Carbon dioxide half-reactions

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]. 

C19-0050. What are the half-reactions for these redox processes (a) Aqueous hydrogen peroxide acts on Co, and the products are hydroxide and Co , in basic solution, (b) Methane reacts with oxygen gas and produces water and carbon dioxide, (c) To recharge a lead storage battery, lead(II) sulfate is converted to lead metal and to lead(IV) oxide, (d) Zinc metal dissolves in aqueous hydrochloric acid to give ions and hydrogen gas. 

If carbon dioxide is reduced directly to give products of interest, the reduction potentials for the half-cell reactions in an aqueous solution of pH 7 are as follows ... 

Still another possibility in the base-catalyzed reactions of carbonyl compounds is alkylation or similar reaction at the oxygen atom. This is the predominant reaction of phenoxide ion, of course, but for enolates with less resonance stabilization it is exceptional and requires special conditions. Even phenolates react at carbon when the reagent is carbon dioxide, but this may be due merely to the instability of the alternative carbonic half ester. The association of enolate ions with a proton is evidently not very different from the association with metallic cations. Although the equilibrium mixture is about 92 % ketone, the sodium derivative of acetoacetic ester reacts with acetic acid in cold petroleum ether to give the enol. The Perkin ring closure reaction, which depends on C-alkylation, gives the alternative O-alkylation only when it is applied to the synthesis of a four membered ring ... 

Carbon Dioxide 0=C=0 The reactions of the metallocene sources 1-6 with carbon dioxide depend strongly on the metal and ligands used. Complex 1 gives, by elimination of half of the alkyne, the dimer 93, which forms the titanafuranone 94 after aerial oxidation [49]. 

Note that Equations 2.3 and 2.4 give the same value for rate, as the actual rate of change of each compound is corrected for stoichiometry. This becomes clear by considering the reaction shown in Equation 2.5, where two moles of methanol react with one mole of carbon dioxide and half a mole of oxygen to produce one mole each of carbonic acid dimethyl ester and water. 

Information concerning the position of the carboxyl groups relative to each other was obtained from the neutralization behavior after reaction with thionyl chloride 35, 47). More base ought to be consumed by such products because additional alkali is used for the neutralization of the hydrochloric acid liberated on hydrolysis. As is shown in Table VII, the additional alkali consumption was equivalent to the quantity of chloride ions found in the solution. However with all the samples which had been activated with carbon dioxide, less sodium ethoxide was consumed than had been expected. The deficit was equivalent to half the NaHCOj neutralization value. This strange behavior can be explained only by... 

Photolytic. A photooxidation rate constant of 6 x 10 " cm /molecule-sec at room temperature was reported for the vapor-phase reaction of benzene with OH radicals in air (Atkinson, 1985). The reported rate constant and half-life for the reaction of benzene and OH radicals in the atmosphere are 8.2 x 10 M/sec and 6.8 d, respectively (Mill, 1982). Major photooxidation products in air include nitrobenzene, nitrophenol, phenol, glyoxal, butanedial, formaldehyde, carbon dioxide, and carbon monoxide (Nojima et al., 1975 Finlayson-Pitts and Pitts, 1986). 

Chemical/Physical. A glass bulb containing air and 1,1-dichloroethane degraded outdoors to carbon dioxide and HCl. The half-life for this reaction was 17 wk (Pearson and McConnell, 1975). Hydrolysis of 1,1-dichloroethane under alkaline conditions yielded vinyl chloride, acetaldehyde, and HCl (Kollig, 1993). The reported hydrolysis half-life at 25 °C and pH 7 is 61.3 yr (Jeffers et al., 1989). 

Chemical/Physical. Hydrolysis in distilled water at 25 °C produced l-chloro-2-propanol and HCl. The reported half-life for this reaction is 23.6 yr (Milano et al., 1988). The hydrolysis rate constant for 1,2-dichloropropane at pH 7 and 25 °C was determined to be 5 x 10 Vh, resulting in a half-life of 15.8 yr. The half-life is reduced to 24 d at 85 °C and pH 7.15 (Ellington et al., 1987). A volatilization half-life of 50 min was predicted from water stirred in an open container of depth 6.5 cm at 200 rpm (Dilling et al., 1975). Ozonolysis yielded carbon dioxide at low ozone concentrations (Medley and Stover, 1983). 

Chemical/Physical. Hydrolysis in distilled water at 25 °C produced 3-chloro-2-propen-l-ol and HCl. The reported half-life for this reaction is only 2 d (Kollig, 1993 Milano et al., 1988). trans-1,3-Dichloropropylene was reported to hydrolyze to 3-chloro-2-propen-l-ol and can be biologically oxidized to 3-chloropropenoic acid which is oxidized to formylacetic acid. Decarboxylation of this compound yields carbon dioxide (Connors et al., 1990). Kim et al. (2003) reported that the disappearance of tra 35-l,3-dichloropropylene in water followed a first-order decay model. At 25 and 35 °C, the first-order rate constants were 0.083 and 0.321/d, respectively. The corresponding hydrolysis half-lives were 8.3 and 2.2 d, respectively. 

When an aqueous solution containing pentachlorophenol (45 pM) and a suspension of titanium dioxide (2 g/L) was irradiated with UV light, carbon dioxide and HCl formed in quantitative amounts. The half-life for this reaction at 45-50 °C is 8 min (Barbeni et al, 1985). When an aqueous solution containing pentachlorophenol was photooxidized by UV light at 90-95 °C, 25, 50, and 75% degraded to carbon dioxide after 31.7, 66.0, and 180.7 h, respectively (Knoevenagel and Himmelreich, 1976). The photolysis half-lives of pentachlorophenol under sunlight irradiation in distilled water and river water were 27 and 53 h, respectively (Mansour et al., 1989). 

Chemical/Physical. Emits toxic phosgene fumes when heated to decomposition (Sax and Lewis, 1987). In a 0.50 N sodium hydroxide solution at 20 °C, chlorpropham hydrolyzed to aniline derivatives. The half-life of this reaction was 3.5 d (El-Dib and Aly, 1976). Simple hydrolysis leads to the formation of 3-chlorophenylcarbamic acid and 2-propanol. The acid is very unstable and is spontaneously converted to 3-chloroaniline and carbon dioxide (Still and Herrett, 1976). 

Today the coupled product is described as being formed by union of two alkyl radicals fonned by loss of one electron and carbon dioxide from the carboxylate ion. Extensive early use of the Kolbe reaction was made for the synthesis of long chain a,co-dicarboxylate esters starting from the half esters of shorter chain a,03-diacids [49]. 

It is seen that at 1000° the reaction is much more uniform through the rod but is still not in the chemical control zone. At this low rate of reaction, it appears that carbon dioxide is diffu.sing sufficiently rapidly between the inner wall of the carbon rod and the ceramic support rod to maintain an appreciable concentration of reactant at the inner exposed surface of the rod. As expected, the minimum porosity (smallest amount of reaction) is found about half-way between the inner and outer radius, that is, at 0.4-cm. radius. 

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