Proton transfer equilibria

Since ions are unstable in the gas phase because of positive-negative ion recombination or discharge on the wall, they must be created by ionizing radiation. The ion solvent molecule interactions or other ion molecule equilibria must be observed within the limited lifetime of the ions before their disappearance. Of interest here are two types of ion equilibria. Proton transfer equilibria involving bases B or acids AH as illustrated by reactions (1) and (2) and clustering equilibria as illustrated by reaction (3) written for the negative ion A and water molecules. 

The available studies imply that general catalysis will be operative in systems involving sulfate monoesters and potential six-membered ring transition states. Salicyl sulfate hydrolyzes at pH 4 via intramolecular carboxyl group participation involving pre-equilibrium proton transfer leading to sulfur trioxide expulsion (Fig. 9)2HH, viz. 

We see here that the mechanism with a pre-equilibrium proton transfer leads to a specific acid catalysis rate law whereas that with a rate-determining proton transfer leads to general acid catalysis. It follows that, according to which catalytic rate law is observed, one of these two mechanisms maybe excluded from further consideration. Occasionally, however, different mechanisms lead to the same rate law and are described as kinetically equivalent (see Chapters 4 and 11) and cannot be distinguished quite so easily. 

In examples such as the above, the rate law establishes the composition of the activated complex (transition structure), but not its structure, i.e. not the atom connectivity, and provides no information about the sequence of events leading to its formation. Thus, the rate law of Equation 1.2 (if observed) for the reaction of Equation 1.1 tells us that the activated complex comprises the atoms of one molecule each of B and X, plus a proton and an indeterminate number of solvent (water) molecules, but it says nothing about how the atoms are bonded together. For example, if B and X both have basic and electrophilic sites, another mechanistic possibility includes a pre-equilibrium proton transfer from AH to B followed by the reaction between HB+ and X, and this also leads to the rate law of Equation 1.2. Observation of this rate law, therefore, allows transition structures in which the proton is bonded to a basic site in either B or X, and distinguishing between the kinetically equivalent mechanisms requires evidence additional to the rate law. 

The absence of scatter in a Bronsted plot for a general base-catalysed reaction can imply that the reaction mechanism involves a rate-limiting proton transfer step. This is because proton transfer to the base in the reaction is closely similar to the equilibrium proton transfer to the base in the reaction which defines the p Ka of the conjugate acid of that base. The observation of scatter, especially for sterically hindered bases (such as 2,6-dimethylpyridine), is evidence that nucleophilic catalysis is operating as opposed to general base catalysis. 

For rate-determining proton transfer (mechanism A-SK2), the determined AV value directly refers to the slow step. For the mechanisms with pre-equilibrium proton transfer, A1 and A2, the experimental AV is the sum of the volume changes of the two steps... 

Consequently, the expected kinetic solvent isotope effect in the case of pre-equilibrium proton transfer [2, 97, 99] is... 

It may be concluded that rate-determining proton transfer in the first step (mechanism A-Se2) is indicated if kH/kD > 1 [2, 4, 99]. On the other hand, pre-equilibrium proton transfer (mechanisms A1 or A2) is indicated if kH/kD < 0.6. 

A distinction between mechanisms A1 and A2 on the basis of solvent isotope effect data is probably feasible if model calculations [102] of isotope effects are carried out with special consideration of the particularities of the reaction under study. In a similar way, it may be possible to distinguish between rate-determining and pre-equilibrium proton transfer with the aid of model calculations for reactions with kH/kD values in the region around 0.7 to 0.9. 

When a base catalyzed reaction with proton transfer in the first step is carried out in D20 solution, the substrate exchanges its acidic hydrogen with the deuterium of the solvent before the reaction takes place if the mechanism is fast pre-equilibrium proton transfer with subsequent slow step. If, on the other hand, hydrogen exchange does not occur prior to the reaction, it may be concluded that proton transfer is the rate-determining step. 

The solvent isotope effect for the acid catalyzed hydrolysis of ethyl diazoacetate (without halide ions) is much smaller than 1 (Table 19, p. 63) as expected for a pre-equilibrium proton transfer mechanism. Furthermore, according to the findings of Roberts et al. [205] the products of ethanolysis of ethyl diazoacetate in C2HsOD solution are C2HS OCHDCOOEt as well as C2 H5 OCD2 COOEt which indicates that H exchange is faster than ethanolysis. 

The solvent isotope effects on individual rate coefficients computed from the experimental results are fe /fep = 2.32, kfJ°/kf2° = 3.68, kf fe jfe5i/fep fe fe i = 0.28. The first two values are of the magnitude expected for rate-determining proton transfer. The third quantity corresponds to a solvent isotope effect on a reaction with pre-equilibrium proton transfer, and the value is of the expected magnitude. It is approximately equal to Ks D /Ks H since fen % /eft. 

The first reaction has a normal kinetic isotope effect (RCO2H reacts faster than RCO2D) while the second has an inverse deuterium isotope effect (RCO2H reacts slower than RCO2D). This suggests that there is a ratedetermining proton transfer in the first reaction but specific acid catalysis in the second (i.e. fast equilibrium proton transfer followed by slow reaction of the protonated species). Protonation occurs at carbon in both reactions, and this can be a slow step. 

The acid-catalyzed hydrolysis of epoxides is an interesting reaction and is worth discussing in some detail. A number of lines of attack have been used in attempts to distinguish between the A-1 and A-2 mechanisms for ethylene, propylene, and isobutylene oxides. The hydrol3Tses show only specific add catalysis (Bronsted et al., 1929) and are about twice as fetst in D,0 as in HgO (Pritchard and Long, 1956b). A pre-equilibrium proton transfer... 

It has been assumed so far that there is known to be a pre-equilibrium proton transfer. If this is not known, then the identification of A-2 mechanisms may be uncertain because a slow proton transfer that has a transition state like... 

The pi e-equilibrium proton transfer is demonstrated by the HjO-DjO solvent effect on the rate of hydrolysis of acetamide in dilute acid (Reitz, 1939), and the slow bimolecular attack of water is demonstrated by the effect of substituents on the rate in dilute acid (see Ingold, 1953, p. 784 for review Leisten, 1959). Hydrolyzing benzamide does not exchange its oxygen with the solvent (Bender and Ginger, 1955 Bender et al., 1954). This observation gives no indication as to whether the attacking water molecule adds or substitutes, but only shows that if it adds then the decomposition of the adduct is fast and does not influence the measured rate. 

Knowing rate data in pure H2O and D2O, having available thermodynamic data on lyonium ion acidities, and by assuming the Bronsted catalysis law, it is an easy matter to calculate rates in mixed solvents based on (a) proton transfer as a rate-determining step, or (b) pre-equilibrium proton transfer. The present system adheres to rate-determining proton transfer, and thus A-2 mechanisms are eliminated. The agreement between theory and experimental values suggests that special features... 

The first evidence for the existence of a discrete 2-propenyl cation in the gas phase was from the results of thermochemical measurements of the heat of formation of this ion by equilibrium proton transfer to propyne. The heat of formation for the 2-propenyl cation derived from these measurements is sufficiently different from that for the allyl cation to suggest that this vinyl cation is stable to rearrangement to its more stable allylic isomer. This rearrangement barrier is calculated to be 74.9 kJ mol , consistent with the high barrier implied by these experiments. This structural conclusion has been confirmed by a number of more direct experimental methods in the gas phase. ... 

Equilibrium proton transfer in the general form is the reaction of the following type ... 

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