Abstraction selectivity from

A teehnique that is a convenient source of radieals for study by EPR involves photolysis of a mixture of di-t-butyl peroxide, triethylsilane, and the alkyl bromide corresponding to the radieal to be studied. Photolysis of the peroxide gives t-butoxy radieals, whieh selectively abstract hydrogen from the silane. This reactive silicon radieal in turn abstracts bromine, generating the alkyl radieal at a steady-state eoncentration suitable for EPR study. 

Hydroxy radical and sulfate radical anion, though they may sometimes give rise to similar products, show quite different selectivity in their reactions with unsaturated substrates. In particular, the sulfate radical anion has a somewhat lower propensity for hydrogen abstraction than the hydroxyl radical. For example, the sulfate radical anion shows little tendency to abstract hydrogen from mcthacrylic acid.232... 

The following are somewhat subjective selections from the vast amount of recent published material on solid proplnts. These short abstracts are grouped under the headings of ignition, combustion, reactivity, detonability safety, analytical procedures data, and miscellaneous. In each grouping the abstracts are arranged chronologically... 

The alkene serves to seavenge any Br- that forms from the reagent. Among the evidence for the mechanism involving 14 are abstraction selectivities similar to those of Ch atoms and the isolation of P-bromopropionyl isocyanate (BrCH2CH2C-ONCO), which is formed by ring opening of 14. 

Evidence-Based Medicine. Evidence-Based Medicine [45] consists of summarized abstracts of articles on family medicine, internal medicine, general surgery, pediatrics, obstetrics, gynecology, psychiatry, and anesthesiology with commentary by clinical experts. More than 50 peer-reviewed medical journals are abstracted. Also included are key selections from ACP Journal Club. Evidence-Based Medicine is commercially available however, its distribution is restricted and does not include North, Central, or South America. 

In this section, we read and analyze abstracts taken from chemistry journal articles. (Later in the chapter, we focus on titles.) We include passages from abstracts (P1-P17) and entire abstracts (excerpts 7B-7H) to illustrate both individual moves and how abstract moves work together as a whole. In part 1, we examine selections move by move. Because abstracts in organic chemistry journals vary slightly from other chemistry journals, we consider them separately at the end of part 1. In part 2, we examine writing practices that span the entire abstract. 

Sentences a-f are all from conference abstracts. Select the correct verb for each... 

In a quest to increase the efficiency of olefin polymerization catalysts and their selectivity in the orientation of the polymerization, the highly effective Group IV metallocene catalysts, M(Cp)2(L)2, have been studied, since they all display high fluxionality. Following methide abstraction, the metallocene catalysts of general formula M(Cp-derivatives)2(CH3)2 (M= Ti, Zr, Hf), were turned into highly reactive M+-CH3 cationic species. The activation parameters for the methide abstraction, derived from variable temperature NMR experiments, establish a correlation between the enthalpies of methide abstraction, the chemical shift in the resulting cation, and the ethylene polymerization activities [149]. 

Triazole has values of 2.19 (as a base) and 10.26 (as an acid). A representative selection of acidity data Hammett treatment to the pXa values of triazoles. The Hammett equation is more satisfactory for acidic than basic values of triazoles <65ZC38l). For triazoles with only annular NH as acidic groups the Hammett a value is 6.00, while for 3-(or 5-) hydroxytriazoles it is 7.62. In both cases constants are most appropriate <65ZC304) this is also true for nitrotriazoles <70KGS558). 

From the different contributions, it may be concluded that, in the oxidation of ammonia, the same type of redox mechanism is operative for metal oxides as in the selective oxidation of hydrocarbons. As a consequence, the hydrogen atoms will be abstracted successively from the NH3 molecule by a stepwise mechanism. 

The synthesis of pyrrolidines by the free radical transformation of (V-chloroamines, the Hofmann-Loeffler-Freytag reaction, is of preparative significance. The key step is the formation of a radical cation which abstracts hydrogen intramolecularly to form a carbon-based radical (Scheme 21(a)). This species then abstracts chlorine from another TV-chloroamine (60JA1657, 50JA2118). The observed positional selectivity for hydrogen abstraction is a consequence of the preferred adoption of a six-membered transition state. A typical conversion achieved is indicated in Scheme 21(b). 

In fact, the rates of y-hydrogen abstraction in 97 must be attenuated severely in the restrictive channels of zeolites. Even in Na-X and Na-Y, where virtually no photoproduct selectivity from valerophenone is observed, its triplet state half-life is 1 /is (vs. 5 ns in fluid isotropic solvents). The narrower channels of Na-ZSM-5 increase the triplet half-life to 15 /is ... 

Perbenzoic acid fcrf-butjl ester (51) is the source — under copper I) bromide catalysis —of a benzoate anion (52) and radical 53. Radical 53 subsequently abstracts a hydrogen atom selective ) from the 11 -position of 55 in a homolytic bond cleavage to give a buta diene system with opening of the cyclopropane ring.Jn... 

Lewis acids such as boron trihalides will also form stronger bonds with fluoride than with the other halogens, and can be used to abstract fluoride selectively from halogenated compounds. Some examples of Friedel-Crafts alkylations with fluoro-haloalkanes in which only fluoride is displaced are sketched in Scheme 4.14. As shown by the last example, however, hydride migrations can readily occur under such strongly acidic reaction conditions. 

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