Hydroxyl group, protonation

The synthesis conditions which lead to weakly branched systems involve the use of an acid catalyst where pH < 2.2 (iso-electric point of silica) and the use of low to moderate water content (r < 10). Hydrolysis (see reactions in Section 7.4) then takes place via a fast protonation of the alkoxide, followed by attack of water, resulting in the substitution of the alkoxy group with an hydroxyl group. Protonation becomes slower when more hydroxyls are present. The hydrolysis rate will therefore decrease with the extent of OH substitution. Acid catalysed condensation reactions proceed analogously where a protonated silanol species is attacked by water. The condensation reaction rate decreases with the number of condensed Si-O-Si groups. 

ABSTRACT. The paper considers spectroscopic methods applied to the study of hydroxyl groups, protonic and Lewis acidity, basicity, cations and extrafiramework phases (oxides, supported metals...). 

For the step which limits the reaction rate (rate limiting step), three mechanisms have been proposed, two of which are intramolecular - denominated intramolecular, as such, and solvolytic - and the other intermolecular (Figure 2.1.18). The first of these implies a covalent bond between water and the atoms of carbon during the whole of the transposition. In the solvolytic mechanism there is an initial rupture from the O-C, bond, followed by a nucleophilic attack of the H2O on the C3. Whilst the intermolecular mechanism corresponds to a nucleophilic attack of H2O on the terminal carbon C3 and the loss of the hydroxyl group protonated of the C. ... 

The overall scheme to generate the supported catalysts is shown in Equation 21.3a. Partially dehydrated silica is treated with a solution of Schrock s tungsten catalyst in pentane. The acidic hydroxyl groups protonate an alkyl group to form the siloxide-supported catalyst. In a similar fashion, partially dehydrated silica is treated with a solution of a bispyr-rolyl version of Schrock s molybdenum catalyst to release pyrrole and generate a catalyst... 

---