Protonation silanol

Bu3Si(OH2)] [H-CBuMesBBrg] 106, which can be rationalized in two canonical forms, namely a Lewis acid-Lewis base adduct of the silylium ion and water, and, alternatively, a protonated silanol (Scheme 18).264... 

These chelates are more reactive than non-chelated chlorosilanes in forming a silyl enol ether from ketones. Moreover, careful hydrolysis of a pentacoordinated chlorosilane was shown to give a pentacoordinate protonated silanol that resembles the intermediate in the aqueous hydrolysis of silanes.271... 

The preparation can also take place under acidic conditions below the isoelectric point of the Si-OH-bearing inorganic species (pH 2) then the silica species are positively charged, that is, protonated silanol groups (Si-OHT) in order to produce an interaction with cationic surfactants, it is necessary to add a mediator ion X- (usually a halide), which gives rise to the S+X I+ pathway (Fig. 3.6b). 

The lower value of for dimers can be explained by simple arguments. The steric requirements of the bulky siloxane group should decrease the reaction rate constant of dimers compared with monomers. Inductive effects should decrease the rate as well. This latter conclusion is based on the mechanism proposed by Pohl and Osterholtz (5), which postulates that in acidic solutions (pH < 4.5), an equilibrium concentration of protonated silanol is rapidly established. The protonated silanol reacts with a neutral silicate species in the rate-determining step, and then, a hydronium ion is eliminated. The more electron-withdrawing siloxane group (compared with Si-OR and Si-OH) would decrease the stability of the protonated species and effectively shift the equilibrium toward the unprotonated form. Therefore, the observed reaction rate constant should decrease. 

The similarity of foi monomers and dimers is unexpected. If the reaction mechanism involves a protonated silanol, as in the water-producing condensation, subsequent reaction rate constants must be proportionately larger to give an observed of the same magnitude. However, the likely leaving group from a protonated silanol species is water, not an alcohol. 

The equilibrium concentration of the protonated silanol is rate-determining in the condensation reaction (Eq. 5). 

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. 

Because gel times decrease below the IEP, it is believed (2, 4, 27, 28) that below about pH 2, condensation occurs by a bimolecular nucleophilic mechanism involving a protonated silanol ... 

It is generally believed that the acid-catalyzed condensation mechanism involves a protonated silanol species. Protonation of the silanol makes the silicon more electrophilic and thus susceptible to nucleophilic attack. The most basic silanol species (silanols contained in monomers or weakly branched oligomers) are the most likely to be protonated. Therefore, condensation reactions may occur preferentially between neutral species and protonated silanols situated on monomers, end groups of chains, etc. 

Additionally, with high pH values a negative absorption band occurs at 980 cm The band is assigned to the bending vibration of protonated silanol groups (Si—OH). It can be interpreted as deprotonation of Si—OH at the surfaces of the silica abrasive and/ or the silica film resulting in Si—0 . Similar behaviour has been found with different types of silica abrasives, for example Stober-sols. 

Cluster-cluster growth is expected to pertain when there is not a continuous source of monomers and when there is no mechanism favoring condensation predominantly between low- and high-molecular-weight species. Re-examination of the Si NMR spectra shown in Fig. 33 shows that these conditions may exist under acidic conditions. At pH 1 the solution is depleted of monomers at = 0.9, yet the concentration of Q species is much less than under neutral conditions. In fact the monomers are essentially depleted for 0.01 [111]. Under strongly acidic conditions the hydrolysis reaction is complete long before the gel point (see Fig. 29), which indicates that the rate of condensation is low with respect to the rate of hydrolysis. Below about pH 3, the depolymerization reaction is minimized. Below about pH 2, the isoelectric point of silica, condensation occurs by a mechanism involving a protonated silanol or alkoxide species. 

Pohl and Osterholtz [50] proposed that below about pD 4.5, the Increased condensation rate of alkylsilanetriol (Fig. 18a) also Involved a deuterated (or protonated) silanol ... 

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