Competing in reactions with

II) The electron competing in reaction with Si, Pi, or H202 the yield will be given by Equation II ... 

The radical generation step is apparently influenced by monomer (M) and solvent molecules (S) which possibly compete in reaction with the initiating complex (I). The radical generation reactions influenced by monomer and solvent may then be expressed as... 

A major limitation of this method is the low pH at which the reactions are performed, which resulted in substantially lower yields in reactions with substrate progenitors of acid-sensitive epoxides, in which competing ring-opening processes effectively reduced the usefulness of the method. As an example, the oxidation of styrene had proceeded with 70% conversion after 3 h at 70 °C, but the observed yield of styrene oxide was only 2% (Table 6.5, Entry 5). 

The other bromine atom comes from another bromine-containing molecule or ion. This is clearly not a problem in reactions with benzylic species since the benzene ring is not prone to such addition reactions. If the concentration is sufficiently low, there is a low probability that the proper species will be in the vicinity once the intermediate forms. The intermediate in either case reverts to the initial species and the allylic substitution competes successfully. If this is true, it should be possible to brominate an alkene in the allylic position without competition from addition, even in the absence of NBS or a similar compound, if a very low concentration of bromine is used and if the HBr is removed as it is formed so that it is not available to complete the addition step. This has indeed been demonstrated. ... 

Among the early second-row transition metals, the chemistry is richest for yttrium. In our early studies of Y + C2H2123 and C2H6,124 several competing product channels were observed, while only H2 elimination was observed for Zr, Nb, or Mo.122 Likewise, in reactions with larger molecules (containing three or more carbons for example), yttrium always led to the greatest number of unique product channels. A likely cause for this result is the presence of an exit-channel barrier for H2 elimination in reactions with Y. Because H2 elimination is usually the most thermodynamically... 

Alkylation of hydroxylamine with primary halides and sulfonates is rarely used nowadays for preparation of A-alkylhydroxylamines due to the competing formation of N,N-dialkylhydroxylamines. A number of older procedures have been reported with low to moderate yields of Al-alkylhydroxylamines. Yet, in many cases the reported low yields can be attributed to workup losses during distillation and crystallization steps rather than to the polyalkylation. Use of excess of hydroxylamine in reactions with primary alkyl halides (e.g. 3) improves the yields of monoalkylation (equation 2). Most of the examples of alkylation of hydroxylamine in good yield involve a substitution of an activated halogen atom at benzylic positions as well as in haloacetamides 4 leading to alkylhydroxylamines such as 5 where dialkylation rates are lower (equation 3). 

Methyl benzoate, anisole, and diphenyl ether each give sandwich compounds with chromium vapor, although in rather low yield (32, 55, 110). Chromium appears to attack alkyl ethers and this deoxygenation probably competes with complexation with the aromatic oxygen compounds. No simple product has been isolated from chromium atoms and aniline, but bis(7V,7V-dimethylaniline)chromium has been prepared (32). The behavior of molybdenum and tungsten vapors closely resembles that of chromium in reactions with oxygen- and nitrogen-substituted arenes (113). 

Because of the relatively slow rates of unimolecular reactions of excited acetone in solution at room temperature, acetone makes a convenient solvent-sensitizer for photosensitizatioh studies, provided that the substrate does not undergo competing chemical reactions with triplet acetone. A recent study of the effects of high-energy radiation on dilute acetone solutions of polynuclear aromatic molecules revealed that the triplet states of these compounds were being formed at close to the diffusion-controlled rate by collision with some pre-... 

The key reaction step for the formation of HCHO in this reaction scheme is the unimoiecular dissociation of the oxy-radical CH2(0H)CH20, i.e., reaction (34). In this experiment, products arising from as much as 20% of the C2H4 reacted could not be fully characterized due to experimental difficulties encountered in the product analysis due to the presence of large amounts of H20 impurity in the HONO samples employed [109], Notably, thermochemical estimates for the unimoiecular reaction (34) indicated the predominant occurrence of the competing bimolecular reaction with 02 under atmospheric conditions [104,108,113]. 

Similar to the reaction of 2-benzopyrylium salts with ammonia (Section III,C,4,a,i), an excess of competing nucleophiles in reactions with secondary amines is undesirable. Thus, the interactions of l-ethyl-3-methyl-substituted salt 183 with 40% aqueous solution of dimethylamine leads to a-naphthol 189 as the major product (79TH1). 

The disconnection underlying this procedure is an alkyl-oxygen fission and not the acyl-oxygen fission as in the reactions discussed above. The non-nucleophilic base used in this reaction is l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) which converts the carboxylic acid into its carboxylate ion but does not interact in a competing substitution reaction with the alkyl halide. The ester-forming reaction may therefore be regarded as an SN2 reaction between the carboxylate ion and the alkyl halide.162... 

The formation of significant amounts of polymeric material, with even a slight excess of TFE, along with formation of dimeric products in reaction with HFP bromine tris (fluorosulfate) is strong evidence in favor of the radical mechanism, which in this reaction may compete with the electrophilic process. 

Kimura el al. (2007) reported the first reproducible production of mass-independent oxygen in iron and silicate smokes from gas-phase reactions (Fig. 4.8). It is possible to misinterpret these data as being associated with ozone, which is known to have a large mass-independent composition. Comparing kinetics of the rates of the two reaction channels competing for reaction with O-atoms O + O2 + M — O3 + M and O + H2 — OH + H, one finds a factor of twenty higher probability for a H2 sink compared to ozone formation. 

Note that the reaction rate constants of the same inhibitors with different radicals differ considerably (by several orders of magnitude) [60, 61] (see Table 1). There are cases when antioxidants that are active for some radicals can not compete in interacting with others and we can not protect cell components from the effects of these radicals because the afiinity of radicals to them will be higher than that of inhibitors introduced. 

As a rule, stoichiometric cuprate reagents have provided the most consistently successful results in reactions with primary alkyl electrophiles. Diethyl ether is the solvent of choice for reactions of alkyl sulfonates, while reactions of alkyl halides appear to be facilitated by THF. The enhanced basicity of the cuprate reagent in THF may be problematic, however, when racemization of an adjacent chiral center or elimination is a competing side reaction. For example, reactions of serine-derived 3-halo esters must be performed in ether, since elimination by-products are the only products isolated when THF is employed as the solvent elimination is more problematic with sulfonate than with halide leaving groups. 

Ion-exchange reactions are reversible therefore, ion-exchange behavior can be described in terms of equilibrium equations. The exchange of two ions, A and B, competing for reaction with the resin is represented by Eq. 5.5a for monovalent ions and by Eq.5. 5b for ions of different charge. 

The parameter K can also, like values in enzyme kinetics, be an agglomeration of rate constants that refers to a steady state, rather than an equilibrium situation. Rapid reaction kinetics, in the case of propane-1,2-diol, established that the intermediate was kinetically competent.The reaction with pinacol, where K is very large and the reaction is cleanly second order over a wide range of pinacol concentration,exhibited general acid and general... 

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