed by one-electron reduction of the phthalimide ring. 

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 results in this paper support an N-C bond cleavage mechanism (Schemes I and II) for the photolysis of both TDI and MDI based polyurethanes. The substituted anilinyl radicals observed no doubt are formed by diffusion from a solvent cage after the primary N-C bond cleavage. Although not specifically shown in this paper, the reported photo-Fries products (6) are probably formed by attack of the carboxyl radical on the phenyl ring before radical diffusion occurs. The solvent separated anilinyl radicals rapidly abstract hydrogens from the solvent to give the reported aromatic amine product (6). 

Thus, the observed aromatic carboxylic acids at chain ends [11, 25] would be accounted for by the hydrogen abstraction by the carboxyl radical (Scheme 18.1, path A). This, of course, generates another radical species, R, (not shown) capable of carrying on further degradative reactions. 

The alkoxy radical of Scheme 18.3 (upper reaction) could scission to produce the same carboxyl radical as seen in the Norrish type 1 path (Scheme 18.1, path A) discussed above. As such, it is an additional source of CO2 but not taken into account in the report by Day and Wiles [25], Not reported but still obvious, the other fragment of this scission is an aliphatic aldehyde that could also have been one of the aldehyde carbonyl IR signals reported [11, 25], Hydrolysis of this chain end would yield the reported glyoxal [21],... 

DR. ESPENSON Carboxylate radical anion from radiation chemistry ought to be well set up in the formation of carbonate ion to meet just this requirement. Yet I m under the impression that the carboxylate radical anion doesn t react with perchlorate. 

It is interesting to note that maleic acid dissociates to two carboxyl radicals and acetylene... 

Under this low-temperature condition the carboxylic radical undergoes attack... 

Carboxyl radicals are formed from one-electron reduction of the phthalimide ring. 

The free radical —O—CH—CH2—O— is supposed to undergo 0-bond scission to form a carboxyl radical and a vinyl end group (Reaction 5). The carboxyl radical then abstracts a hydrogen atom from another repeat... 

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. 

For instance, the carboxyl radical will react with alkali such as sodium hydroxide lo form sails with phosphorus peniachloride to form acid... 

Stepwise decarboxylation also occurs, particularly in reactions in which the carboxylate radical (RC02 ) is formed. This radical usually decomposes to a hydrocarbon radical (R-) and C02- The overall decarboxylation product is determined by what R- reacts with If a good hydrogen donor is present, RH is formed if a halogen donor such as Br2 is present, RBr is formed ... 

Carboxylate radicals can be generated in several ways. One is the thermal decomposition of diacyl peroxides, which are compounds with rather weak 0-0 bonds ... 

Another method involves electrolysis of sodium or potassium carboxylate solutions, known as Kolbe electrolysis, in which carboxylate radicals are formed by transfer of an electron from the carboxylate ion to the anode. Decarboxylation may occur simultaneously with, or subsequent to, the formation of carboxylate radicals, leading to hydrocarbon radicals, which subsequently dimerize ... 

The mechanism of this reaction seems to involve formation of carboxylate radicals through decomposition of an acyl hypobromite intermediate, 12 ... 

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 ... 

Buxton GV, Sellers RM (1973) Acid dissociation constant of the carboxyl radical. Pulse radiolysis studies of aqueous solutions of formic acid and sodium formate. J Chem Soc Faraday Trans 1 69 555-559... 

Flyunt R, Schuchmann MN, von Sonntag C (2001) Combination vs. proton-catalysed disproportionation of the carboxyl radical anion, CCV-, in aqueous solution. Chem Eur J 7 796-799 Fojtik A, Czapski G, Flenglein A (1970) Pulse radiolytic investigation of the carboxyl radical in aqueous solution. J Phys Chem 74 3204-3208... 

Diacyl peroxides are another important source of free-radicals and, consequently, are also commonly used as initiators of free-radical reactions. There is a vast amount of data pertaining to the kinetics and mechanism of decomposition of these compounds in conventional solvents there are a number of side reactions, both radical and ionic in nature, that complicate the kinetics of their decomposition. Generally, these compounds decompose by initial 0-0 bond cleavage that generates carboxyl radicals (RC02 ), which subsequently decarboxylate yielding R (Scheme 4.7)... 

The utilization of perfluorodiacyl peroxides for this purpose has been more widely developed. The rate of decomposition of perfluorodiacyl peroxides in the presence of electron-rich benzene derivatives is enhanced by a significant factor via a process of electron-transfer [66, 280], As can be seen by the contrasting examples below [281], highly reactive arenes are capable of trapping the per-fluoroalkyl carboxyl radical before it decarboxylates to RF, a result which can diminish the synthetic utility of this process. 

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