Exchange reactions between hydroxyl protons

Figure 24. Subsequent configurations in the chain of proton exchange reactions between neighboring OH and H2O molecules leading to an effective shift in the position of the surface hydroxyl. The Rh(l 11) surface is in the x-y plane.

This means, that glass electrodes show an application range in both, acidic and alkaline solutions with one and the same function, but one has to remember that, above pH 7 hydroxyl ions are involved in the chemical reaction. Baucke has shown that the formation of a Galvani potential difference can also be derived from a kinetic point of view. Eigure II.9.6, in principle, shows the proton exchange reaction between the surface of the glass and the solution. Both the backward and the forward reactions are second-order reactions. The exchange current can be expressed as follows ... 

In ethanol, the hydroxyl peak should be split by spin-spin interaction with the methylene protons. However, the high-resolution NMR spectrum shows only a single hydroxyl peak. The lack of apparent spin-spin coupling in this peak is due to the fact that this spectrum was taken on a sample that contained a small amount of water. In the NMR spectrum of pure ethanol, the hydroxyl peak will be a triplet as we expect. However, if a small amount of water is present, a rapid proton-exchange reaction between the —OH proton in ethanol and H30 in the water will occur that has the effect of broadening the three peaks in the —OH triplet so that they coalesce into a single observed peak. 

The protonated aluminol sites are the most effective fluoride sorption sites and are usually responsible for the rapid kinetics due to coulombic attraction between the positively charged sites and the negatively charged fluoride species. The reaction with non-protonated sites involves ligand exchange, leads also to the formation of inner-sphere complexes, releases hydroxyl ions, is slow and characterized by a higher activation energy. 

In the adsorption of a humic molecule to a mineral surface, several reactions may occur simultaneously formation of inner-sphere complexes between carboxylate groups and the mineral through ligand-exchange reactions, formation of outer-sphere complexes with carboxylate (RCOO ) and hydroxyl (RCOH) groups, and protonation-deprotonation reactions. The overall adsorption reaction for an HS—for example a fulvic acid molecule—is written as (Filius et al. 2001, 2003)... 

Busch, J. H., and de la Vega, J. R., Symmetry and tunneling in proton transfer reactions. Proton exchange between methyloxonium ion and methyl alcohol, methyl alcohol and methoxide ion, hydronium ion and water, and water and hydroxyl ion, J. Am. Chem. Soc. 99,2397-2406 (1977). 

The recent studies on the relationship between activation temperature and carbonium ion type catalytic activity of both decationized and cation exchanged zeolites show that at arid above the temperature required for the removal of all observable hydroxyls with vibrational frequencies between 3700-3500 cm" the activity sharply declines. The lowest concentration of acidic lattice hydroxyl required for carbonium ion activity seems to depend on the reaction involved. For example, dehydroxylation of La-exchanged Y to a level at which hydroxyl content was unobservable by currently-used infrared techniques led to total loss of activity to crack n-butane, but only partial loss of activity to crack cumene (vide infra) and to alkylate toluene with propylene (74). The activity and hydroxyl content lost on dehydroxylation can be restored upon subsequent treatment with water (11). Furthermore, alkali metal zeolites, which have little or no carbonium ion type activity can be made to show strong activity by the addition of a proton source, such as alkyl chlorides (51, 58). The similarity of the products obtained with the... 

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