Proton transfer extent

The conductivity of DC1 in D20 solution depends to a large extent on the ease with which a deuteron can jump from a (D30)+ ion to an adjacent D20 molecule. From the value given for DC1 in Table 7 it is clear that such deuteron transfers take place with greater difficulty than the corresponding proton transfers in H20 see Sec. 79. 

Ammonium salts of the zeolites differ from most of the compounds containing this cation discussed above, in that the anion is a stable network of A104 and Si04 tetrahedra with acid groups situated within the regular channels and pore structure. The removal of ammonia (and water) from such structures has been of interest owing to the catalytic activity of the decomposition product. It is believed [1006] that the first step in deammination is proton transfer (as in the decomposition of many other ammonium salts) from NH4 to the (Al, Si)04 network with —OH production. This reaction is 90% complete by 673 K [1007] and water is lost by condensation of the —OH groups (773—1173 K). The rate of ammonia evolution and the nature of the residual product depend to some extent on reactant disposition [1006,1008]. 

Although thermodynamics can be used to predict the direction and extent of chemical change, it does not tell us how the reaction takes place or how fast. We have seen that some spontaneous reactions—such as the decomposition of benzene into carbon and hydrogen—do not seem to proceed at all, whereas other reactions—such as proton transfer reactions—reach equilibrium very rapidly. In this chapter, we examine the intimate details of how reactions proceed, what determines their rates, and how to control those rates. The study of the rates of chemical reactions is called chemical kinetics. When studying thermodynamics, we consider only the initial and final states of a chemical process (its origin and destination) and ignore what happens between them (the journey itself, with all its obstacles). In chemical kinetics, we are interested only in the journey—the changes that take place in the course of reactions. 

A low ion pair yield of products resulting from hydride transfer reactions is also noted when the additive molecules are unsaturated. Table I indicates, however, that hydride transfer reactions between alkyl ions and olefins do occur to some extent. The reduced yield can be accounted for by the occurrence of two additional reactions between alkyl ions and unsaturated hydrocarbon molecules—namely, proton transfer and condensation reactions, both of which will be discussed later. The total reaction rate of an ion with an olefin is much higher than reaction with a saturated molecule of comparable size. For example, the propyl ion reacts with cyclopentene and cyclohexene at rates which are, respectively, 3.05 and 3.07 times greater than the rate of hydride transfer with cyclobutane. This observation can probably be accounted for by a higher collision cross-section and /or a transmission coefficient for reaction which is close to unity. 

Besides water, the most common weak base is ammonia, NH3, whose proton transfer equilibrium with water appears in Section 16-. Many other weak bases are derivatives of ammonia called amines, hi these organic compounds, one, two, or three of the N—H bonds in ammonia have been replaced with N—C bonds. The nitrogen atom in an amine, like its counterpart in ammonia, has a lone pair of electrons that can form a bond to a proton. Water does not protonate an amine to an appreciable extent, so all amines are weak bases. Table 17-4 lists several examples of bases derived from ammonia. 

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]. 

However, it is not known into which part of the potential energy surface these species couple. The reactions of H2CO+ with OH and CH2OH+ with O-atoms would also access the surface although these are not experimentally very tractable. The surface is also accessed to a limited extent by the gas kinetic proton transfer from HCO+ to H20 yielding H30+. 

The values of 3/(NH,H) coupling constant observed for imine proton can be helpful in detection of the proton transfer processes and determination of mole fractions of tautomers in equilibrium. For NH-form, this value is close to 13 Hz, lower values usually indicate the presence of tautomeric equilibrium. It should be mentioned that the values below 2.4 Hz have not been reported. The chemical shift of C—OH (C-2 for imines, derivatives of aromatic ortho-hydroxyaldehydes or C-7 for gossypol derivatives) carbon to some extent can be informative, however, this value depends on type of substituents and should be interpreted with caution. 

The extent to which the effect of changing substituents on the values of ks and kp is the result of a change in the thermodynamic driving force for the reaction (AG°), a change in the relative intrinsic activation barriers A for ks and kp, or whether changes in both of these quantities contribute to the overall substituent effect. This requires at least a crude Marcus analysis of the substituent effect on the rate and equilibrium constants for the nucleophile addition and proton transfer reactions (equation 2).71-72... 

To what extent is the partitioning of simple aliphatic and benzylic a-CH-substituted carbocations in nucleophilic solvents controlled by the relative thermodynamic driving force for proton transfer and nucleophile addition reactions It is known that the partitioning of simple aliphatic carbocations favors the formation of nucleophile adducts (ksjkp > 1, Scheme 2) and there is good evidence that this reflects, at least in part, the larger thermodynamic driving force for the nucleophilic addition compared with the proton transfer reaction of solvent (A dd U Scheme 6).12 21,22,24... 

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 computational results show that transition structures derived from hydroperoxo Re complexes lie slightly higher in energy than those obtained for the corresponding peroxo complexes, nevertheless their involvement in the epoxidation reaction cannot be excluded. However, for neither MoVI nor Revn evidence Get alone preference) for hydroperoxo reaction pathways is as clear as for TiIV complexes. Of course, more complex mechanisms involving intermolecular proton transfer and/or hydrogen bonded intermediates may change this picture to some extent. 

The proton-transfer behaviour of [47] differs in one important respect from that of the other proton sponges. It is found that the rates of equilibration of the protonated amine with the free amine in mixtures of the two in [ Hg]Me2SO at 30°C is fast on the nmr timescale and averaged proton signals are observed (Zirnstein and Staab, 1987). For mixtures of [33] and the free amine, separate proton signals are observed. Quantitative information about the extent to which the rates of proton transfer for [47] differ from those of the other proton sponges must await detailed kinetic studies. 

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