Prepared under Acidic Conditions

The epoxy cresol—novolak resins (2) are prepared by glycidylation of o-cresol—formaldehyde condensates in the same manner as the phenol—novolak resins. The o-cresol—formaldehyde condensates are prepared under acidic conditions with formaldehyde—o-cresol ratios of less than unity. 

Organically modified porous silicates have been prepared under acidic conditions, by direct reaction of a mixture of phenyltriethoxysilane (PTES) and tetraethoxysilane (TEOS), and an aqueous solution of cetyltrimethylammonium bromide (CTAB). For a 1 4 molar ratio between PTES and TEOS, the hexagonal (2d, p6m) phase, but also a cubic phase analogous to the already reported SBA-1 phase (Pm3n), can be prepared. The surfactant can then be efficiently removed by calcination at 350°C, leading to phenyl-functionalized microporous silicates with two types of architecture. The influence of several parameters (PTES/TEOS ratio ethanol content) that affect the organization of the samples, will be discussed. 

Most of the one-pot syntheses of organically functionalized mesoporous silicates have been done under basic conditions. Only hexagonal phases (2d, p6m) were reported so far, except one very recent example of a phenyl-functionalized cubic phase [17], analogous to the bicontinuous MCM-48 phase (la3d) [8]. The cubic phase prepared under acidic conditions from PTES and TEOS is indeed related to a different type of cubic mesophases, micellar mesophases, reported in the literature for various surfactant/solvent systems [23] as well as for lipid-containing systems [24]. 

Thioacetals and thioketals are the sulphur equivalents of acetals and ketals and are also prepared under acid conditions (Following fig.). These can also be used to protect aldehydes and ketones, but the hydrolysis of these groups is more difficult. Moreover, the thioacetals and thioketals can be removed by reduction and this provides a method of reducing aldehydes and ketones. 

Although aliphatic ethers and especially symmetrical ethers can be prepared under acidic conditions when activated alcohols are introduced or symmetric products formed, " it is more common to substitute chlorine. bromine, """ iodine, or sulfonates under basic conditions. Dimethyl sulfate is regularly used to form methyl ethers.Typical... 

Tributyltin alkoxides can also be prepared under acid conditions by the alcoholyis of tributyltin hydride or allyltributyltin in the presence of a triflic acid catalyst presumably the reaction involves the tin triflate as an intermedate.13... 

The epoxy-cresol—novolaks resins are prepared by glyci-dylation of o-cresol-formaldehyde condensates in the same manner as phenol-novolak resins, o-cresol-formaldehyde condensates are prepared under acidic conditions with HCHO-o-cresol ratios of less than unity. The o-cresol novolacs of commercial significance possesses degrees of polymerization, n, of 1.7-4.4 and the epoxide functionality of the resultant glycidylated resins varies from 2.7 to 5.4. Softening points (Durrain s) of the products are 35-99°C [31]. 

SBA-15 is prepared under acidic conditions using polyethylene oxide)-poly(propylenc oxide)-poly(elhylene oxide) triblock copolymers, (EO)n(PO)7o(EO)n, as templates, e.g. Pluronic P123 (n = 20), Pluronic L121 (n - 5) or Pluronic F127 (n = 106). In a typical synthesis [54,551 based on the original procedures [33], 4.0 g of (EO)2o(PO)7o(EO)2o copolymer was dissolved in 150 g of 1.6 M HC1. To this solution was added 8.50 g of TEOS, and the resulting mixture was stirred until TEOS was dissolved. The mixture was heated in an oven for 24 h at 308 K. and then 24 h more at 373 K. under static condition. Silica products were filtered, dried (without washing), and calcined at 823 K. 

PANl can be prepared by the polymerization of aniline in an oxidizing medium, and the resulting polymer is found to be in a partially oxidized state [295]. Its molecular constitution is intermediate between that of poly(para-phenylene amine), which represents the so-called leucoemeraldine form, and that of poly(par o-phenylenc imine), known as the pemi-graniline form (Figure 1.45). Since the polymer is prepared under acidic conditions (HCl), it is also partly... 

Figure 10.9 Pickles are prepared under acidic conditions. Why do they resist spoilage ...

Out of the concern of lanthanide hydrolysis to produce intractable precipitates of lanthanide oxides and/or hydroxides, the coordination of the lanthanide ions with amino acids has historically been carried out under rather acidic conditions. A large number of such complexes have been obtained under low-pH (1-4) conditions and structurally characterized. Following a summary of the unique features of lanthanide coordination chemistry and the structural and functional characteristics of amino acids, the coordination modes of the amino acid ligands and the salient stmctural features of the complexes prepared under acidic conditions are discussed. 

Rf (DOBAA) —Rp/Si02 nanocomposite, which was prepared under acidic conditions, showed a clear weight loss (25%) at 800°C, whose value is quite similar to that (24%) of elementary analyses of fluorine in this composite [43-45], However, Rp—(DOBAA) —Rp/Si02 nanocomposite, which was prepared under alkaline conditions, exhibited no weight loss eharacteristic even after caleination at 800° C, although the content of Rp—(DOBAA) —Rp oligomer in this nanocomposite was estimated to be 23% by elementary analyses of fluorine (see Figure 4.5) [46]. 

Liu, L., Li, H., and Zhang Y. (2005). Effect of synthesis parameters on the chromium content and catalytic activities of mesoporous Cr-MSU-x prepared under acidic conditions, J. Phys. Chem. B, 110, pp. 15478-15485. 

Aerogels prepared under acidic conditions are composed of primary particles having a very small size. These particles aggregate to form clusters which stick together to build up the homogeneous network. 

Depending on the molar ratio of the two reactants we can obtain products which range from syrupy liquids at low formaldehyde levels to completely crosslinked thermosetting resins at high levels of formaldehyde. Novolaks used in tire cord adhesive are RF resins of low formaldehyde content (mol ratio of resorcinol to formaldehyde 1.00/.40 to 1.00/.75) and are prepared under acidic conditions. These resins are manufactured and sold commercially as... 

Yang, Y. and Sayari, A. (2007) Molecularly ordered biphenyl-bridged mesoporous organosilica prepared under acidic conditions. Chem. Mata"., 19, 4117-4119. 

Phenolic resins are obtained by reacting phenols (P) with formaldehyde (F) under different reaction conditions, such as in the presence of acidic or basic catalysts or at different P/F ratios. Depending upon the reaction conditions, phenolic resins are classified as novolac or resol resins (also termed nonheat- or heat-reactive resins, respectively). Resols are usually obtained when condensation is carried out in the presence of excess formaldehyde and under alkaline conditions, whereas novolacs are prepared under acidic conditions in the presence of excess phenol. The chemistry and technology of phenolic resins has recently been reviewed in the literature. ... 

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