Equilibrium constants atom abstraction

We might be tempted to say that methoxide is a much better nucleophile because it is much more basic. This would be a mistake because basicity and nucleophilicity are different properties. Basicity is defined by the equilibrium constant for abstracting a proton. Nucleophilicity is defined by the rate of attack on an electrophilic carbon atom. In both cases, the nucleophile (or base) forms a new bond. If the new bond is to a proton, it has reacted as a base if the new bond is to carbon, it has reacted as a nucleophile. Predicting which way a species will react may be difficult most (but not all) good nucleophiles are also strong bases, and vice versa. 

The RS radicals are able to abstract H-atoms, though in a thermodynamically unfavorable process. The equilibrium constant involving penicillamine (PenSH) and the 2-propanol radical was estimated to be around 10 [reaction (10)] ... 

Examination of the measured second order rate constants for the dye oxidations shows that the reaction is significantly favoured by electron-donating substituents the 4-methoxylated compound (9) reacts 274 times faster than the 4-nitro analogue (3). Furthermore, the excellent linear Hammett correlation of log(second order rate constant) with and the p value of-1.66 confirm this conclusion and show that all the substrates are oxidised by the same mechanism. However, the size of the p value is larger than that obtained from the earlier oxidation of phenols (p -1.10) and is larger than expected for a mechanism involving H atom abstraction in the rate determining step. A possible explanation is that the substituents have two effects on the reaction which reinforce each other they influence H atom abstraction by the electrophilic oxidant, as noted previously this is favoured by electron-donation, and they shift the position of the azo-hydrazone equilibrium. 

CH3O. The rate coefficients recommended by Gray et al. (1967) and by Heicklen (1968), based on their reviews of the early literature, are given in Table 18 for the abstraction of H atoms by CH3O. The rate-coefficient parameters reported by Gray et al. (1967) were obtained fixim the rate coefficients of the reverse reaction and the equilibrium constants. Those reported by Heicklen (1968) were obtained from the ratio k2sk 2lk -i for the reactions... 

Transition metal complexes functioning as redox catalysts are perhaps the most important components of an ATRP system. (It is, however, possible that some catalytic systems reported for ATRP may lead not only to formation of free radical polymer chains but also to ionic and/or coordination polymerization.) As mentioned previously, the transition metal center of the catalyst should undergo an electron transfer reaction coupled with halogen abstraction and accompanied by expansion of the coordination sphere. In addition, to induce a controlled polymerization process, the oxidized transition metal should rapidly deactivate the propagating polymer chains to form dormant species (Fig. 11.16). The ideal catalyst for ATRP should be highly selective for atom transfer, should not participate in other reactions, and should deactivate extremely fast with diffusion-controlled rate constants. Finther, it should have easily tunable activation rate constants to meet sped c requirements for ATRP monomers. For example, very active catalysts with equilibrium constants K > 10 for styrenes and acrylates are not suitable for methacrylates. 

The BEP correlation between rates and driving force for HAT is very valuable, but it applies only to a specific set of similar reactions, for instance Mn04 abstracting H from hydrocarbons. In addition, the a and jS parameters are defined only with the context of the correlation and have no independent meaning. In contrast, cross relation uses three independently measurable parameters the equilibrium constant Xxh/y (which is equal to e constants for the hydrogen atom self-exchange reactions... 

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