Alkoxide Hydrolyze

EflFectofpH. The structure of the silica polymers generated in these reaction depends primarily on the relative rates of the hydrolysis and condensation reactions (11). These rates, in turn, depend on the solution pH, the water concentration, and the alkoxide used. Lower molecular weight alkoxides hydrolyze faster than higher molecular weight alkoxides, and obviously, higher water concentrations favor hydrolysis. The major factor is the solution pH. The hydrolysis reaction is catalyzed by both acids and bases, and so the minimum in the hydrolysis rate occurs at a nominally neutral pH. The condensation reaction, however, is fastest at a pH of about 5-6... 

One major problem in producing gels containing homogeneous mixtures of a variety of oxides is that the precursors may not all hydrolyze at the same rate. In particular, transition metal alkoxides hydrolyze much more rapidly than silicon alkoxides. The controlled hydrolysis of low-molecular-weight homometallic species described in the previous section can be adapted to prepare mixed alkoxides. For example, pre-hydrolysis of metal alkoxide followed by reaction with the silicon alkoxide gives a mixed dimeric species such as ... 

Evidence for molecular siloxane networks is abundant in numerous 29Si NMR spectroscopy studies of alkoxides hydrolyzed under acidic conditions... 

The alkoxides hydrolyze vigorously in water. The fe/7-butoxide is a cyclic dimer (9-V) in solvents, whereas the isopropoxide is tetrameric (9-VI) at ordinary temperatures but trimeric at elevated temperatures. Terminal and bridging alkoxyl groups can be distinguished by nmr spectra. Other alkoxides can exist also as dimers and trimers. 

Zirconium alkoxides are used for cross-linking and hardening of isocyanate, epoxy, silicon, urea, melamine, and terephthalate resins in the sol-gel process as catalysts in condensation and as water repellents. Zirconium alkoxides hydrolyze in moist air, but more slowly than titanium alkoxides. 

Evidence for molecular siloxane networks is abundant in numerous Si NMR spectroscopy studies of alkoxides hydrolyzed under acidic conditions [35-40]. Figure 47.4 [41] shows a sequence of Si NMR spectra of TEOS hydrolyzed with 2 mol of water under acidic conditions in ethanol and for comparison a Si NMR spectrum of a commercial aqueous silicate (Ludox). With time the TEOS sol becomes more highly condensed, as evident from the disappearance of monomer (Q ) and the progressive formation of end groups and di-, tri-, and tetrasubstituted silicate species (Q -Q" species, respectively). However, even after 14 days, di- and trisubstituted species appear more prevalent than tetrasubstituted species. By comparison, Si NMR spectra of aqueous silicates (Figure 47.4d) and base-catalyzed alkoxides (Figure 47.5) are dominated by monomer and tetrasubstituted species. 

Most metal alkoxides hydrolyze readily in the presence of water, so that stringent conditions must be maintained to achieve powders with controlled characteristics. The reactions are sensitive to the concentration of the reactants, the pH, and the temperature. Oxide or hydrated oxide powders are produced. The precipitated particles are commonly amorphous and can be agglomerates of much finer particles (Fig. 2.20a). 

Figure 17 summarizes the avadable sol—gel processes (56). The process on the right of the figure involves the hydrolysis of metal alkoxides in a water—alcohol solution. The hydrolyzed alkoxides are polymerized to form a chemical gel, which is dried and heat treated to form a rigid oxide network held together by chemical bonds. This process is difficult to carry out, because the hydrolysis and polymerization must be carefully controlled. If the hydrolysis reaction proceeds too far, precipitation of hydrous metal oxides from the solution starts to occur, causing agglomerations of particulates in the sol. 

Mixing. In method 1, a suspension of colloidal powders, or sol, is formed by mechanical mixing of colloidal particles in water at a pH that prevents precipitation (step A in Fig. 1) (8). In method 2 or 3, a Hquid alkoxide precursor such as Si(OR)4, where R is CH (TMOS), C2H (TEOS), or C Hy, is hydrolyzed by mixing with water (eq. 2). 

The effects of both pH and temperature of aluminum alkoxide hydrolysis on gelation is shown in Eigure 8. Addition of acid into the mixture hydrolyzed at 90°C, and by consequence reduction of pH, reduces the gelation time of the samples, whereas in mixtures hydrolyzed at room temperature, acidic addition increases gelation time. 

Zirconium tetrachloride is instantly hydrolyzed in water to zirconium oxide dichloride octahydrate [13520-92-8]. Zirconium tetrachloride exchanges chlorine for 0x0 bonds in the reaction with hydroxylic ligands, forming alkoxides from alcohols (see Alkoxides, METAl). Zirconium tetrachloride combines with many Lewis bases such as dimethyl sulfoxide, phosphoms oxychloride and amines including ammonia, ethers, and ketones. The zirconium organometalLic compounds ate all derived from zirconium tetrachloride. 

Alkoxides. Zirconium alkoxides are part of a family of alcohol-derived compounds (219). The binary zirconium compounds have the general formula ZRX — (OR). They are easily hydrolyzed and must be prepared under anhydrous conditions. They are prepared by the reaction of zirconium tetrahahdes and alcohols ... 

Zirconium alkoxides readily hydrolyze to hydrous zirconia. However, when limited amounts of water are added to zirconium alkoxides, they partially hydrolyze in a variety of reactions depending on the particular alkoxide (222). Zirconium tetraisopropoxide [2171 -98-4] reacts with fatty acids to form carboxjiates (223), and with glycols to form mono- and diglycolates (224). 

The hydrides can also be used to form primary alcohols from either terminal or internal olefins. The olefin and hydride form an alkenyl zirconium, Cp2ZrRCl, which is oxidized to the alcohol. Protonic oxidizing agents such as peroxides and peracids form the alcohol direcdy, but dry oxygen may also be used to form the alkoxide which can be hydrolyzed (234). 

Reactions of the Side Chain. Benzyl chloride is hydrolyzed slowly by boiling water and more rapidly at elevated temperature and pressure in the presence of alkaHes (11). Reaction with aqueous sodium cyanide, preferably in the presence of a quaternary ammonium chloride, produces phenylacetonitrile [140-29-4] in high yield (12). The presence of a lower molecular-weight alcohol gives faster rates and higher yields. In the presence of suitable catalysts benzyl chloride reacts with carbon monoxide to produce phenylacetic acid [103-82-2] (13—15). With different catalyst systems in the presence of calcium hydroxide, double carbonylation to phenylpymvic acid [156-06-9] occurs (16). Benzyl esters are formed by heating benzyl chloride with the sodium salts of acids benzyl ethers by reaction with sodium alkoxides. The ease of ether formation is improved by the use of phase-transfer catalysts (17) (see Catalysis, phase-thansfer). 

Sol-gel primers use inorganic or metal-organic precursors (generally aluminum, silicon or titanium alkoxides) whose chemistry is closely related to the silane coupling agents discussed previously. These precursors are dissolved in alcohol, then hydrolyzed by the addition of water ... 

An isolated acetoxyl function would be expected to be converted into the alkoxide of the corresponding steroidal alcohol in the course of a metal-ammonia reduction. Curiously, this conversion is not complete, even in the presence of excess metal. When a completely deacetylated product is desired, the crude reduction product is commonly hydrolyzed with alkali. This incomplete reduction of an acetoxyl function does not appear to interfere with a desired reduction elsewhere in a molecule, but the amount of metal to be consumed by the ester must be known in order to calculate the quantity of reducing agent to be used. In several cases, an isolated acetoxyl group appears to consume approximately 2 g-atoms of lithium, even though a portion of the acetate remains unreduced. Presumably, the unchanged acetate escapes reduction because of precipitation of the steroid from solution or because of conversion of the acetate function to its lithium enolate by lithium amide. 

The reactions of 2- and 4-cyanoquinazolines are similar to those of the chloro compounds. Thus the cyano group can be replaced by alkoxide, phenoxide, substituted amino, and hydrazino groups substitution of the 4-cyano takes place more readily than that of the 2-cyano group.The nitrile substituent can also be hydrolyzed to an alkoxycarbonyl and amide group/ ... 

Factor b above is discussed in Sections II, B, 1 II, B, 4 and II, C. A hydrogen-bonded structure such as 221 can account for the facile reaction of 5-bromouracil or for the unique, so-called hydrolyzability of carboxymethylthio-azines (237). The latter may also react via the intramolecular mechanism indicated in 136. The hydrogen-bonded transition state 238 seems a reasonable explanation of the fact that 3,4,6- and 3,4,5-trichloropyridazines react with glacial acetic acid selectively to give 3-pyridazinones while other nucleophiles (alkoxides, hydrazine, ammonia, or sulfanilamide anion) react at the 4- and 5-positions. In this connection, 4-amino-3,5-dichloro-pyridazine in liquid hydrazine gives (95°, 3hr, 60%yield)the isomer-... 

Even polyalkoxy-s-triazines are quite prone to nucleophilic substitution. For example, 2,4,6-trimethoxy-s-triazine (320) is rapidly hydrolyzed (20°, dilute aqueous alkali) to the anion of 4,6-dimethoxy-s-triazin-2(l )-one (331). This reaction is undoubtedly an /S jvr-4r2 reaction and not an aliphatic dealkylation. The latter type occurs with anilines at much higher temperatures (150-200°) and with chloride ion in the reaction of non-basified alcohols with cyanuric chloride at reflux temperatures. The reported dealkylation with methoxide has been shown to be hydrolysis by traces of water present. Several analogous dealkylations by alkoxide ion, reported without evidence for the formation of the dialkyl ether, are all associated with the high reactivity of the alkoxy compounds which ai e, in fact, hydrolyzed by usually tolerable traces of water. Brown ... 

This reaction, called the oxy-Cope rearrangement has proved highly useful in synthesis." The oxy-Cope rearrangement is greatly accelerated (by factors of 10 -10 ) if the alkoxide is used rather than the alcohol.In this case the direct product is the enolate ion, which is hydrolyzed to the ketone. 

Two kinds of solution were prepared in advance. Solution A was a water solution containing an Si source, which was obtained by hydrolyzing metal alkoxide (tetraethylorthosilicate, TEOS) with a dilute tetrapropylammoniumhydroxide (TPA-OH)/water solution at room temperature. The molar ratio of Si to the template was 3. In peparation of ZSM-S zeolite nanoerystals, aluminium isopropoxide as an A1 source and sodium chloride were added into solution A. Solution B was an oi mic solution containing surfectant Nonionie surfactants, poljraxyethylene (15) cxslylether (C-15), polyoxyethylene (15) nonylphenylether (NP-15), and polyoxyethylene (15) oleylether (O-15), and ionic surfoctnnts, sodium bis(2-ethylhexyl) sulfosucdnate (AOT) and... 

The basic sol-gel reaction can be viewed as a two-step network-forming polymerization process. Initially a metal alkoxide (usually TEOS, Si(OCIl2CH )4) is hydrolyzed generating ethanol and several metal hydroxide species depending on the reaction conditions. These metal hydroxides then undergo a step-wise polycondensation forming a three-dimensional network in the process. The implication here is that the two reactions, hydrolysis and condensation, occur in succession this is not necessarily true (8.9). Depending on the type of catalyst and the experimental conditions used, these reactions typically occur simultaneously and in fact may show some reversibility. 

Fig. 3.3 The common two-stage sol-gel process used to entrap biopolymers in a silica matrix (see Scheme 3.1). The first stage serves to hydrolyze alkoxide Equation (2) in the acidic or alkaline media. This is also attended with condensation reactions Equations (3) and (4) resulting in the formation of oligomeric silica that self-organizes in the form of sol nanoparticles. Biopolymers are entrapped in the...

The sol-gel process involves hydrolysis of alkoxide precursors under acidic or basic conditions, followed by condensation and polycondensation of the hydroxylated units, which lead to the formation of porous gel. Typically a low molecular weight metal alkoxide precursor molecule such as tetramethoxy silane (TMOS) or tetra ethoxysilane (TEOS) is hydrolyzed first in the presence of water, acid catalyst, and mutual solvent... 

Because of the role of precursor structure on film processing behavior (consolidation, densification, crystallization behavior), the reaction pathways are typically biased through the use of the catalyst, which is simply an acid or a base. This steers the reaction toward an electrophilic or nucleophilic attack of the M—OR bond.1,63 Hydrolysis sensitivity of singly or multiply hydrolyzed silicon alkoxides is also influenced by the catalyst, which contributes to the observed variations in oligomer length and structure. Figure 2.3b illustrates... 

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