Proton transfer reactions constants

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... 

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. 

It is possible to measure equilibrium constants and heats of reaction in the gas phase by using mass spectrometers of special configuration. With proton-transfer reactions, for example, the equilibrium constant can be determined by measuring the ratio of two reactant species competing for protons. Table 4.13 compares of phenol ionizations. 

Table 4-1. Rate Constants for Proton Transfer Reactions in Water ...

A large red shift observed in polar solvents was indicative of the intramolecular charge transfer character of the triplet state. The change of dipole moment accompanying the transition Tj - Tn, as well as rate constants for electron and proton transfer reactions involving the T state of a-nitronaphthalene, were determined. The lower reactivity in polar solvents was attributed to a reduced n-n and increased charge transfer character of the triplet state... 

What Are the Key Ideas Bronsted acids are proton donors Bronsted bases are proton acceptors. The composition of a solution of an acid or base immediately adjusts to satisfy the values of the equilibrium constants for all the proton transfer reactions taking place. 

Like all chemical equilibria, this equilibrium is dynamic and we should think of protons as ceaselessly exchanging between HCN and H20 molecules, with a constant but low concentration of CN and H30+ ions. The proton transfer reaction of a strong acid, such as HCl, in water is also dynamic, but the equilibrium lies so strongly in favor of products that we represent it just by its forward reaction with a single arrow. 

At this point several assumptions must be made. The first requires the absence of isotope effects on the rate of Reaction O—i.e., the rate constants of Reactions O, U + V, and W are identical this is termed k, and the plausibility of the assumption is indicated by the absence of isotope effects on the rate constant of Reaction I. The second assumption requires that in proton transfer reactions like Reaction O, a deuteron... 

Using either of the above approaches we have measured the thermal rate constants for some 40 hydrogen atom and proton transfer reactions. The results are tabulated in Table II where the thermal rate constants are compared with the rate constants obtained at 10.5 volt cm.-1 (3.7 e.v. exit energy) either by the usual method of pressure variation or for concurrent reactions by the ratio-plot technique outlined in previous publications (14, 17, 36). The ion source temperature during these measurements was about 310°K. Table II also includes the thermal rate constants measured by others (12, 13, 33, 39) using similar pulsing techniques. 

The occurrence of proton transfer reactions between Z)3+ ions and CHa, C2H, and NDZ, between methanium ions and NH, C2HG, CzD , and partially deuterated methanes, and between ammonium ions and ND has been demonstrated in irradiated mixtures of D2 and various reactants near 1 atm. pressure. The methanium ion-methane sequence proceeds without thermal activation between —78° and 25°C. The rate constants for the methanium ion-methane and ammonium ion-ammonia proton transfer reactions are 3.3 X 10 11 cc./molecule-sec. and 1.8 X 70 10 cc./molecule-sec., respectively, assuming equal neutralization rate constants for methanium and ammonium ions (7.6 X 10 4 cc./molecule-sec.). The methanium ion-methane and ammonium ion-ammonia sequences exhibit chain character. Ethanium ions do not undergo proton transfer with ethane. Propanium ions appear to dissociate even at total pressures near 1 atm. 

The equilibrium constant for the proton transfer reaction of benzoic acid, determined in Example, is 6.4 X 10. Calculate the equilibrium concentration of benzoic acid and benzoate anions in a 5.0 X 10 M aqueous solution of the acid. 

This proton transfer reaction involves the second acidic hydrogen atom of carbonic acid, so the appropriate equilibrium constant is. a 2 > whose p is found in Appendix E p. a 2 — 10.33. Because this is a buffer solution, we apply the buffer equation ... 

As an example, consider an early calculation of isotope effects on enzyme kinetics by Hwang and Warshel [31]. This study examines isotope effects on the catalytic reaction of carbonic anhydrase. The expected rate-limiting step is a proton transfer reaction from a zinc-bound water molecule to a neighboring water. The TST expression for the rate constant k is... 

The absolute values of the individual rate constants ks and kp for the nucleophile addition and proton transfer reactions. 

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 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. 

ApA < 1. In Fig. 2 the region of curvature is much broader and extends beyond — 4 < ApA < + 4. One explanation for the poor agreement between the predictions in Fig. 3 and the behaviour observed for ionisation of acetic acid is that in the region around ApA = 0, the proton-transfer step in mechanism (8) is kinetically significant. In order to test this hypothesis and attempt to fit (9) and (10) to experimental data, it is necessary to assume values for the rate coefficients for the formation and breakdown of the hydrogen-bonded complexes in mechanism (8) and to propose a suitable relationship between the rate coefficients of the proton-transfer step and the equilibrium constant for the reaction. There are various ways in which the latter can be achieved. Experimental data for proton-transfer reactions are usually fitted quite well by the Bronsted relation (17). In (17), GB is a... 

Another situation in which an already well-studied proton transfer reaction serves as a probe of a physical phenomenon has been suggested by Knight, Goodall and Greenhow (43, 44). They ionized water with single photons of Nd glass laser infrared radiation and measured an ion recombination rate constant for the reaction... 

Variations in dielectric constant should alter the relative strength of acids of different charge types, since the amount of electrical work involved in a proton-transfer reaction must vary with the dielectric constant of the medium. Since a lowering in dielectric constant increases the work required to separate the ions (for example, to ionize an uncharged acid one must create an anion and a cation), any addition of organic solvent should lead to an increase of pXa values. 

Class (3) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (1) reactions. Class (4) reactions include proton-transfer reactions in trans-decalin and cis-trans decalin mixtures [77]. Proton transfer from the decalin hole to aliphatic alcohol results in the formation of a C-centered decalyl radical. The proton affinity of this radical is comparable to that of a single alcohol molecule. However, it is less than the proton affinity of an alcohol dimer. Consequently, a complex of the radical cation and alcohol monomer is relatively stable toward proton transfer when such a complex encounters a second alcohol molecule, the radical cation rapidly deprotonates. Metastable complexes with natural lifetimes between 24 nsec (2-propanol) and 90 nsec (tert-butanol) were observed in liquid cis- and tra 5-decalins at 25°C [77]. The rate of the complexation is one-half of that for class (1) reactions the overall decay rate is limited by slow proton transfer in the 1 1 complex. The rate constant of unimolecular decay is (5-10) x 10 sec for primary alcohols, bimolecular decay via proton transfer to the alcohol dimer prevails. Only for secondary and ternary alcohols is the equilibrium reached sufficiently slowly that it can be observed at 25 °C on a time scale of > 10 nsec. There is a striking similarity between the formation of alcohol complexes with the solvent holes (in decalins) and solvent anions (in sc CO2). 

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