Proton transfer reasonable

Table 4-1 lists some rate constants for acid-base reactions. A very simple yet powerful generalization can be made For normal acids, proton transfer in the thermodynamically favored direction is diffusion controlled. Normal acids are predominantly oxygen and nitrogen acids carbon acids do not fit this pattern. The thermodynamicEilly favored direction is that in which the conventionally written equilibrium constant is greater than unity this is readily established from the pK of the conjugate acid. Approximate values of rate constants in both directions can thus be estimated by assuming a typical diffusion-limited value in the favored direction (most reasonably by inspection of experimental results for closely related... 

Tliere are several reasons for this great interest in the tautomerism of porphyrins (which could justify its own review) (1) their biological significance, (2) their applications in material science ( hole burning is related to their tautomerism), (3) the simplicity of the system (annular tautomerism involving intramolecular proton transfer both in solution and in the solid state), and (4) the possibility of elucidating the kinetic processes in great detail. 

There are, in fact, two reasons why we should prefer to discuss proton transfers of class I. In concentrated solutions the average electrostatic forces between the ions will be intense. Only in proton transfers of class I does the number of positive and negative charges in the solution remain unaltered when the proton is transferred only here do we find the possibility that the contribution from the interionic forces will remain almost unchanged in a proton transfer. At the same time, although the number... 

A reasonable mechanism is shown in equation 23 methyllithium attacks the sulfur atom, giving the secondary carbanion 36 by cleavage of the four-membered ring. A rapid proton transfer produces the sulfonyl-stabilized carbanion 37 which reacts with the added electrophile to give the product (equation 23)3. 

To realize the reason for this result from a simple intuitive point of view it is important to recognize that the ionized form of Aspc is more stable in the protein-active site than in water, due to its stabilization by three hydrogen bonds (Fig. 7.7). This point is clear from the fact that the observed pKa of the acid is around 3 in chymotrypsin, while it is around 4 in solution. As the stability of the negative charge on Aspc increases, the propensity for a proton transfer from Hisc to Aspc decreases. 

On the basis of the examples given above, it is reasonable to suggest that the underlying principles for optimization of the overall reaction rate with respect to the choice of metal ion are similar. That is, there are basically three states along the reaction pathway which determine the most suitable choice of metal ion. These are (1) the reactant state with bound metal and substrate before the proton transfer step, (2) the intermediately created free OH nucleophile and, (3) the subsequent transition state associated with... 

Each successive proton transfer reaction of a polyprotic acid has its own value for, and each successive value is approximately 10 times smaller than its predecessor. We explain the reasons for this trend in Section 17-1 and show how to treat calculations involving polyprotic acids in Section 17-1. 

The same molecular reasoning shows that a buffer solution can absorb added hydronium ions. Consider what happens when some hydronium ions are added to the acetic acid-acetate buffer solution described in Example. The hydronium ion is a strong acid, and the acetate anion is a weak base, so proton transfer from CH3 CO2 to H3 goes essentially to completion ... 

In view of the fact that complete methylation of F N- HX to give (CH3)3N- -HX leads to an increased extent of proton transfer from HX to the base when X is Cl and essentially complete transfer when X is I, it seemed reasonable to seek a more significant contribution from the ionic valence bond structure [(CH3)3NC1] + - F in (CT N- ClF by examining properties similarly derived from its rotational spectrum [68]. 

The exchange proreeds in three steps. The first step is substitution of the free ligand at one end of the chelate to give the intermediate I(fca). The second is intramolecular proton transfer between the unidentate ligand in l(kb). The third is the reverse of the first (k a). Consequently, application of the steady-state approximation to the intermediate 1, whose concentration is reasonably assumed to be very low, provides... 

Large numbers of reactions of interest to chemists only take place in strongly acidic or strongly basic media. Many, if not most, of these reactions involve proton transfer processes, and for a complete description of the reaction the acidities or basicities of the proton transfer sites have to be determined or estimated. These quantities are also of interest in their own right, for the information available from the numbers via linear free energy relationships (LFERs), and for other reasons. 

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