Proton-transfer reactions generalization

Proton transfer reactions generally satisfy a LFER. Although they are not directly related to ligand substitution processes, proton transfer steps are often involved and usually treated as rapidly mainudned equilibria ... 

The values listed in Tables 8.7 and 8.8 are the negative (decadic) logarithms of the acidic dissociation constant, i.e., — logj, For the general proton-transfer reaction... 

A1C13, or S02 in an inert solvent cause colour changes in indicators similar to those produced by hydrochloric acid, and these changes are reversed by bases so that titrations can be carried out. Compounds of the type of BF3 are usually described as Lewis acids or electron acceptors. The Lewis bases (e.g. ammonia, pyridine) are virtually identical with the Bransted-Lowry bases. The great disadvantage of the Lewis definition of acids is that, unlike proton-transfer reactions, it is incapable of general quantitative treatment. 

It may be instructive to again consider the energetics of a proton transfer reaction of the type involved in the first step of the examples above, in solution. Under the influence of a possible general base as the proton acceptor and a possible metal ion assisting as a catalyst we can write... 

Proton transfers between oxygen and nitrogen acids and bases are usually extremely fast. In the thermodynamically favored direction, they are generally diffusion controlled. In fact, a normal acid is defined as one whose proton-transfer reactions are completely diffusion controlled, except when the conjugate acid of the base to which the proton is transferred has a pA value very close (differs by g2 pA units) to that of the acid. The normal acid-base reaction mechanism consists of three steps ... 

This type of charge reduction by charge transfer to the solvent molecule occurs in general when SI are polar solvent molecules of aprotic character such as dimethyl-sulfoxide, dimethyl formamide, and acetonitrile. Protic solvents such as water lead to charge reduction which involves an intracluster proton transfer reaction ... 

To what extent are the variations in the rate constant ratio /cs//cpobserved for changing structure of aliphatic and benzylic carbocations the result of changes in the Marcus intrinsic barriers Ap and As for the deprotonation and solvent addition reactions It is not generally known whether there are significant differences in the intrinsic barriers for the nucleophile addition and proton transfer reactions of carbocations. 

The general features discussed so far can explain the complexity of these reactions alone. However, thermodynamic and kinetic couplings between the redox steps, the complex equilibria of the metal ion and/or the proton transfer reactions of the substrate(s) lead to further complications and composite concentration dependencies of the reaction rate. The speciation in these systems is determined by the absolute concentrations and the concentration ratios of the reactants as well as by the pH which is often controlled separately using appropriately selected buffers. Perhaps, the most intriguing task is to identify the active form of the catalyst which can be a minor, undetectable species. When the protolytic and complex-formation reactions are relatively fast, they can be handled as rapidly established pre-equilibria (thermodynamic coupling), but in any other case kinetic coupling between the redox reactions and other steps needs to be considered in the interpretation of the kinetics and mechanism of the autoxidation process. This may require the use of comprehensive evaluation techniques. 

Eight generalizations are given arising from world-wide studies of proton transfer reactions in aqueous media carried out over the past twenty-five years. Future directions of research on proton transfer kinetics are predicted, and recent kinetic studies by the authors on proton transfer in nonaqueous media (methanol, acetonitrile, and benzonitrile) are reviewed. 

Inorganic solution chemistry often involves proton transfers to and from solvated metal ions as well as to and from the acids and bases that complex metal ions. Eight generalizations are presented below that attempt to summarize the insights regarding proton transfer reactions that have emerged in the past quarter century. The masterful reviews by Eigen (1) and Bell (2) provide much more extensive analysis of most of these points. 

General Discussion—Proton-Transfer Reaction Rates and Mechanisms... 

General Discussion—Proton-Transfer Reactions in Organometallic Chemistry... 

One way to classify EGBs is according to the atom to which proton transfer takes place, typically nitrogen, oxygen or carbon. As in other proton transfer reactions, proton transfer to/from heteroatoms (N or O) is generally much faster than to/from carbon when comparing reactions... 

Five-membered carbonyl ylide derivatives form with ease, but tend to suffer from proton-transfer reactions to the carbonyl more readily than their six-membered counterparts. Generally, disubstitution of the position a to the carbonyl led to smooth carbonyl ylide formation and subsequent dipolar cycloaddition (35,77). 

According to Supuran and co-workers (199,204), an enzyme-activator complex may be formed in which the activator facilitates the proton transfer. A general reaction scheme is presented in Scheme 9. 

In Chapter 8, we addressed proton transfer reactions, which we have assumed to occur at much higher rates as compared to all other processes. So in this case we always considered equilibrium to be established instantaneously. For the reactions discussed in the following chapters, however, this assumption does not generally hold, since we are dealing with reactions that occur at much slower rates. Hence, our major focus will not be on thermodynamic, but rather on kinetic aspects of transformation reactions of organic chemicals. In Section 12.3 we will therefore discuss the mathematical framework that we need to describe zero-, first- and second-order reactions. We will also show how to solve somewhat more complicated problems such as enzyme kinetics. 

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