Hydrogen abstraction reaction rates

It is thus anticipated that compressive stress inhibits while tensile stress promotes chemical processes which necessitate a rehybridization of the carbon atom from the sp3 to the sp2 state, regardless of the reaction mechanism. This tendency has been verified for model ring-compounds during the hydrogen abstraction reactions by ozone and methyl radicals the abstraction rate increases from cyclopropane (c3) to cyclononane (c9), then decreases afterwards in the order anticipated from Es [79]. The following relationship was derived for this type of reactions ... 

Reaction step 5 in Scheme 3.1 can be rnled ont becanse the flnoranil ketyl radical (FAH ) reaches a maximum concentration within 100 ns as the triplet state ( FA) decays by reaction step 2 while the fluoranil radical anion (FA ) takes more than 500 ns to reach a maximum concentration. This difference snggests that the flnoranil radical anion (FA ) is being produced from the fluoranil ketyl radical (FAH ). Reaction steps 1 and 2 are the most likely pathway for prodncing the flnoranil ketyl radical (FAH ) from the triplet state ( FA) and is consistent with the TR resnlts above and other experiments in the literatnre. The kinetic analysis of the TR experiments indicates the fluoranil radical anion (FA ) is being prodnced with a hrst order rate constant and not a second order rate constant. This can be nsed to rnle ont reaction step 4 and indicates that the flnoranil radical anion (FA ) is being prodnced by reaction step 3. Therefore, the reaction mechanism for the intermolecular hydrogen abstraction reaction of fluoranil with 2-propanol is likely to predominantly occur through reaction steps 1 to 3. 

The reactions of fluorine atoms with hydrocarbons are similar to those of active nitrogen in that they provide an essentially universal response. Fluorine atoms abstract H atoms from hydrocarbons at near-collisional reaction rates. Reactions with fluorine are highly exothermic, forming strong H—F (=570 kJ mol-1) and C—F (=485 kJ mol-1) bonds while breaking much weaker C—H (=414 kJ mor1) and C—C (=368 kJ mol-1) bonds. The hydrogen abstraction reaction... 

Time-resolved PAC has also been applied to investigate the thermochemistry of reactions involving the cleavage of C-H, O-H, and S H bonds by a method similar to the one described by equations 13.23 and 13.24 [304-310]. In these cases, the hydrogen abstraction reactions were too slow to be examined by non-time-resolved PAC. Other advantages of using TR-PAC were described by Correia et al. [308]. Finally, it is noted that TR-PAC can also be used to derive rate constants of hydrogen abstraction reactions [311],... 

P. M. Nunes, C. F. Correia, R. M. Borges dos Santos, J. A. Martinho Simoes. Time-resolved Photoacoustic Calorimetry as a Tool to Determine Rate Constants of Hydrogen Abstraction Reactions. Int. J. Chem. Kinet. 2006, 38, 357-363. 

The alkyl radical may also dissociate thermally to form an alkene and a smaller alkyl radical. The mechanism that is initiated by these reactions is chain propagating rather than chain branching and for this reason the overall oxidation rate of the fuel decreases. Also there is a change from OH to HO2 as the main chain carrier, and as we have seen, the HO2 radical is much less reactive than OH. The HO2 radical is formed both from alkyl + O2 hydrogen abstraction reactions such as (R69) and from recombination of hydrogen atoms with O2, H + O2 + M HO2 + M (R5). Under lean conditions any hydrogen atoms formed will primarily react with oxygen. At intermediate temperatures the reaction H + O2 O + OH (Rl) is still too slow to compete with (R5). 

Reid DL, Armstrong DA, Rauk A, Nese C, Schuchmann MN, Westhoff U, von Sonntag C (2003) H-atom abstraction by C-centered radicals from cyclic and acyclic dipeptides. A theoretical and experimental study of reaction rates. Phys Chem Chem Phys 5 3278-3288 Roberts BP (1996) Understanding the rates of hydrogen abstraction reactions empirical, semi-em-pirical and ab initio approaches. J Chem Soc Perkin Trans 2 2719-2725 Russell GA (1973) Reactivity, selectivity, and polar effects in hydrogen atom transfer reactions. In Kochi JK (ed) Free radicals. Wiley, New York, pp 275-331 Russo-Caia C, Steenken S (2002) Photo- and radiation-chemical production of radical cations of methylbenzenes and benzyl alcohols and their reactivity in aqueous solution. Phys Chem Chem Phys 4 1478-1485... 

This is a case for which only stabilization is of interest. It has not been treated explicitly as an excitation reaction. Its rate has been measured directly103-105 at low temperatures (300-450 °C) as have those for several other radical combinations. Without doubt, the combination of methyl radicals is the most important reaction of this type for chemical kinetics. It serves as a reference standard for the measurement of rates of many hydrogen abstraction reactions, but very little is known about the temperature dependence of its rate coefficient. 

Our earlier studies of the bimolecular alkene addition reactivity of a,a-difluoro alkyl radicals indicated that they exhibited little philicity , reacting with styrene and pentafluorostyrene (IPs 8.43 and 9.20, respectively) at virtually the same rate [70]. Their significantly greater reactivity in bimolecular additions, hydrogen-abstraction reactions and unimolecular cyclizations can therefore be largely attributed to their pyramidal nature, with some possible thermodynamic con-... 

The different reactivity of phosphorus azides might be interpreted in terms of conjugation between phenyl groups and sulfur or between phenyl groups and the P=N=0 double bond in these cases the rate of the phenyl migration decreases and hydrogen abstraction reactions dominate. 

Further stereoelectronic effects on hydrogen abstraction reactions have been claimed by Malatesta and Ingold (1981) on the basis of their failure to observe the e.s.r. signal for the appropriate radical. Their necessary assumption, however, that rates of decomposition of product radical are the same as those of the reference radical is merely a pious hope. 

Results from this study confirms the experimental findings that the reaction of n-bromopropane with OH radicals should be slower than the reaction with Cl atoms. The present results show that pre- and post-reaction complexes are important in the hydrogen abstraction reactions. A detailed study of these rate constants that incorporates the contribution of pre-reactive complexes, the multi-chaimel nature of these reactions, and temperature dependence is necessary. The results also find that there are subtle reaction preferences for the abstraction of site specific hydrogen on n-bromopropane. While knowledge of the dominant products of the... 

For most of the reactions, rate constants have been determined. In dense clouds, CH is efficiently destroyed by the hydrogen abstraction reaction... 

The model compounds (III,a), (V,f), (VI,a) and NB described in Sect. B.l h) and the kinetic results obtained from the attack of t-butoxy radical on isooctane solutions of these products have been used to clarify some aspects of the mechanism of ter-polymer cross-linking. The reactivity of isooctane, expressed as the hydrogen abstraction reaction by t-butoxy radical relative to the scission rem tion of (CH3)300 (see k2/ki values in Table 7), has bear converted into the rate cmstant for the H abstraction from ethylene-propylene units (kj) Iqy means of Pryor s... 

The PVK copolymer blend shows a lower rate and less sensitization of degradation in the homqpolymer. The PVK copolymer is known to have a low yield of type I reaction 0 and this supports the idea that tiie sensitization of photooxidation is due to free radical formation rather than more elaborate mechanisms such as via formation of singlet oxygen. It also appears that the hydrogen abstraction reaction of excited phenone groups (1 which is the basis of the use of benzo-phenone as a prodegradant in polystyrene, is not as important in this system. 

R Is negligible with respect to ROO because of Its swift reaction with oxygen. The rate of propagation, then. Is governed by the hydrogen abstraction reaction, kp. Termination la k Is more likely than crosslinking because of the low concentration of R radicals. 

C.L. Clifton, R.E. Huie (1993). Rate constants for some hydrogen abstraction reactions of the carbonate radical. Int. J. Chem. Kinet., 25,199-203. 

---