Hydroperoxy radicals, rate constants

Because of the importance of hydroperoxy radicals in autoxidation processes, their reactions with hydrocarbons arc well known. However, reactions with monomers have not been widely studied. Absolute rate constants for addition to common monomers are in the range 0.09-3 M"1 s"1 at 40 °C. These are substantially lower than kL for other oxygen-centered radicals (Table 3.7). 454... 

Reaction 2-6 is sufficiently fast to be important in the atmosphere. For a carbon monoxide concentration of 5 ppm, the average lifetime of a hydroxyl radical is about 0.01 s (see Reaction 2-6 other reactions may decrease the lifetime even further). Reaction 2-7 is a three-body recombination and is known to be fast at atmospheric pressures. The rate constant for Reaction 2-8 is not well established, although several experimental studies support its occurrence. On the basis of the most recently reported value for the rate constant of Reaction 2-8, which is an indirect determination, the average lifetime of a hydroperoxy radical is about 2 s for a nitric oxide concentration of 0.05 ppm. Reaction 2-8 is the pivotal reaction for this cycle, and it deserves more direct experimental study. 

The methyl radical rapidly (in 10 s) combines with oxygen to form the methylperoxy radical, CH3O3. A recent study has confirmed that nitric oxide is oxidized by methylperoxy, although the rate constant is still unknown. The meffioxy radical, CH3O, should then react predominantly with oxygen to form formaldehyde, CHjO, and hydroperoxy radical. The net result of this sequence is the oxidation of one molecule of nitric oxide to nitrogen dioxide and the conversion of an alkyl radical into a hydro-... 

The participation of hydroxyl and hydroperoxy radicals in the oxidation of nitric oxide raises the possibility that these radicals might also attack hydrocarbons. In the case of hydroxyl these reactions are known to be fairly rapid. On the basis of e rate constants that have been measured and estimates of those which have not the rates of attack of hydroxyl and hydroperoxy radicals appear to be large enough to explain the excess consumption of propylene shown in Figure 2-5. 

Direct determinations of rate constants are needed for almost all the reactions of hydroperoxy radical and ROj. 

Laser magnetic resonance, which has already been used to detect the free hydroxyl, methynyl (CH), hydroperoxy, formyl (HCO), and amino radicals in low-pressure gases and could be used to determine rate constants for the reactions of the smaller free radicals. 

Rate constants alkylperoxy radicals, 3 hydroperoxy radicals, 3, 31 ozone reactions, 76... 

First, hydroperoxy radicals must react predominantly via Reaction 8 and not via Reaction 7. It is difficult to assess this competition because of the uncertain energetics of these reactions. Assuming that k7 — k3, Reaction 8 is faster than Reaction 7 when the hydrogen abstracted in 7 is primary, the rate constants are approximately equal when it is secondary, and 7 predominates when it is tertiary. Only under conditions where the yields of alkenes are considerable and the alkane has no tertiary C—H bonds will Reaction 8 be important. Even then, abstraction of allylic hydrogen from the alkene by HO2 will compete strongly with Reaction 8. 

To account for these observations, it has been proposed that hydroperoxy radicals are produced during the oxidation of alcohols containing an a-hydrogen [22-26]. These radicals have a high termination rate constant (>10 L moF s the reaction is said to be diffusion-controlled [14, 27, 28]) they remove radicals from the system rapidly (eq. (9)) ... 

Several studies have been done to establish the chain-carrying species in the oxidation of alcohol (29). The absence of peroxide in the photooxidation of 2-propanol suggests either that the chain carrier is a hydroperoxy radical and not the 2-hydroxy-2-propylperoxy radical or that 2-hydroperoxy-2-propanol is too unstable to accumulate, if formed under these conditions. Only the hydroperoxy radical could be trapped by the spin-trapping reagent phenyl-N-f-butylnitrone in the oxidation of 2-propanol at 25°C (28). The reported high rate constant (30) for the decomposition of the 2-hydroxy-2-propylperoxy radical to acetone and hydroperoxy radical also implies that the hydroperoxy radical is the predominant chain carrier. On the other hand, Schenck and his co-workers (31) isolated 2-hydroperoxy-2-propanol in the photooxidation of 2-propanol with 313-nm light. 

Depending on the reaction solvent and conditions, the disproportionation of hydroperoxy radical (Eq. (14.5)) may contribute to the reaction (estimated rate constant 10 -10 s ) [23]. While no autocatalytic effects were observed in... 

The total 296 K rate constant was 1.6 x 10" cm s", with abstraction of O2 to yield the hydroperoxy radical (reaction (9)) noted as the major pathway. Exact branching ratios could not be defined since the product ions were also found to be very reactive towards H atoms, preventing any quantitative diagnostics. 

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