Hydrolysis of Silicon Compounds

According to Brinsmead and Brown (111) a silicic acid sol containing 43% SiOj was made by refluxing a mixture of ethyl silicate and isopropyl alcohol to which a dilute aqueous solution of acid was slowly added to furnish a stoichiometric amount of water for hydrolysis. There was no evidence that hydrolysis was complete, as would be shown by complete miscibility with water, nor any indication that discrete silica particles had been formed. 

Sodium silicate can be hydrolyzed if sufficiently diluted. Thus a dilute solution of high ratio sodium silicate can be hydrolyzed in an autoclave to form colloidal particles which coagulate to a precipitate if the sodium ion concentration is sufficiently high. Schnurch and Koster (112) report that when a solution of sodium silicate with a SiOj Na,0 weight ratio of 3.89 1 is diluted to 20 g 1 of Si02 and heated 1.5 hr at 150°C, 38% of the silica is precipitated as fine silica. 

---

Many productive methods have been developed for the preparation of silica sol including acidification/121 electrolysation-electrodialysis,[13] ion-exchange,[14] peptization/111 and hydrolysis of silicon compounds/101 which can be grouped into two main types. One is called the aggregation method that contains two steps the polymerization of silicate ions and the aggregation of these polysilicate anions via condensation reaction between the hydroxy groups of the particles. The other one is called the peptization method, i.e., dispersal of a precipitate of Si02 to form colloid. The acidification method will be discussed in detail below. 

The hydrolysis of silicon compounds constitutes the primary step in the formation of silica gel. Numerous reaction routes are possible. 

The most significant difference between the alkoxysilanes and siUcones is the susceptibiUty of the Si—OR bond to hydrolysis (see Silicon compounds, silicones). The simple alkoxysilanes are often operationally viewed as Hquid sources of siUcon dioxide (see Silica). The hydrolysis reaction, which yields polymers of siUcic acid that can be dehydrated to siUcon dioxide, is of considerable commercial importance. The stoichiometry for hydrolysis for tetraethoxysilane is... 

Starting from (OC)5MnSiR2H (R = Me, Ph, Cl), the p-silylene complex 70 is accessible via the oxidative addition of the Si —H bond to Pt(C2H4.)(PPh3)2 and Pt(PPh3)4, respectively. Structure 70 can be functionalized by displacement of the phosphine ligands alcoholysis and hydrolysis of the compound 70 leads to silicon-free complexes [175]. 

An important reaction of silicon compounds containing electronegative substituents, such as the halogens or a Group 15 or 16 element, is hydrolysis, which often occurs very readily even with atmospheric moisture. The primary reaction in silicone production is the hydrolysis of halosilanes, usually the readily available chlorosilanes, to give silanols, which then undergo acid-catalyzed condensation with loss of water, as shown in Scheme 1. 

Moisture levels are also expected to influence the degradation process of the silicone surfactants. The degradation of surfactants on soils and peat is known to increase with soil moisture content [61], and this will also be expected for the primary degradation (hydrolysis) of silicone surfactants. Further studies of these compounds under different... 

The wide range of silicon compounds that are readily hydrolyzed means that the siloxane H3SiOSiH3 is formed (probably via H3 SiOH) in numerous reactions (Scheme 35). Similarly, hydrolysis of halodisilanes tends to give siloxane rather than silanol products (Scheme 36). ... 

No silicon compounds containing multiple bonds to silicon are known. The hydrolysis of halogen compounds of silicon (discussed later) does not yield the silicon analogues of ketones and carboxylic acids by elimination of water from each molecule. Instead, water is eliminated from two or more molecules leading to polymerized products. 

Intensive studies of the products of hydrolysis of substituted chloro-silanes and esters have led to an important new branch of chemistry. Many organo-silicon compounds were prepared by Kipping and his school in the early years of this century and it was observed that glue-like products often resulted from the hydrolysis of these compounds, as well as intractable solids of the kinds already mentioned. No study was made of these polymeric products which are now the basis of silicone chemistry. 

Migration and Accumulation of Silicon Compounds in Soil-Water Systems Silicon is permanently entering soil solution and ground waters as a result of hydrolysis of dispersed alumosilicate and dissolution of various silicon minerals, like quartz. 

Note that in the compound (CH3)2Si(OH)2 the silicon atom can hold two OH groups, unlike carbon. It is this property that makes the existence of silicones possible. By variation of the compounds and conditions of hydrolysis, straight chains, rings and cross-linked polymers are obtained, for example ... 

---