Second order disproportionation

In a DISP 2 mechanism the second-order disproportionation step is rate limiting (see Section 2). An example of such a process involves the photoreduction of the dye fluorescein in basic aqueous solutions at mercury electrodes (Compton etal., 1988b). The photoreduction of benzophenone (86) and fluorobenzophenone in acetonitrile also proceeds via a DISP 2 type mechanism as verified by channel electrode voltammetry (Leslie et al., 1997). The rate-limiting step is electron transfer (86c) between photoexcited radical anion and the initial anionic species formed on electron transfer at the electrode surface. This process is further complicated by significant con-proportionation (86e) and quenching of the benzophenone excited state (86f). 

The disproportionation of HOOH occurs via a concerted transfer of the two hydrogen atoms from a second HOOH to the Fe Cl3(HOOH) adduct. This dehydrogenation of HOOH is a competitive process with the Fe Cfi/substrate/HOOH reactions. The controlled introduction of dilute HOOH into the Fe Cfi/substrate solution limits the concentration of HOOH and ensures that the substrate/HOOH reaction can be competitive with the second-order disproportionation process. The substrate reaction efficiencies in Table 11 are proportional to the relative rates of reaction ( RH/kHoon)-The mode of activation of HOOH by Fe Cls is analogous to that of Fe (MeCN)4 + both are strong electrophiles in ligand-free dry MeCN and induce HOOH to monooxygenate organic substrates. 

A linear relationship was obtained between the reciprocal of the yield of polymer and time in accord with a second order disproportionation reaction but the slopes of these lines were inversely proportional to [Ti]o. 

The ascorbate free radical decays by a strictly second-order disproportionation process to ascorbic acid and dehydroascorbic acid, independent of generation method. A product analysis (22) with l-ascorbate-l- C showed dehydroascorbate to be the only new product in an irradiated solution. 

Table 5-3 Rate Constants for the Second-Order Disproportionation (fcj) of HOO-in Various Solvents at 25°C...

The degradation behaviour in this case cannot be obtained from the preceding equations, which are restricted to first-order termination or second-order disproportionation termination. Another method [2] restricted to a most probable type initial distribution has shown that in this case, in the absence of transfer... 

The monomeric radical cations are generally excellent oxidants, better than their dimeric conjugates. Even the resonance stabilized aromatic radical cations are still powerful enough to initiate oxidation reactions. Two values have been reported for the redox potential of the (C6H5-S -CH3)/C6H5SCH3 couple, namely, 1.45 and 1.53 V [131, 132]. Other decay processes include hydroxyla-tion, that is, reaction with OH" and H2O, second order disproportionation and, whenever possible, deprotonation as well. 

Preliminary work on the FD + ED reaction, indicating this reaction to be kinetically second-order in [ED] (57) was confirmed by later work of Clyne and Watson (45). The second order rate constant was determined bo be (8.5 2.8) x 10 cm molecule" s at 298 K. The mechanism of this reaction is believed to involve regeneration of P atoms and to be analogous to the simileu second-order disproportionation reaction of CIO + CIO and BrO + BrO, i.e. ... 

For the above mechanism to occur, the accumulation of Cr(IV) intermediate concentration should be high enough to make the following second-order disproportionation possible (Eq. 12.14). This could happen only if reaction (12.15) was quite slow. The objection of the above mechanism to occur based on the high equilibrium constant of disproportionation (7.6 x 10 ) estimated from the redox potentials of various Cr(V)/Cr(lV) (+1.34 V) and Cr (lV)/Cr(lll) (+2.1 V) couples. Again, Hasan and Rocek [36] reported that the formation of Cr(IV) as an intermediate is completely avoided in redox reactions that involve the formation of stable complexes such as in the following redox reaction ... 

Xylene isomerization reactions can be accomplished by contacting a hot gas stream with a solid catalyst. Under these conditions the isomerization reactions may be regarded as reversible and first-order. Unfortunately, the catalyst also catalyzes disproportionation reactions. These reactions may be regarded as essentially second-order and irreversible. If one desires to achieve an equilibrium mixture of isomers with minimal material losses due to disproportionation, what do you recommend concerning the mode in which one should operate a continuous flow reactor ... 

One example was reported by Tolman and coworkers (78) who found that the copper(I) complex C Tp112 (TpR2=tris(3-(R2)-5-methylpyrazol-l-yl)hydroborate) promotes NO disproportionation via a weakly bound CuITpR2(NO) intermediate (formally a MNO 11 species). The products are N20 and a copper(II) nitrito complex (Eq. (36)). The rate law established the reaction to be first-order in copper complex concentration and second-order in [NO], and this was interpreted in terms of establishment of a pre-equilibrium between NO and the Cu(I) precursor and the Cux(NO) adduct, followed by rate-limiting electrophilic attack of a second NO molecule (mechanism B of Scheme 5) (78b). 

The reaction was second order in acid and first order in substrate, so both rearrangements and the disproportionation reaction proceed via the doubly-protonated hydrazobenzene intermediate formed in a rapid pre-equilibrium step. The nitrogen and carbon-13 kinetic isotope effects were measured to learn whether the slow step of each reaction was concerted or stepwise. The nitrogen and carbon-13 kinetic isotope effects were measured using whole-molecule isotope ratio mass spectrometry of the trifluoroacetyl derivatives of the amine products and by isotope ratio mass spectrometry on the nitrogen and carbon dioxide gases produced from the products. The carbon-12/carbon-14 isotope... 

Second-order irreversible chemical reaction following a reversible electron transfer disproportionation. The disproportionation reaction can be represented as ... 

Diagnostic criteria to identify an irreversible disproportionation reaction following a reversible electron transfer. Once again the dependence of the parameters of the cyclic voltammetric response from the concentration of the species Ox preliminarily reveals the second-order complication. 

The 40s conversion of the 600nm intermediate to RbOgray (Scheme 10.1) might seem too low to compete with spontaneous disproportionation of superoxide, which occurs with a second-order rate constant of 5 x lO M s at pH 7 (Bielski and Cabelli 1991). However, in aerobic bacterial cells such as E. coli, the steady-state concentration of superoxide is esti-... 

Fig. 1.15 Second-order superoxide disproportionation constant vs pH at 25 °C. Potassium superoxide ( 1 mM) in pH a 12 was mixed in a stopped-flow apparatus with buffers at various pH s and the change in absorbance at 250 nm monitored. The decays were second-order and data were treated in a similar manner to that described in Fig. 1.3. The full line fits Eqn. (1.231) using the parameters given in the text. Reprinted with permission from Z. Bradid and R. G. Wilkins, J. Am. Chem. Soc. 106, 2236 (1984). (1984) American Chemical Society.

The green tetrahedral ion MnO " is stable in basic solution. It can be prepared by reducing MnOj with Fe(CN)g . There is uncertainty about the MnO "—H2O exchange rate. The ion disproportionates in acid and the kinetics have been studied by stopped-flow. At 610 nm where loss of MnO is monitored, the reaction is first-order. At 520 nm where formation of Mn04 is observed, the reaction is second-order. These observations and the H dependency suggest a mechanism... 

Would suspect disproportionation to be second-order. Confirm by examining first three entries. Calculate 2nd order rate constant for each entry. Since acid and base forms, NiLH and NiL, are unreactive, then maximum rate close to pH 4 arises from reaction of NiLtf + with NiL + (k). Use equation analogous to (1.231) and confirm k = (3.4 0.5) X lO M- s. ... 

It is noted that the microporous effect was greater in the disproportionation of 1,2,4-TrMB than in the cracking of cumene. As shown in the previous paper [14], the disproportionation of 1,2,4-TrMB at 200°C proceeds via a bimolecular transition state and obeys the second order kinetics. In contrast, the cracking of cumene is the first order kinetics with respect to cumene concentration. Thus, it seems that the microporous effect is exerted more significantly in the second order reaction (disproportionation) than in the first order reaction (cracking) if pore structure plays an important role in localizing concentration of reactant molecules. 

This reaction scheme involves the second-order homogeneous disproportionation of NO2. It was stated that the rate of the latter process is independent of the electrode material, the supporting electrolyte, the presence of oxygen, and the pH of the solution. 

The fates of the radicals produced in the second and third equations are matters for conjecture. Since the ESR signals attributed to them decay by second-order processes (12, 13), disproportionation reactions such as... 

The reaction exhibits a 3 2 stoichiometry of Eq. (46) (205), and kinetics that are second order in the total concentration of HN02, and independent of [L2(H20)Rh002+], consistent with the disproportionation of HN02 being rate determining. 

Experimentally, Scheme 14 is unrealistic as written, because NO is itself oxidized by 02, Eq. (68), which makes these two reagents incompatible for extended periods of time. For the scheme to work, NO should be produced in situ at low concentrations to slow the autoxidation which exhibits second-order dependence on NO 195), Eq. (69). The bimolecular disproportionation of HN02 in the reverse of Eqs. (41)-(42) turned out to be a good source of NO for this purpose. As described above, the other disproportionation product, N02, should not interfere except for the weak, reversible binding to a portion of the catalyst, Craq002 +. 

Kinetic traces exhibited rapid initial absorbance changes, identified as the conversion of a fraction of Craq002 + to Craq00N022 +. This step was followed by a well-behaved, second order reaction characterized by the same rate constant and same acid dependence as those obtained earlier for the disproportionation of HN02, Eqs. (47)-(48) (205). There was no dependence on [Craq002+], [02], or [PhCH2OH]. All the data fit the mechanism in Scheme 15 and Eqs. (72) and (73). 

Small alkylperoxy and alkoxy radicals can decompose uni-molecularly, though their rate constants are often in the second-order region. They abstract hydrogen atoms from alkanes, aldehydes, esters, and acids, add to olefins, and may react with 02. Furthermore, interactions with other radicals can lead to disproportionation or combination. These reactions are reviewed, and particular attention is given to CH 02 and CH30 a number of rate constants are estimated. 

---