Reactivity of the Carbonate Radical

Reactivity of The Carbonate Radical. —Rate constants for the reaction of C63H radicals with several complexes have been measured. Of the labile aquo-ions Co +, Zn +, Ni +, Cu , and Mn +, only Co (A =2.8x 10 M cm ), Cu + ( 4.5 X 10 M s 0 and Mn (A =1.5x 10 M s ) react at measurable rates the oxidation potential of the metal ion appears to be a governing factor. Two Co -macrocycle complexes, [Coi (2)] and [Co (3)] (see below), whose equatorial co-ordination sites are substitution inert and axial sites have labile HjO, react 200 times faster than Colq, and here the rate is probably substitution limited.  

Complexes of the composition [M (NH3)5(OH)] (M = Cr, Co, Rh, or Ru) are more reactive than their conjugate acids and, except for Co , than the corresponding [M (NH3)5C1] + complexes. This seems to rule out oxidation of the ligand by direct electron or H-atom transfer. 

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Larson, R. A., and R. Zepp, Reactivity of the carbonate radical with aniline derivatives , Environ. Toxicol. Chem., 7, 265-274(1988). 

Chen S, Hoffmann M Z (1975) Reactivity of the carbonate radical towards aromatic compounds in aqueous solutions, Journal of Physical Chemistry 79 1911-1912. 

Chen S-N, Hoffman MZ. (1975) Effect of pH on the reactivity of the carbonate radical in aqueous solution. Radiat Res 62 18-27. 

The bicarbonate radical, HC03 , is more acidic than HC03 (pKa = 10.3 vs. 10, respectively), but the pK for equilibrium (16) is poorly constrained with reported values between 7.0-9.6 [150-152]. In many fresh water systems the predominant species will be the bicarbonate radical, while both the bicarbonate and carbonate radicals will be important in seawater. As may be expected, the reactivity of the carbonate radical with many organic and inorganic species will be pH dependent [153]. 

S.-N. Chen, M.Z. Hoffman, G.H. Parsons, Jr. (1975). Reactivity of the carbonate radical toward aromatic compounds in aqueous solution. J. Phys. Chem., 79, 1911-1912. 

Persistent and stable silyl radicals have attracted considerable attention [42]. Bulky aryl or alkyl groups that generally make carbon-centred radicals persistent [43,44] have a much weaker effect on the silyl radicals. The high reactivity of the Ph3Si radical contrary to the stable Ph3C radical is mentioned above. The decay of the trimesitylsilyl radical at 63°C follows a first-order kinetics with a half-life of 20 s [37]. Tri-tert-butylsilyl radical is also not markedly persistent showing the modest tendency of tert-h Ay groups to decrease pyramidalization... 

Here it should be mentioned that the chemical structure of carbonate radical is still under debate. The reaction of the carbonate radical has been intensively investigated by pulse radiolysis [154-156] and laser photolysis [157], and the pAa of HCO3 radical was determined to be 7.6 [158] and 9.6 [157,159,160] from the change of the reactivity as a function of pH. However, recent studies of Raman spectroscopy [160] and pulse radiolysis [161] of carbonate solution proposed that the carbonate radical is a strong acid and keeps a chemical form of CO3 even at below pH 0. Furthermore, dimer formation mechanism has also been proposed [78]. 

Consequent upon the orbitals preferred for the unpaired electron, a high spin population occurs at C-4 in each of 1,52, and 53. These are the anion-radicals found to exhibit the least persistence.35 Relative reactivities of the anion-radicals have been estimated by a cyclic voltammetric method as 1 > 1,3,5-triazine anion-radical > 52 > 53 > 22 > 54, which corresponds with qualitative observations on persistence and accords very closely with the ranking order of maximum spin populations at carbon.119 120 Dimerization of the radicals at positions of high spin population at carbon is proved for 1 and 52, although the mechanism is by no means clear for 52 and other modes of reaction can also occur, e.g., proton abstraction from solvent or adventitious water.30,35,121-127... 

However, if the reactivity of the attacking radical is reduced sufficiently, then some degree of selectivity may be introduced. An example of this is that the ratio of chlorination at a primary, secondary or tertiary carbon atom is 1 4.4 6.7 while for bromination the ratio is 1 80 1600. 

Rate constants for reaction of the carbonate radical with a variety of organic compounds in aqueous solution range from less than 10 to 10 1 mol s (Table 5). A comprehensive tabulation of rate constants is given in Neta et al. [158]. Many of these reactions are temperature dependent characterized by an Arrhenius response [156]. Competing with DOM, the carbonate radical is fairly reactive towards inorganic species in aqueous solution (Table 5) including 02 and H202[159] ... 

Carbon-13 NMR spectroscopy may be used for detailed studies of the microstructure to test the predictions of the sequence lengths from the simple copolymer equation. Refinements to the equation include the consideration of the effect of the penultimate group in the chain on the reactivity of the terminal radical with the monomers (Tirrell, 1989). These studies do not provide information regarding the molecule-to-molecule variation in composition that may occur, since again these sequence distributions are the average over the whole population. 

The reactivity of the carbon atoms in the heterocycles relevant to this chapter can be exclusively restricted to the class of 2,3-dihydro-1,3-diboroles. Here the only atom of interest is C-2. Its tendency to lose a hydrogen atom and to form the 1,3-diborolyl radical is described in detail in Sections... 

In this molecule the possibility of resonance with structures containing trivalent carbon stabilizes the molecule and reduces the reactivity of the free radical. 

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