Water alkoxides

Water Alkoxide ion ft om nucleophilic addition Hydroxide ion Aldol... 

The oxygen nucleophiles that are of primary interest in synthesis are the hydroxide ion (or water), alkoxide ions, and carboxylate anions, which lead, respectively, to alcohols, ethers, and esters. Since each of these nucleophiles can also act as a base, reaction conditions are selected to favor substitution over elimination. Usually, a given alcohol is more easily obtained than the corresponding halide so the halide-to-alcohol transformation is not used extensively for synthesis. The hydrolysis of benzyl halides to the corresponding alcohols proceeds in good yield. This can be a useful synthetic transformation because benzyl halides are available either by side chain halogenation or by the chloromethylation reaction (Section 11.1.3). 

The oxygen nucleophiles that are of primary interest in synthesis are the hydroxide ion (or water), alkoxide ions, and carboxylate anions, which lead, respectively, to alcohols, ethers, and esters. Because each of these nucleophiles can also act as a base, reaction conditions must be selected to favor substitution over elimination. 

Silica membranes have also been studied by several investigators for use in gas separation and membrane reactors. They arc thermally very sublc up to about 500°C. Sintering and densification temperatures of silica membranes depend on the water/alkoxide ratio in the sol-gel process for making the membranes (Langlct et al., 1992]. Crystallization of amorphous silica particles in the membranes takes place at temperatures around 1,000°C [Larbot et al., 1989]. However, pore growth can gradually... 

Benzenesulfonylquinazolin-4(l//)-ones 1 react smoothly with a series of nucleophilic compounds, namely water, alkoxides, thiols, and organometallic compounds, to afford quinazo-line-2,4(117,3//)-dione and 2-substituted quinazolin-4(l//)-ones (NB tautomerism, cf. PP 13,51). 

T3.8.2.3 Obtained in Microemulsion Ammonium hydroxide was added to a solution containing Igepal Co520 (surfactant) and cyclohexane, and the mixture was shaken to obtain a microemulsion. Solutions of zirconium n-propoxide in acetylacetone and n-butanol (l 0.5 3) and of tetraethyl orthosilicate in water and ethanol (1 1 4) were mixed for 3 h, and the mixture was added to the microemulsion with stirring, and hydrolyzed for 3 d. The final concentrations of alkoxides were 0.0075 M, the water surfactant molar ratio was 0.8, and the water alkoxide molar ratio was 10. The powder was washed with acetone and heated at 900°C for 2 h. 

The sol-gel titania-silica mixed oxide contained 20 wt% titania. The catalyst was synthesized imder acidic conditions [5]. The acidic hydrolysant was added to an isopropanohc solution of tetraisopropoxytitanitam(IV) modified by acetylacetone (molar ratio alkoxide acetylacetone =1 1) and tetramethoxysilicon(IV). The water alkoxide acid molar ratio was 5 1 0.09. The resulting gel was dried by semicontinuous extraction with supercritical COg at 40 °C and 240 bar, and stored in a closed vessel under Ar. After calcination in flowing air at 600 °C, the BET smface area was 648 m g and the specific pore volume 2.9 cm g". Details on the synthesis and characterization of the sol-gel titania-sihca by means of FTIR, Raman and UV-vis spectroscopies, Ng-physisorption, XRD, XPS, TEM and thermal analysis have been reported previously [5, 6, 14, 15]. 

As in the case of inorganic silica systems [Eqs. (8)-(ll)], both hydrolysis and condensation of alkoxides may be acid- or base-catalyzed [9,24,37]. The relative contributions of the four reactions—hydrolysis, olation, oxolation, and alcoxolation—determine the physical and chemical characteristics of the resulting oxide material. These contributions are in turn determined by the nature of the metal alkoxide (i.e., nature of the metal atom, nature of the alkyl groups) and the characteristics of the chosen experimental conditions (i.e., the water/alkoxide molar ratio, the nature and concentration of the catalyst, the nature of the solvent, and temperature). A major factor is the hydrolysis ratio, h, defined as the water/alkoxide molar ratio ... 

Recently, Moran et al. [264] made a detailed study of nanoparticle formation in reverse micelles in the system AOT/cyclohexane/water with tetrapropyl titanate or its solution in cyclohexane added into it. One of their observations, relevant for the present discussion, was that with a given water/alkoxide molar ratio the Ti02 crystallite size increased with increasing water/surfactant molar ratio (= w), while for a given value of vw the crystallite size increased with decreasing values of water /alkoxide molar ratio. 

The effect of the water/alkoxide ratio (hydrolysis ratio) on the surface area is almost the same in the two conditions. At low hydrolysis ratio, about 4, the hydrolisis is not favoured, however the particles association is favoured. As a consequence cross-linking is favoured and the surface area decreases. At hydrolysis ratio higher than 10 the diluition could lead to a... 

Some authors perform hydrolysis in the presence of a large excess of water, using hydrolysis ratios h — [water]/[alkoxide] up to ib = 50. Released alcohol is then highly diluted, gelation occurs almost entirely in water and xerogels with surface areas in excess of1000 cm are obtained (Conroy, 2000). 

Many factors influence the kinetics of the hydrolysis and condensation reactions in the sol-gel process, which include the water/alkoxide ratio, temperature, the nature of solvent, and so forth [107]. 

In a typical hydrolytic xerogel synthesis, the silicon and titanium alkoxide precursors are mixed with the solvent in the presence of an acid or base catalyst under vigorous stirring at ambient condition. Typically applied solvents are alcohols, such as ethanol or isopropanol, due to their ability to effectively homogenize the metal precursors. Since Si(OR)4 is by far less reactive toward hydrolysis and condensation reactions than Ti(OR)4, phase separation is expected, as already discussed [23,42]. As one possibility to reduce the difference in reactivity of the various alkoxides, Yoldas proposed to prehydrolyze the less reactive silicon alkoxide prior to the addition of the titanium alkoxide [26]. Keeping in mind that the water/alkoxide ratio is the key parameter, which is typically kept below 2 to increase the time for gelation, thus favoring Si—O—Ti bond formation, this route is pursued in many of the reported studies [27,45]. 

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