Hydroxyl groups ratios

The alkoxlyated lignin requires an isocyanate to hydroxyl group ratio of 1.5 to form effective networks. This is a significantly higher ratio of expensive isocyanate to hydroxyl units than is used in commercial practice. The work... 

Figure Bl.25.9(a) shows the positive SIMS spectrum of a silica-supported zirconium oxide catalyst precursor, freshly prepared by a condensation reaction between zirconium ethoxide and the hydroxyl groups of the support [17]. Note the simultaneous occurrence of single ions (Ff, Si, Zr and molecular ions (SiO, SiOFf, ZrO, ZrOFf, ZrtK. Also, the isotope pattern of zirconium is clearly visible. Isotopes are important in the identification of peaks, because all peak intensity ratios must agree with the natural abundance. In addition to the peaks expected from zirconia on silica mounted on an indium foil, the spectrum in figure Bl. 25.9(a)...

Commonly used isocyanates are toluene dhsocyanate, methylene diphenyl isocyanate, and polymeric isocyanates. Polyols used are macroglycols based on either polyester or polyether. The former [poly(ethylene phthalate) or poly(ethylene 1,6-hexanedioate)] have hydroxyl groups that are free to react with the isocyanate. Most flexible foam is made from 80/20 toluene dhsocyanate (which refers to the ratio of 2,4-toluene dhsocyanate to 2,6-toluene dhsocyanate). High-resilience foam contains about 80% 80/20 toluene dhsocyanate and 20% poly(methylene diphenyl isocyanate), while semi-flexible foam is almost always 100% poly(methylene diphenyl isocyanate). Much of the latter reacts by trimerization to form isocyanurate rings. 

Representative results are given in Table 5.4. From column 7, it is seen that the ratio iV,/ Afj - - N/,) is in the region of 1 2 (in contrast to the 1 1 found with silica) suggesting that each molecule of water in the physisorbed monolayer is bonded to two surface hydroxyl groups. 

This chemical bond between the metal and the hydroxyl group of ahyl alcohol has an important effect on stereoselectivity. Asymmetric epoxidation is weU-known. The most stereoselective catalyst is Ti(OR) which is one of the early transition metal compounds and has no 0x0 group (28). Epoxidation of isopropylvinylcarbinol [4798-45-2] (1-isopropylaHyl alcohol) using a combined chiral catalyst of Ti(OR)4 and L-(+)-diethyl tartrate and (CH2)3COOH as the oxidant, stops at 50% conversion, and the erythro threo ratio of the product is 97 3. The reason for the reaction stopping at 50% conversion is that only one enantiomer can react and the unreacted enantiomer is recovered in optically pure form (28). 

The basic carbohydrate molecule possesses an aldehyde or ketone group and a hydroxyl group on every carbon atom except the one involved in the carbonyl group. As a result, carbohydrates are defined as aldehyde or ketone derivatives of polyhydroxy alcohols and their reaction products. A look at the formula for glucose shows that it contains hydrogen and oxygen atoms in the ratio in which they are found in water. The name carbohydrate... 

As with the other aminoplastics, the chemistry of resin formation is incompletely understood. It is, however, believed that under acid conditions at aniline-formaldehyde ratios of about 1 1.2, which are similar to those used in practice, the reaction proceeds via p-aminobenzyl alcohol with subsequent condensation between amino and hydroxyl groups (Figure 24.10). 

Many aromatic steroids submitted to the Birch reduction contain hydroxyl groups which are deprotonated to the corresponding alkoxides during the reduction, particularly if a tertiary alcohol is used as the proton donoi. The steroidal alkoxides and the one derived from the proton donor often precipitate and cause foaming of the reaction mixture, as was noted by Wilds and Nelson. These alkoxides can be kept in solution by adding an excess of the proton donor alcohol to the mixture the alcohol also assists in dissolving the starting hydroxylic steroid. A particularly useful reaction medium for hydroxylic steroids contains ammonia, tetrahydrofuran and -butyl alcohol in the volume ratio of 2 1 (Procedure 2, section V). This mixture... 

Treatment of a- or P hydroxyacids with sulfur tetrafluoride leads to conver Sion of the carboxylic group into the tiifluoromethyl group, but the hydroxyl group undergoes either fluorination, fluorosulfination, esterification, or dehydration to form esters, ethers, or alkenes The ratio of the products depends on P substitution [209, 210] (equations 103 and 104)... 

Ratio of Zwittbrion Form to Hydroxy Form (pXr) fob Pyridines and Azines in Which a Benzo Ring Carries a Hydroxyl Group... 

The ratios of these slopes for L- and D-esters are shown in Table 12. The kL/kD values of the acylation step in the CTAB micelle are very close to those in Table 9, as they should be. It is interesting to note that the second deacylation step also occurs enantioselectively. Presumably it is due to the deacylation ocurring by the attack of a zinc ion-coordinated hydroxide ion which, in principle, should be enantioselective as in the hydroxyl group of the ligand. Alternatively, the enantioselectivity is also expected when the free hydroxide ion attack the coordinated carbonyl groups of the acyl-intermediate with the zinc ion. At any rate, the rates of both steps of acylation and deacylation for the L-esters are larger than those for the D-esters in the CTAB micelle. However, in the Triton X-100 micelle, the deacylation step for the D-esters become faster than for the L-esters. 

Unlike the 4,6-O-benzylidene derivatives, methyl 2,3-O-benzylidene-5-0-methyl-/ -D-ribofuranoside (99) (9) in which the acetal spans two secondary hydroxyl groups, reacts with NBS to give (48) the isomeric bromobenzoates 100 and 101. These are formed in a ratio of 2 1, and,... 

This dicarboxy-terminated oligomer is prepared by reacting excess adipic acid with 1,2-edianediol in die bulk until hydroxyl group conversion is complete (Scheme 2.47). The molar mass of final polymer depends on the initial molar ratio of monomers (procedure similar to that described in ref. 401). 

Side reactions involving branching through a secondary hydroxyl group can also occur. The extent of these side reactions should decrease as the ratio of epoxy to phenol decreases since phenolate anions are significantly more nucleophilic titan aliphatic hydroxyl groups. 

The diastereomeric ratio was determined by the NMR integral ratio of the methine protons adjacent to the hydroxyl group (3.33 ppm for the (//-isomer 3.44 ppm for the waw-isomer). 

Under 50 mbar of H2 and 50 °C, SnBu4 reacts selectively on the Pt surface to form surface complexes of average formula Pts[SnBux] /. The empirical formula (values of x and y) depend on the reaction time and on the Snint/Pts ratio (Fig. 6). Note that under these conditions SnBu4 does not chemically react with the silica surface, but it is fully physisorbed on the support [114]. In fact, when silica is contacted with SnBu4, IR spectroscopy shows a shift of the v(0 - H) band of silica to lower wave numbers, i.e. from 3747 cm to ca. 3700 cm which results from van der Waals interactions between the hydroxyl groups of the support and the butyl chains of adsorbed SnBu4 (Scheme 32). 

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