Chemical sterically hindered

The same chemical mechanisms and driving forces presented for phenol-formaldehyde resins apply to resorcinol resins. Resorcinol reacts readily with formaldehyde to produce resins (Fig. 2) which harden at ambient temperatures if formaldehyde is added. The initial condensation reaction, in which A-stage liquid resins are formed, leads to the formation of linear condensates only when the resorcinol/formaldehyde molar ratio is approximately 1 1 [119]. This reflects the reactivity of the two main reactive sites (positions 4 and 6) of resorcinol [120]. However, reaction with the remaining reactive but sterically hindered site (2-positiori) between the hydroxyl functions also occurs [119]. In relation to the weights of resorcinol-formaldehyde condensates which are isolated and on a molar basis, the proportion of 4- plus 6-linkages relative to 2-linkages is 10.5 1. However, it must be noted that the first-mentioned pair represents two condensa-... 

Boronic esters are easily prepared from a diol and the boronic acid with removal of water, either chemically or azeotropically. (See Chapter 2 on the protection of diols.) Sterically hindered boronic esters, such as those of pinacol, can be prepared in the presence of water. Boronic esters of simple unhindered diols are quite sensitive to water and hydrolyze readily. On the other hand, very hindered esters, such as the pinacol and pinanediol derivatives, are exceedingly difficult to hydrolyze and often require rather harsh conditions to achieve cleavage. 

The (Z)-isomers do not show a clear sequence (Table 7-2). This result is consistent with a planar structure for the ( >diazoates and a nonplanar, sterically hindered structure for the (Z)-diazoates, which is an obvious cause for their lower chemical stability. 

The success of this transformation depends upon the oxidation potential of the ESE group (Eox 1.5 V), which is lower than that of the alkyl silyl ether group (Eax 2.5 V). Recently, Schmittel et al.35 showed (by product studies) that the enol derivatives of sterically hindered ketones (e.g., 2,2-dimesityl-1-phenyletha-none) can indeed be readily oxidized to the corresponding cation radicals, radicals and a-carbonyl cations either chemically with standard one-electron oxidants (such as tris(/>-bromophenyl)aminium hexachloroantimonate or ceric ammonium nitrate) or electrochemically (equation 10). 

VA Roginskii. Oxidation of polyolefines inhibited by sterically hindered phenols, Doctoral Thesis Dissertation, Institute of Chemical Physics, Chernogolovka, 1983. 

As observed with alkenes, bromine addition to sterically hindered dienes shows a peculiar behavior. Highly substituted dienes, existing predominantly in non-planar conformations, often present a chemical reactivity distinctly different from that of planar... 

Specific control of the stereochemistry of the chemical reaction is better achieved using chiral phase-transfer catalysts. These catalysts interact specifically with the substrate and sterically hinder the approach of nucleophile to one face of the reactive site. Experimental procedures are essentially the same as those employed in reactions using achiral catalysts where there is no stereochemical control and, in subsequent sections, reference is made back to the appropriate Chapter unless variations in the procedure differ significantly. 

The polarimetric method, in combination with the results of chemical correlation, made it possible to determine the optical purity of a range of chiral sulftnates (105-107), thiosulfinates (35,105), and sulfinamides (83) with the sulfur atom as a sole center of chirality. These compounds were converted by means of Grignard or alkyl-lithium reagents into sulfoxides of known specific rotations. This approach to the determination of optical purity of chiral sulfinyl compounds has at least two limitations. The first is that it cannot be applied to sterically hindered compounds [e.g., t-butyl /-butanethio-sulfinate 72 does not react with Grignard reagents]. Second, this... 

Unwanted degradation and oxidation processes can be avoided or at least suppressed for some time either by structural modiflcation of the polymer or by special additives. In practice, the addition of so-called antioxidants is particularly effective. Chemical substances that slow down oxidations and the following aging phenomena serve for this purpose. Antioxidants are sufficiently effective even in concentrations below 1 wt% and are added as early as possible to the polymer to be protected, e.g., already during the drying of powdery polymeric materials or during the preparation of granulates. Some of the most important so-called primary antioxidants are sterically hindered phenols and secondary aromatic amines secondary antioxidants are thioethers as well as phosphites and phosphonites. 

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