Single-step proton transfer

Table 10.2. These results can be interpreted either as the kinetic effects measured for a single-step proton transfer or as the inverse thermodynamic isotope effects in fast preequilibria [5]. Unfortunately, in contrast to the isotope effects measured for classical hydrogen bonds [24], the effects of denterinm on the thermodynamics of dihydrogen bonding are still nnknown.

On the other hand,it is obvions that nnder psendo-first-order conditions, the position of the kinetic preeqnilibrinm shonld be shifted completely toward dihydrogen bonds. Therefore, the hypothesis of a single-step proton transfer via a late transition state could be successful for interpreting the isotopic effects observed. A structure similar to that of the contact ion pairs (see below) with almost complete formation of new bonds could represent such a transition state. The knlk-c values cau then be calculated by [5]... 

The observed rate coefficient for exchange (L = H, D, or T) is fe bs = k k2 /(kh. ] + k2)- If the primary isotope effect on k2 is different from that on k1 and k... j it is argued that the experimental isotope effects feob s tklb s and feobs/ ob s will not be related by the Swain—Schaad relation, kH/kT = (feH/feD)1442 which is derived with reference to a single-step proton transfer [115, 128]. The size of the discrepancy will depend upon the value of /e, /fe , the amount of internal return. In the analysis of isotope effects for triphenylmethane exchange it is assumed that k2=k2 = k2 since this represents a diffusion step. By introducing aT = k- i /k2 and Kl = k /k j eqns. (82) and (83) are obtained. A third equation (84)... 

It is well known that base-induced elimination reactions can proceed either by a single, concerted step (E2), or by two steps, proton transfer and leaving group expulsion, with a carbanion intermediate (ElcB) to yield an alkene. " The... 

Returning to single-stage proton-transfer reactions, we have seen in Chapter 9 that proton transfer from carbon acids to bases is the ratedetermining step in many reactions of these compounds, and during the last fifteen years or so many studies of isotope effects have been carried... 

The mechanism includes two single electron transfers (steps 1 and 3) and two proton transfers (steps 2 and 4) Experimental evidence indicates that step 2 is rate determining and it is believed that the observed trans stereochemistry reflects the dis tribution of the two stereoisomeric alkenyl radical intermediates formed in this step... 

The mechanism by which the Birch reduction of benzene takes place (Figure 118) IS analogous to the mechanism for the metal-ammonia reduction of alkynes It involves a sequence of four steps m which steps 1 and 3 are single electron transfers from the metal and steps 2 and 4 are proton transfers from the alcohol... 

The proton transfer equilibrium that interconverts a carbonyl compound and its enol can be catalyzed by bases as well as by acids Figure 18 3 illustrates the roles of hydroxide ion and water m a base catalyzed enolization As m acid catalyzed enolization protons are transferred sequentially rather than m a single step First (step 1) the base abstracts a proton from the a carbon atom to yield an anion This anion is a resonance stabilized species Its negative charge is shared by the a carbon atom and the carbonyl oxygen... 

The entity marked with a double dagger is regarded by the authors as an activated complex. Its breakdown (19) may well consist of a sequence of rapid steps rather than the single step implied, which involves a three-electron transfer and double protonation of a transition state subsequent to its formation. Steps (22)-(24 were invoked to explain the complete oxidation of S(IV) to S(VI) at higher Cr(VI) concentrations. 

Of hundreds of theoretically possible pathways, the list can be trimmed to four using linear sweep voltammetry (LSV) and chemical arguments [22]. The LSV method is an exceptionally powerful one for analyzing electrochemical processes [24-27]. From LSV studies, it was concluded that a single heterogeneous electron transfer precedes the rate-determining step, cyclization is first order in substrate, and that proton transfer occurs before or in the rate-determining step. The candidates include (a) e-c-P-d-p (radical anion closure). 

For aminophenols, one-electron oxidation and the proton elimination can run together in one stage. This leads to a cation-radical containing O and +NH3 fragments within one and the same molecular carcass (Rhile et al. 2006). Such concerted reactions are classified as proton-coupled electron transfer (Mayer 2004). Proton-coupled electron transfer differs from conventional one-electron redox reaction in the sense that proton motion affects electron transfer. Because the transfers of a proton and an electron proceed in a single step, we can say about the hydrogen-atom transference, (H+ -I- e)=H. It is the fundamental feature of proton-coupled electron-transfer reactions that the proton and electron are transferred simultaneously, but from different places (see Tanko 2006). 

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