Proton transfer potential

FIGURE 4 7 Potential energy diagram for proton transfer from hydrogen chio ride to tert butyl alcohol... 

In Section 8, the material on solubility constants has been doubled to 550 entries. Sections on proton transfer reactions, including some at various temperatures, formation constants of metal complexes with organic and inorganic ligands, buffer solutions of all types, reference electrodes, indicators, and electrode potentials are retained with some revisions. The material on conductances has been revised and expanded, particularly in the table on limiting equivalent ionic conductances. 

Under these circumstances, a distinct contribution to the overall rate will be seen for each potential hydrogen-bond donor D—H. General acid catalysis is also observed when a ratedetermining proton transfer occurs fiom acids other than the solvated proton ... 

Three-dimensional potential energy diagrams of the type discussed in connection with the variable E2 transition state theory for elimination reactions can be used to consider structural effects on the reactivity of carbonyl compounds and the tetrahedral intermediates involved in carbonyl-group reactions. Many of these reactions involve the formation or breaking of two separate bonds. This is the case in the first stage of acetal hydrolysis, which involves both a proton transfer and breaking of a C—O bond. The overall reaction might take place in several ways. There are two mechanistic extremes ... 

Fig. 8.3. Three-dimensional potential energy diagram for addition of a proton and nucleophile to a caibonyl group, (a) Proton transfer complete before nucleophilic addition begins (b) nucleophilic addition complete before proton transfer begins (c) concerted proton transfer and nucleophilic addition.

The relative importance of the potential catalytic mechanisms depends on pH, which also determines the concentration of the other participating species such as water, hydronium ion, and hydroxide ion. At low pH, the general acid catalysis mechanism dominates, and comparison with analogous systems in which the intramolecular proton transfer is not available suggests that the intramolecular catalysis results in a 25- to 100-fold rate enhancement At neutral pH, the intramolecular general base catalysis mechanism begins to operate. It is estimated that the catalytic effect for this mechanism is a factor of about 10. Although the nucleophilic catalysis mechanism was not observed in the parent compound, it occurred in certain substituted derivatives. 

Proton Transfer Standard Reduction Potential Group Transfer... 

Since the final proton transfer is essential for a successful condensation, it is important to understand what factors drive the proton transfer. Examine the electrostatic potential map of the carbanion, and draw all of the resonance contributors that are needed to describe this ion. How does this ion differ from the others Which product, if either, would be expected from the following condensation Explain. 

Which proton transfer is easier energetically, Q —> QH or QH- QH2 (The energy of H is given at right.) Use electrostatic potential maps of Q and QH anion (QH ) to explain this difference. 

Let the initial distance between the particles AH and B be denoted by r. The mutual potential energy of the two charged particles is —c2/r, as in the simple ionic dissociation depicted in Fig. 8a. If the value of r is sufficiently great, the energy associated with the electrostatic fields will not depend appreciably on r. For the proton transfer there is thus a characteristic quantity similar to D or. 

VB Potential Surface for Proton Transfer Reactions in Solutions... 

FIGURE 2.6. An EVB-LD potential surface for proton transfer between an acid R"COO/t H and an ROsR molecule in solution. The independent coordinates r1 and r3 are the distances between the proton and Oa and Os, respectively. Regions of the potential surface that have more than 50% ionic character are dotted (see Ref. 6 for more details). 

FIGURE 5.8. A downhill trajectory for the proton transfer step in the catalytic reaction of trypsin. The trajectory moves on the actual ground state potential, from the top of the barrier to the relaxed enzyme-substrate complex. 1, 2, and 3 designate different points along the trajectory, whose respective configurations are depicted in the upper part of the figure. The time reversal of this trajectory corresponds to a very rare fluctuation that leads to a proton transfer from Ser 195 to His 57. 

Exercise 5.3. Construct an EVB potential surface for a proton transfer from Cys-25 to an active site of water molecules in papain. 

FIGURE 9.1. The potential surface for proton transfer reaction and the effect of constrainir the tiA B distance. The figure demonstrates that the barrier for proton transfer increasi drastically if the A — B distance is kept at a distance larger than 3.5 A. However, in solutic and good enzymes the transfer occurs through pathway a where the A - B distance is arour 2.7 A. 

Transition state theory, 46,208 Transmission factor, 42,44-46,45 Triosephosphate isomerase, 210 Trypsin, 170. See also Trypsin enzyme family active site of, 181 activity of, steric effects on, 210 potential surfaces for, 180 Ser 195-His 57 proton transfer in, 146, 147 specificity of, 171 transition state of, 226 Trypsin enzyme family, catalysis of amide hydrolysis, 170-171. See also Chymotrypsin Elastase Thrombin Trypsin Plasmin Tryptophan, structure of, 110... 

The living nature of ethylene oxide polymerization was anticipated by Flory 3) who conceived its potential for preparation of polymers of uniform size. Unfortunately, this reaction was performed in those days in the presence of alcohols needed for solubilization of the initiators, and their presence led to proton-transfer that deprives this process of its living character. These shortcomings of oxirane polymerization were eliminated later when new soluble initiating systems were discovered. For example, a catalytic system developed by Inoue 4), allowed him to produce truly living poly-oxiranes of narrow molecular weight distribution and to prepare di- and tri-block polymers composed of uniform polyoxirane blocks (e.g. of polyethylene oxide and polypropylene oxide). 

The principles outlined above are, of course, important in electro-synthetic reactions. The pH of the electrolysis medium, however, also affects the occurrence and rate of proton transfers which follow the primary electron transfer and hence determine the stability of electrode intermediates to chemical reactions of further oxidation or reduction. These factors are well illustrated by the reduction at a mercury cathode of aryl alkyl ketones (Zuman et al., 1968). In acidic solution the ketone is protonated and reduces readily to a radical which may be reduced further only at more negative potentials. 

As is the case for cationic polymerisation, anionic polymerisation can terminate by only one mechanism, that is by proton transfer to give a terminally unsaturated polymer. However, proton transfer to initiator is rare - in the example just quoted, it would involve the formation of the unstable species NaH containing hydride ions. Instead proton transfer has to occur to some kind of impurity which is capable for forming a more stable product. This leads to the interesting situation that where that monomer has been rigorously purified, termination cannot occur. Instead reaction continues until all of the monomer has been consumed but leaves the anionic centre intact. Addition of extra monomer causes further polymerisation to take place. The potentially reactive materials that result from anionic initiation are known as living polymers. 

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