Solvents derivatives

The degree of duorination can be limited by the thermal stabiUty of the solvent or by its reaction with basic potassium duoride through proton abstraction. Such solvent-derived by-products can subsequentiy react with the starting material and/or main product. 

Most solvents for hydroperoxides are not completely inert to radical attack and, consequendy, react with radicals from the hydroperoxide to form solvent-derived radicals, either by addition to unsaturated sites or by hydrogen- or chlorine-atom abstraction. In equation 15, S—H represents solvent and S is a solvent radical. 

Such solvent-derived radicals can induce the decomposition of the hydroperoxide or react with oxygen in the system to form peroxidic solvent molecules. They may also react with other radicals either by coupling or disproportionation. 

Solution Polymerization. Solution polymerization of vinyl acetate is carried out mainly as an intermediate step to the manufacture of poly(vinyl alcohol). A small amount of solution-polymerized vinyl acetate is prepared for the merchant market. When solution polymerization is carried out, the solvent acts as a chain-transfer agent, and depending on its transfer constant, has an effect on the molecular weight of the product. The rate of polymerization is also affected by the solvent but not in the same way as the degree of polymerization. The reactivity of the solvent-derived radical plays an important part. Chain-transfer constants for solvents in vinyl acetate polymerizations have been tabulated (13). Continuous solution polymers of poly(vinyl acetate) in tubular reactors have been prepared at high yield and throughput (73,74). 

The cation [NSO(NPCl2)2] (14.11) is the proposed intermediate in this ring-opening polymerization process. This cation is extremely reactive, as illustrated by the isolation of the solvent-derived product 14.12 when it is generated by halide abstraction from the cyclic precursor with AICI3 in l,2-dichloroethane. °... 

The reactions of the radicals (whether primary, secondary, solvent-derived, etc.) with monomer may not be entirely regio- or ehemoseleetive. Reactions, such as head addition, abstraction or aromatic substitution, often compete with tail... 

For the case of MMA polymerization with a source of f-butoxy radicals (DBPOX) as initiator and toluene as solvent, most initiation may be by way of solvent-derived radicals"1"" (Scheme 3.9). Thus, a high proportion of chains (>70% for 10% w/v monomers at 60 °C22) will be initiated by benzyl rather than 1-butoxy radicals. Other entities with abstractable hydrogens may also be incorporated as polymer end groups. The significance of these processes increases with the degree of conversion and with the (solvent or impurity) monomer ratio. 

In order to safely identify k0 with intramolecular carbenic reactions (e.g., k and the formation of alkene 4 in Scheme 1), product analysis should demonstrate that the yield of intramolecular products exceeds 90%, while dimer, azine, and solvent-derived (intermolecular) carbene products should be absent or minimal. If these conditions are not met, mechanistic interpretation is often ambiguous, a result that is well illustrated by the saga of benzylchlorocarbene (see below, Section IV.C). Less desirably, k0 can be corrected for competitive intermolecular carbenic reactions. Bimolecular reactions like dimerization and azine formation can be minimized by working at low carbene precursor concentrations, and careful experimental practice should include quantitative product studies at several precursor concentrations to highlight potential product contamination by intermolecular processes. 

Recently, poly(vinyl alcohol)-isocyanate reactions have been run successfully in dimethylsulfoxide solutions by several lab- oratories, including ours. While there are reports of solvent derived products from the reactions of isocyanates in DMSO (60), these reactions do not seem to pose a problem in the reaction with poly(vinyl alcohol). This solvent has been used to react PVA with methoxymethyl isocyanate (61.62) and a series of isocyanates including methyl, ethyl, isopropyl, phenyl and 1-naphthyl (63). This reaction is illustrated in Equation 17 where R is... 

The solvent isotope effect produces an A-ratio (HOH/DOD) of three with isotope-independent A// of 17-18 kJ/mol. This result is more difficult to interpret, because it is unknown how many isotopic sites in the enzyme or water structure contribute to the isotope effect of 2-3. If a single site should be the origin of the effect, then the site could reasonably be a solvent-derived protonic site of the enzyme involved in general-acid catalysis of the hydride transfer, most simply by protonic interaction with the carbonyl oxygen of cyclohexenone or possibly by proton transfer to an olefinic carbon of cyclohexenone. 

The second procedure we employed involved the use of a switchable polarity solvent derived from a low-boiling secondary amine and carbon dioxide (6) [40]. 

In the more strongly ionizing and more nucleophilic solvent MeOH containing silver trifluoroacetate, a greater proportion of solvent-derived products were observed.The product 18a accounted for 70% of 14a under these conditions, while 19c and N-terr-butyl-para-anisidine accounted for 45% of 14c. ° If X was sufficiently electron withdrawing, the solvent-derived products were suppressed. Both 14e and 14g yielded only the rearranged products 15 and the reduction products 16. ... 

The presence of 61 (15%) in the hydrolysis of 46c in 0.5 M Kl, conditions that completely suppress the solvent-derived products of hydrolysis of 46c, revealed another pathway for production of 60 that appears to account for... 

The mechanism of Scheme 34 quantitatively explains the yields of rearrangement, solvent-derived and N3-derived products of hydrolysis of hydroxylamine or hydroxamic acid esters that yield selective nitrenium ions (log 5 One of the characteristics of these hydrolysis reactions is the... 

An example of a case in which all three processes contribute to the N3 trapping is provided in Scheme 40 and Fig. 3. The significant decrease in the yield of the rearrangement product in the NJ concentration range required to trap most of the solvent-derived products indicates that NJ trapping must occur at some other point in the mechanism in addition to the free ion. The theoretical curves of Fig. 3 were obtained from fitting the experimental product yield data to the mechanism of Scheme 39 with... 

A systematic investigation of the copper-catalyzed reaction between 2-bromobenzoic acid and the anions of 1,3-dicarbonyl compounds has established the optimum conditions for the direct arylation of the /3-dicarbonyl moiety (75T2607). The use of sodium hydride as the base and copper(I) bromide as catalyst is recommended. The absence of a protic solvent ensures that competitive attack on the bromobenzoic acid by a solvent-derived base leading to a salicylic acid is eliminated. For larger scale reactions the addition of toluene offers some practical advantages. 

The Bronstcd relation, statistical factors, and the acidity of solvent-derived species (H and H20) 345 Mechanism of the uncatalyzed reaction 348 The Marcus model of proton transfer 350... 

THE BR0NSTED RELATION, STATISTICAL FACTORS, AND THE ACIDITY OF SOLVENT-DERIVED SPECIES (H AND H20)... 

Thus, firstly, the choice of the pure solvent as the reference state for the definition of activities of solutes in fact impairs a fair comparison of the activity of dilute solutes such as general adds to the activity of the solvent itself. Secondly, the observed first-order rate constants k or k0 for the reaction of a solute with the solvent water are usually converted to second-order rate constants by division through the concentration of water, h2o = oA iho, for a comparison with the second-order rate coefficients HA. Again, it is questionable whether the formal h2o coefficients so calculated may be compared with truly bimolecular rate constants kUA for the reactions with dilute general acids HA. It is then no surprise that the values for the rate coefficients determined for the catalytic activity of solvent-derived acids scatter rather widely, often by one or two orders of magnitude, from the regression lines of general adds.74... 

This is a useful and informative situation, and solvolytic experiments of this kind have a particular value if an absolute value for the second-order rate constant, ki, for the reaction of the trap with the intermediate is known. In that case, an absolute value of the first-order rate constant, k2, for the conversion of the intermediate into the solvent-derived product maybe obtained, and hence an estimate of its lifetime under the reaction conditions. Measurements yielding values less than the vibrational limit (1.7 x 10 13 s at 25°C) indicate clearly that I has no real lifetime and hence is not a viable intermediate, and an alternative mechanism is required. For non-solvolytic reactions in a solution where the forward reaction of the reactive intermediate (other than with T) is bimolecular/second order, its lifetime will be diffusion controlled and the limit is likely to be closer to 10 10 s (though dependent upon the concentration of its co-reactant). 

This acutely toxic solvent derived from wood, natural gas, or petroleum can cause blindness. 

Tire burning etopped until renew penalt to burn whole tlrea public opposition to solvent-derived fuels working their copy through the penalt procees References 2, 7, 16, and 17... 

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