Proton transfer during hydration

During the course of the work, we also conducted further studies of the reactions of proton hydrates with CH3COCH3 and CH3COOCH3. The reaction mechanisms were found to change from proton transfer to ligand switching and ultimately to an association process, which would be equivalent to adsorption in the case of bulk systems. 

First of all, what was considered were bare hydronium H3+0 ions with three equivalent protons, a hydrated hydronium ion with three strongly bound water molecules (i.e., Eigen cluster H904+), and the symmetric H502+ complex in which a proton is shared between two water molecules (i.e., the Zundel ion). Many intermediates or more-complex states of the hydrated proton, H+(H20) , may also exist. All clusters have a finite lifetime and transform between each other during charge transport. Due to the variation of the relative abundance of these three basic states, proton transfer may occur via different pathways. 

The foregoing base-catalyzed dehydration of a hydrate is an example of specific-base catalysis. In specific-base catalysis, the proton is completely removed from the reactant before the slow step of the reaction begins. In general-base catalysis, on the other hand, the proton is removed from the reactant during the slow step of the reaction. Compare the extent of proton transfer in the slow step of the preceding specific-base-catalyzed dehydration with the extent of proton transfer in the slow step of the following general-base-catalyzed dehydration ... 

Phenylacetylene and l-phenylprop)T e hydrate in solutions containing about 50% sulfuric acid in water, and even acetylene can be hydrated to acetaldehyde in sufficiently acidic solutions. A solvent deuterium isotope effect on the acid-catalyzed hydration of 1-phenylpropyne indicates that proton transfer occurs during the rate-determining step. The data are therefore consistent with the intermediacy of a vinyl cation, as illustrated for 1-phenylpropyne in equation 9.73. 

In the active site of a hydroxylase, an OH group can be transferred from the peroxide to a suitable substrate (Eq. 18-42). Although radical mechanisms are likely to be involved, such hydroxylation reactions can also be viewed as transfer of OH+ to the substrate together with protonation on the inner oxygen atom of the original peroxide to give a 4a - OH adduct. The latter is a covalent hydrate which can be converted to the oxidized flavin by elimination of H20. This hydrate is believed to be the third spectral intermediate identified during the action of p-hydroxybenzoate hydroxylase 286 287 290... 

If ion symbols in the reacting solutions have been successfully drawn (see Fig. 7.10) and neutralization reaction is then discussed based on these drawings, one automatically comes to the correct interpretation that those hydronium ions, through the transfer of protons to hydroxide ions, react with each other to form water molecules. During the reactions, the hydrated sodium ions Na + (aq) and hydrated chloride ions Cl (aq) are not involved (see Fig. 7.14). These ions are known as spectator ions they exist as spectators ... 

Superoxide is believed to react as an effective hydrogen atom acceptor. Thermodynamically, it is one of the weakest oxidants in Table 9.10, but the electron transfer alternative is highly disfavored by the inaccessibility of 022 in aqueous solution Taube estimates that H02- has a pXa value of 21.110 Thus, a HAT mechanism is inferred for the oxidations of [Con(sep)]2 +, [Fen(tacn)2]2 +, [Run(tacn)2]2 +, and [Run(sar)]2 + nl in these reactions the ligands are the hydrogen atom donors, but it is the metal centers that are oxidized. The bimolecular disproportionation of H02 is another likely example of hydrogen atom transfer this mechanism may also apply to the disproportionation via the reaction of H02 with 02, 27 Curiously, the reaction of 02- with the hydrated electron is fast and seems not to be pH dependent. This latter reaction may yield genuine 022- as an intermediate, or it may be a form of PCET where the solvent donates a proton during the addition of the electron. 

In recent times the concept of crystal polymorphism has expanded beyond its original boundaries to encompass crystal forms of the same molecule with different molecular partners. These may be solvent molecules in solvates [20], or counterions if the molecule can be made non-neutral (by say proton or electron transfer [21]), or other molecules in co-crystals [22]. It is worth noting that the formation of solvate and hydrated forms is commonly observed during polymorph screening, therefore the use of the term pseudopolymorphism to describe solvate forms of a given molecular crystal ought to be discouraged, at least because solvates may, in turn, be polymorphic [23-25]. 

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